*Article* **Secondary Hyperparathyroidism in Children with Mucolipidosis Type II (I-Cell Disease): Irish Experience**

**Ritma Boruah 1,\*, Ahmad Ardeshir Monavari 1,2, Tracey Conlon 2,3, Nuala Murphy 2,3, Andreea Stroiescu 4, Stephanie Ryan 4, Joanne Hughes 1, Ina Knerr 1,2, Ciara McDonnell <sup>3</sup> and Ellen Crushell 1,2**


**Abstract:** Mucolipidosis type II (ML II) is an autosomal recessive lysosomal targeting disorder that may present with features of hyperparathyroidism. The aim of this study was to describe in detail the clinical cases of ML II presenting to a tertiary referral centre with biochemical and/or radiological features of hyperparathyroidism. There were twenty-three children diagnosed with ML II in the Republic of Ireland from July 1998 to July 2021 inclusive (a 23-year period). The approximate incidence of ML II in the Republic of Ireland is, therefore, 1 per 64,000 live births. Medical records were available and were reviewed for 21 of the 23 children. Five of these had been identified as having biochemical and/or radiological features of hyperparathyroidism. Of these five, three children were born to Irish Traveller parents and two to non-Traveller Irish parents. All five children had radiological features of hyperparathyroidism (on skeletal survey), with evidence of antenatal fractures in three cases and an acute fracture in one. Four children had biochemical features of secondary hyperparathyroidism. Three children received treatment with high dose Vitamin D supplements and two who had antenatal/acute fractures were managed with minimal handling. We observed resolution of secondary hyperparathyroidism in all cases irrespective of treatment. Four of five children with ML II and hyperparathyroidism died as a result of cardiorespiratory failure at ages ranging from 10 months to 7 years. Biochemical and/or radiological evidence of hyperparathyroidism is commonly identified at presentation of ML II. Further studies are needed to establish the pathophysiology and optimal management of hyperparathyroidism in this cohort. Recognition of this association may improve diagnostic accuracy and management, facilitate family counseling and is also important for natural history data.

**Keywords:** mucolipidosis type II; ML II; I-cell disease; hyperparathyroidism

#### **1. Introduction**

Mucolipidosis type II (ML II) (OMIM #252500) or inclusion cell disease (I-cell disease) is a rare autosomal recessive lysosomal enzymetargeting disease due to deficiency of uridine diphosphate-N-acetylglucosamine: lysosomal enzyme N-acetylglucosamine-1 phosphotransferase (GlcNac-1-phosphotransferase). This enzyme is involved in the first step of the mannose 6-phosphate signal, which allows specific targeting of lysosomal acid hydrolase from the trans-Golgi network to lysosomes. The enzyme deficiency precludes the generation of the common phosphomannosyl recognition marker of lysosomal enzymes [1]. Subsequently, newly synthesized lysosomal enzymes are secreted into the extracellular

**Citation:** Boruah, R.; Monavari, A.A.; Conlon, T.; Murphy, N.; Stroiescu, A.; Ryan, S.; Hughes, J.; Knerr, I.; McDonnell, C.; Crushell, E. Secondary Hyperparathyroidism in Children with Mucolipidosis Type II (I-Cell Disease): Irish Experience. *J. Clin. Med.* **2022**, *11*, 1366. https:// doi.org/10.3390/jcm11051366

Academic Editors: Karolina M. Stepien and Sylvia Lee-Huang

Received: 6 January 2022 Accepted: 25 February 2022 Published: 2 March 2022

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

space rather than targeted to the lysosomes. Thus, affected lysosomes are secondarily deficient in most acid hydrolases; undigested junk materials accumulate within the lysosomes [1,2]. ML II was first described as inclusion-cell (I-cell) disease by Leroy and Demars in 1967 [3], because the fibroblasts derived from patients contain abundant 'inclusions' (now recognized as swollen lysosomes) within the cytoplasm. These inclusions are observed not only in cultured skin fibroblasts, but also in a variety of other cell types in vivo, including peripheral blood lymphocytes [2].

The term "mucolipidosis" was introduced in 1970 by Spranger and Wiedemann to describe several conditions with features both of mucopolysaccharidoses (MPS) and sphingolipidoses [4]. ML II is a progressive multi-organ disease, usually with prenatal clinical onset and fatal outcome within the first decade of life due to cardiopulmonary complications [5]. It is characterized by coarse facial features, short stature, hyperplastic gums, organomegaly, retarded psychomotor development and skeletal deformities, which may include shortened limbs, flexion contractures and talipes [6,7]. Secondary hyperparathyroidism is a recognized feature of ML II [1,7–11]. Reported biochemical features of secondary hyperparathyroidism include elevated parathyroid hormone (PTH), serum calcium (Ca), alkaline phosphatase (ALP) levels and low levels of phosphate (P) [8,11–14], Radiographic findings in neonates resemble changes of rickets and/or hyperparathyroidism. These changes include osteopenia, subperiosteal resorption, poor cortical delineation, periosteal new bone formation with 'cloaking' (linear periosteal new bone parallel to the shaft of the bone but widely separated from the bone), metaphyseal irregularity and submetaphyseal lucent bands and later develop into Hurler-type dysostosis multiplex. Congenital long bone and rib fractures are rare and likely the result of severe osteopenia and disorganized bone formation [9,12,15–17]. Bone changes can precede elevations in biochemical markers [12]; therefore, regular monitoring in infancy is required and skeletal radiographs should be performed regardless of initial biochemical findings.

Clinical suspicion is the first step in establishing a diagnosis of ML II, with typical clinical features often apparent at birth or otherwise manifesting in the first year of life [12]. An indirect diagnosis is usually established by measurement of lysosomal hydrolases, both in white blood cells, where their levels should be low, and their surrounding extracellular environment (e.g., plasma), where their levels should be high [18]. The diagnosis is confirmed by *GNPTAB* gene molecular analysis; this is particularly important in cases when biochemical testing is inconclusive or carrier detection is required [11,19]. There are at least 258 mutations reported in the *GNPTAB* gene, the most prevalent being c. 3503\_3504del. Despite increased prevalence of homozygous mutations, particularly in highly consanguineous populations, the autosomal recessive inheritance of MLII means a high number of compound heterozygous *GNPTAB* sequence alterations [5]. ML II is a multi-ethnic disease. It has been identified in many different ethnic groups [9,11,19–25], with reported prevalence as follows: Portugal—approximately 1:123, 500 live births [22], Japan—1:252, 500 live births [23] and 1:625, 500 live births in Netherlands [24].

ML II has a very high incidence of 1 per 909 live births in the Irish Traveller community [26]. Irish Travellers are an endogamous grouup who have cultural values and customs quite distinct from that of the "settled community", i.e., the non-Traveller Irish population. Cultural traditions within the community include a preference to marry within their own community (often resulting in consanguineous unions), young age at marriage and large families [26].

Hyperparathyroidism is not universal but has been observed in patients with MLII [1,7–12,14,27]. The biochemical and radiological features of hyperparathyroidism in infants with ML II vary in the literature [9,10,12,15,16,27,28], with resolution of these findings observed in many cases, even in the absence of active management [14,17,27]. It is known that, following this initial early period where features of hyperparathyroidism or rickets may be observed, children with ML II experience a progressive osteodystrophy [9]. Recognition and active management of hyperparathyroidism may prevent complications such as bone fractures and, thus, improve quality of life for the affected children. It is also

important to recognize hyperparathyroidism in this cohort for natural history data. Here, we describe clinical, biochemical, radiological and molecular findings in five children with ML II and hyperparathyroidism from 5 unrelated families.

#### **2. Materials and Methods**

This study was performed in The National Centre for Inherited Metabolic Diseases, Children's Health Ireland (CHI) at Temple Street in the Republic of Ireland. A retrospective chart review of ML II patients born between July 1998 and July 2021 inclusive was performed, providing a twenty-three-year cohort. A database was compiled documenting clinical features, focusing on those consistent with hyperparathyroidism. For those diagnosed with hyperparathyroidism, biochemical, radiological and molecular data were recorded. This study was approved by the Research and Ethics Committee of CHI at Temple Street (protocol code 21. 013).

#### **3. Results**

We identified 23 patients with ML II from 14 families, born between 1 July 1998 and 1 July 2021. This gives an approximate national incidence of 1 per 64,000 live births in the Republic of Ireland. Medical records were available for 21 out of 23 children. Of the 23 identified, 19 were from the Irish Traveller community, confirming the very high incidence within the Traveller community, in line with previously reported figure of 1 in 909 [26]. In this cohort, five patients from five families had biochemical and/or radiological evidence of hyperparathyroidism, three of these children were born to Irish Traveller parents.

#### *3.1. Diagnosis of ML II—Biochemical and Molecular Genetic Features*

The diagnosis of ML II was based on clinical features and biochemical testing by detecting increased activity of alpha mannosidase and beta hexosaminidase in plasma. In all cases, the diagnosis was confirmed by molecular genetic testing; these patients were found to be homozygous for a common mutation c.3503\_3504delTC (p.L1168Qfs\*5) in *GNPTAB* gene.

#### *3.2. Biochemical Features of Hyperparathyroidism*

All five patients had been tested for hyperparathyroidism within the first few weeks of life and four had increased levels of parathyroid hormone (PTH). Calcium (Ca) and phosphate (P) levels were normal; however, alkaline phosphatase (ALP) levels were markedly raised in four of five cases (Patients 2, 3, 4, 5). Vitamin D levels were checked in four cases (Patients 2, 3, 4, 5) and were normal in three, with one (Patient 5) having a suboptimal level of 30 nmol/L (normal range is >50 nmol/L).

#### *3.3. Radiological Abnormalities Identified*

Skeletal radiographs in the first week of life were available for four patients and at 2 months of age for one patient (Patient 3). Follow up radiographs were available for four out five patients. Early radiographs already showed marked changes of hyperparathyroidism in all five infants including osteopenia, subperiosteal resorption, poor cortical delineation, periosteal new bone formation with 'cloaking' (linear periosteal new bone parallel to the shaft of the bone but widely separated from the bone), metaphyseal irregularity and submetaphyseal lucent bands (Figures 1 and 2).

**Figure 1.** Acute findings of hyperparathyroidism and rickets. Patient 1, left leg radiograph day 1 demonstrates features of hyperparathyroidism including reduced bone density, subperiosteal resorption and poor cortical delineation (see medial tibia) as well as diaphyseal cloaking of the femur and tibia (arrowheads). Features of rickets are seen with cupped, splayed and frayed metaphyses especially in the distal femur. Diaphyseal angulation consistent with antenatal fractures is seen in the distal femur and tibia. Additionally, talocalcaneal stippling, a feature of I-cell disease is also present (white arrow).

**Figure 2.** Acute findings of hyperparathyroidism and of rickets. Patient 2, right leg radiograph day 4 (**a**) frontal and (**b**) lateral views show features of hyperparathyroidism (best seen on the lateral view) including reduced bone density, subperiosteal resorption and poor cortical delineation as well as diaphyseal cloaking of the tibia (arrowheads). Features of rickets are seen with cupped, splayed and frayed metaphyses in all the bones. A submetaphyseal lucent band is seen in the tibia (white arrow). Talocalcaneal stippling is also present.

All early radiographs also showed features of rickets, with metaphyseal cupping, fraying and splaying (Figures 1 and 2). There was evidence of antenatal long bone fractures in Patients 1, 2 and 4. Patient 4 also had an acute fracture of the proximal left humeral neck (Figure 3). Further radiographic findings of ML II including talocalcaneal stippling were identified in Patients 1, 2 and 5, and an abnormal appearance of the vertebral bodies with increased height; rounding and sclerosis were seen in Patients 1, 2 and 4.

**Figure 3.** Acute fracture with healing at 4 months. Patient 4 (**a**) radiograph of left humerus on day 1 shows a transverse fracture of the left humerus. (**b**) Follow-up radiograph 4 months later showing interval healing of the fracture and resolution of the periosteal cloaking. There are already emerging features of dysostosis multi-plex with widening of the shaft and short length of the humerus and coarse trabecular markings as well as thickening of the ribs.

Follow-up radiographs in all but the most recent patient showed resolution of the features of hyperparathyroidism and rickets, with interval healing of the fractures. There was progression of skeletal features to those of dysostosis multiplex, the constellation of radiographic abnormalities classically seen in mucopolysaccharidoses (MPS), including coarse trabecular markings, broadening of the ribs (oar/paddle shaped ribs), flared iliac wings, constricted inferior iliac bodies and dysplastic femoral heads (Figures 3 and 4).

**Figure 4.** Progression to dysostosis multiplex. Patient 4 (**a**) Radiograph of pelvis on day 1 shows a reduced bone density and irregularity of the proximal femora with periosteal cloaking. (**b**) Follow-up radiograph at 2 years old shows normal bone density. The pelvis now has a typical shape of dysostosis multiplex with constriction of the lower part of the iliac bones. There has been interval healing of the rickets of the proximal femora and resolution of the periosteal cloaking.

Follow up radiographs in Patient 1 showed resolution of the acute changes, including the periosteal reaction and subperiosteal erosion, but development of a progressive erosive osteodystrophy with erosion of the humeral and femoral necks and also erosion of the necks of the ribs was observed (Figure 5).

**Figure 5.** Progressive erosive osteodystrophy in Patient 1 at 6 years. Resolution of the acute changes seen in Figure 1. Development of a progressive erosive osteodystrophy with erosion of the heads and necks of the ribs, erosion of the lower part of the iliac bones, erosion of the ischial and pubic bones and of the femoral necks.

#### *3.4. Management and Clinical Course*

Two of the infants who had evidence of antenatal fractures (Patients 1, 4), including the one with an acute fracture (Patient 1), received treatment with increased Vitamin D supplementation of 600 IU per day and guidance around minimal handling, including a lie flat car seat, lying supported on the side and avoidance of walkers and bouncers. In the case of Patient 4, the previously abnormal biochemical parameters normalized within five months of treatment with vitamin D; the dose was then reduced to the standard supplementation dose of 200 IU/day, with handling liberalized successfully. A followup left humeral radiograph 4 months later showed interval healing of the humeral neck fracture (Figure 3). Patient 1 was followed up at a local hospital and vitamin D dose was reduced to 200 IU/day and handling liberalized at 1 year.

Patients 2 and 3 were monitored without specific treatment. In Patient 2, PTH level had spontaneously returned to normal at 10 months. In the case of Patient 3, the previously abnormal biochemical marker (ALP) had normalized at 10 months of age (during an admission to Pediatric Intensive Care unit with respiratory failure). No further fractures were observed. Patient 5 was started on high dose Vitamin D supplementation of 1000 IU/day with interval re-evaluation planned.

Four of the five affected infants died from progressive cardiopulmonary decline at ages ranging from 10 months to 7 years. Summary of clinical, biochemical, radiological features of secondary hyperparathyroidism, Vitamin D levels at diagnosis and treatment can be found in Table 1.


**Table 1.**

Summary Summary of clinical,

biochemical,

 radiological

 features of secondary

hyperparathyroidism

 and Vitamin D levels at diagnosis

#### **4. Discussion**

Patients with ML II have been reported to present in the neonatal period with features of ''metabolic" bone disease [9,27]. While features of dysostosis multiplex (the constellation of radiographic abnormalities classically seen in mucopolysaccharidoses) are seen in older children with ML II, the skeleton in ML II in the young infant is characterized by an osteodystrophy which has clinical and radiographic features of hyperparathyroidism and rickets and these changes have been reported in ML II as early as 19 weeks of gestation [28].

Osteoporosis, fractures, periosteal new bone formation and cupped epiphyses have been described in neonates and infants [9]. Radiological and histological features of both rickets and hyperparathyroidism (subperiosteal bone resorption and loss of bone mass) have been documented in babies with I-cell disease [15,16]. Biochemical evidence of hyperparathyroidism is more variable but had been reported in some affected neonates [9].

In our study, none of the patients had abnormal serum calcium or phosphate levels. Similar findings were reported by David-Vizcarra et al. [9]. Four out of five of our patients had increased levels of ALP and PTH. All children had early skeletal x-rays with radiographic evidence of hyperparathyroidism, including osteopenia, subperiosteal bone resorption, poor cortical delineation, periosteal new bone formation, metaphyseal irregularity and submetaphyseal lucent bands. Three had evidence of antenatal fractures. All early radiographs also had features of rickets, with metaphyseal cupping, fraying and splaying.

Sathasivam et al. [10] previously described similar findings in a female neonate with neonatal hyperparathyroidism and rickets-like radiographic changes. Alfadhel et al. [11] reported two children with raised PTH and ALP, normal calcium levels who also had radiographic changes consistent with rickets/hyperparathyroidism. Some authors speculate that the presence of severe skeletal changes related to secondary hyperparathyroidism indicate that the abnormal elevation of PTH starts in utero [8].

The exact etiology of secondary hyperparathyroidism in ML II remains unclear. It has been suggested that the active transplacental transport of calcium is interrupted by ML II. The syncytiotrophoblastic layer where active transplacental calcium transport is regulated demonstrates generalized cytoplasmic vacuolization in patients with ML II. This suggests that the enzymatic abnormalities related to ML II interfere in some way with transplacental calcium transport. PTH secretion is thought to increase to maintain extracellular calcium at the expense of the skeleton [27]. This, however, does not explain skeletal changes in those with normal PTH levels.

David-Vizcarra et al. [9] observed that, following birth, biochemical hyperparathyroidism in ML II resolves, but a progressive erosive osteodystrophy develops after 4 months of age. We saw this progressive erosive osteodystrophy in one patient whose early PTH levels were not increased. They proposed that tissue hypersensitivity to circulating PTH ("pseudohyperparathyroidism") may be a factor. They confirmed that circulating levels of parathyroid related protein (PTHrP) were normal and postulated that, postnatally, the radiographic features could be consistent with an increased sensitivity of skeletal tissue to normal circulating levels of PTH. A more recent study by Kollmann et al. [29], however, refuted tissue hypersensitivity to PTH as a pathogenetic mechanism for the osteopenia, since the secretion of Rankl (pro-osteoclastogenic cytokine) in osteoblasts from ML II mice was not affected in response to PTH stimulation.

The above demonstrates the need for further studies regarding the pathophysiology of bone disease in patients with ML II.

Optimal management of the secondary hyperparathyroidism in this patient cohort is also controversial, as changes may be self-limiting and might represent the natural history of ML II [14,17,27].

Unger et al. [27] described three patients with bone disease, increased serum PTH and ALP, but normal calcium levels. Two were treated with Vitamin D and calcium supplements, while one received no treatment and secondary hyperparathyroidism resolved in all cases. Another report by Leyva et al. [8] described a patient with secondary hyperparathyroidism (with biochemical and radiographic changes), in whom a spontaneous normalization of

previously elevated PTH was observed in the absence of treatment, however the ALP level remained high and radiographic follow up was not reported.

In contrast, a patient reported by Khan et al. [12] had low serum phosphate, normal vitamin D and PTH levels at birth and radiographic findings of rickets/hyperparathyroidism. Initially, after parents declined vitamin D supplementation, ALP and PTH levels rose significantly. At 4 months of age, parents agreed to vitamin D supplementation and, within a month, serum phosphate, PTH and ALP levels normalized. This indicates that bone disease can precede the elevations in biochemical markers and highlights the importance of regular biochemical monitoring, even if initial PTH levels are normal. We recommend checking for biochemical and radiographic features of secondary hyperparathyroidism at diagnosis of ML II by checking levels of PTH, Ca, P, ALP, Vitamin D and by performing skeletal survey. We suggest monitoring of biochemical markers at least 6–12 monthly during infancy; however, such monitoring intervals are arbitrary and would depend on the initial levels. Similarly, the frequency of radiological monitoring would depend on initial clinical and radiographic findings, e.g., sooner re-imaging for those with bone fractures. Rapid normalization of biochemical markers post commencement of vitamin D supplementation may indicate that this supplement has a role in the treatment of secondary hyperparathyroidism in some patients with ML II. Currently, in our centre, we recommend high dose Vitamin D supplements in those with biochemical features of secondary hyperparathyroidism and/or bone fractures and low/suboptimal Vitamin D levels. Minimal handling, including a lie flat car seat, lying supported on the side and avoidance of walkers and bouncers would be recommended for children with bone fractures.

Antiresorptive therapy has also been suggested as a therapeutic option, but this is usually reserved for individuals with a high fracture risk and this treatment is controversial in ML II, given the multisystem involvement and overall poor prognosis [29].

Unger et al. [27] summarized that most children with a neonatal presentation of ML II and perinatal bone disease have a shortened life expectancy, i.e., most die before the age of 2 years. In our cohort with secondary hyperparathyroidism, all patients had perinatal bone disease confirmed by skeletal survey and two died before the age of 2 years.

Given the rarity of ML II, extensive sequential assessment of five patients with secondary hyperparathyroidism from a clinical, radiological and biochemical perspective provides important information regarding natural history of the condition. Our paper also provides recommendations on diagnostics and management of secondary hyperparathyroidism in this cohort; we believe that these would help to improve diagnostic accuracy and to optimize management of these patients. In addition, our paper contains interesting figures depicting clinical and radiological course of the disease and provides an up-to-date incidence of ML II in Republic of Ireland.

While we identified five members of the cohort as having secondary HPT, not all infants were systematically investigated for the same; therefore, it is likely that there is under-ascertainment.

#### **5. Conclusions**

Children with ML II may have varying degrees of radiological and biochemical features of hyperparathyroidism at presentation. It is important to recognize this association, as this may improve diagnostic accuracy and management. It is also important for appropriate family counseling and natural history data. In this cohort, resolution of abnormal biochemical findings was observed in all cases, irrespective of management. Further studies are needed to establish the etiology and pathophysiology of the bony changes observed in ML II and the potential benefit of Vitamin D supplementation in this cohort.

**Author Contributions:** Conceptualization, E.C. and R.B.; methodology, R.B.; data curation, R.B.; clinical data—I.K., J.H. and C.M.; writing—original draft preparation, R.B.; radiology images and legends—S.R. and A.S.; writing—review and editing, E.C., N.M., T.C., A.A.M., S.R. and A.S.; supervision, E.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** The study was conducted in accordance with the Declaration of Helsinki and approved by the Research and Ethics Committee of Children's Health Ireland at Temple Street, Dublin (protocol code 21. 013).

**Informed Consent Statement:** This study was approved as a retrospective chart review under the Health Research Regulations, 2018 and patient consent is not required.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy restrictions.

**Acknowledgments:** We acknowledge the care given by colleagues on multidisciplinary teams at NCIMD and across the country who look after these special children. We thank staff at Willink laboratory in Manchester for diagnostic assistance, and last, but not least we thank our patients and their parents as without them this manuscript would not be possible.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Nadia Ali 1,\*, Amanda Caceres 2, Eric W. Hall <sup>3</sup> and Dawn Laney <sup>1</sup>**


**Abstract:** The present pilot study examines subjective reported symptoms of attention-deficit/ hyperactivity (AD/H) in adults with Fabry disease (FD) in comparison with existing normative control data. Existing data from 69 adults with FD via the Achenbach System of Empirically Based Assessment Adult Self-Report questionnaire were analyzed. The results demonstrated a higher prevalence of AD/H symptoms in adults with FD than in the general United States population, with a roughly equal endorsement of Inattention/Attention Deficit symptoms (AD), Hyperactivity-Impulsivity (H-I) symptoms, and Combined Inattention/hyperactivity-impulsivity (C) symptoms. No gender differences were observed. While all subjects endorsing H-I symptoms fell into the symptomatic range on the AD/H scale, only two-thirds of subjects endorsing AD did so. This suggests that attention difficulties with FD are not solely explained by ADHD. Adults with FD who endorsed the AD, H-I, and C symptoms were also more likely to report mean adaptive functioning difficulties. These findings support the growing literature regarding attention difficulties in adults with FD, as well as suggesting a previously unrecognized risk of AD/H symptoms. Future research involving the objective assessment of ADHD in adults with FD is recommended. When serving adults with FD clinically, healthcare professionals should address multiple areas of care, including physical, psychological, and cognitive arenas.

**Keywords:** Fabry disease; attention; Attention Deficit/Hyperactivity; cognition

#### **1. Introduction**

Fabry disease (FD) is an X-linked lysosomal storage disorder (LSD) caused by mutations in the *GLA* gene, leading to a deficiency of α-galactosidase A (α-gal A; EC 3.2.1.22) and resulting in the storage of globotriaosylceramide (GL3) and related lipids in the lysosome. Its incidence has historically been estimated at 1:40,000 male live births; however recent data suggests as high as 1:3000 [1], with a range of 1250–117,000 worldwide [2]. The symptoms and complications include acroparesthesia, fatigue, anhidrosis, angiokeratomas, gastrointestinal symptoms, kidney failure, cardiovascular problems, and stroke [3–7]. The standard of care treatment is enzyme replacement therapy (ERT) or chaperone therapy (in individuals with amenable *GLA* mutations).

Historically, research has focused on somatic manifestations of FD, with less attention paid to neuropsychological manifestations. However, recent research suggests difficulties with cognitive functioning, particularly in the realm of attention and concentration, with implications for central nervous system (CNS) functioning in patients with FD.

The initial neuropsychological screening studies of patients with FD reported contradictory results due to varying testing methods and small sample sizes. One initial study found patients with FD performed marginally better on tasks of attention than normal

**Citation:** Ali, N.; Caceres, A.; Hall, E.W.; Laney, D. Attention Deficits and ADHD Symptoms in Adults with Fabry Disease—A Pilot Investigation. *J. Clin. Med.* **2021**, *10*, 3367. https:// doi.org/10.3390/jcm10153367

Academic Editors: Karolina M. Stepien, Christian J. Hendriksz and Gregory M Pastores

Received: 26 June 2021 Accepted: 24 July 2021 Published: 29 July 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

controls and slightly worse on tasks measuring language skills, with unimpaired performances in other cognitive domains [8], while another found patients with FD performed mildly worse on tasks of attention than normal controls [9]. Although the patients initially appeared to perform worse on executive functioning tasks, this difference disappeared once corrected for the effects of depression and remained absent in a subset of patients eight years later [10]. The subsequent early research found that patients with FD performed worse on some tests of attention (especially those involving information processing speed) [11–13], as well as some measures of executive functioning [11,13].

The first study to examine neurocognitive functioning in FD using comprehensive and well-validated neuropsychological measures rather than screening tools found that males with FD demonstrated a slower information processing speed and reduced performance on measures of executive functioning compared to both females with FD and 15 age-matched normal controls [14]. However, several confounds were present. None of the females with FD had experienced a stroke or transient ischemic attacks compared to 33% of the males. Males with FD were also more likely to report symptoms of anxiety and depression, which is known to have delirious effects on cognition, including attention, memory, and executive functioning [15]. Taken together with a low sample size and correlational analyses suggesting a link between the cognition and clinical measures of disease severity, these confounds compromised the generalizability.

A more recent study found 29.3% of Danish patients with FD to have cognitive difficulties, with attention, psychomotor speed, and executive functioning once again being the most frequently impaired [16]. Neither depression, disease severity, nor gender predicted objective cognitive impairment; however, depression was associated with the subjective perception of cognition. The subjective perception of cognition was lower than the actual cognitive performance among subjects.

In comparison, subjective perceptions of cognitive impairment among Dutch subjects with FD were found to be much greater (64%) than the objective evidence of impairment (16%) [17]. Objective impairment was found primarily in males, especially those with classical FD. Follow-up testing one year later, however, demonstrated a worsening objective cognitive impairment in only 5.3% of subjects and was found more often among women (three women and one man) [18]. Subjective impairment was prevalent in both genders and correlated with depression [16,17].

Given the increasing evidence of the role of FD in aspects of attention, anecdotal patient reports regarding the use of medication for Attention Deficit Hyperactivity Disorder (ADHD) should perhaps not come as a surprise. Previously referred to as Attention Deficit Disorder (ADD) in the Diagnostic and Statistical Manual of Mental Disorders 3rd Edition (DSM III) [19], one of the core symptoms is a deficiency in attention. The updated label of ADHD in DSM IV and DSM-5 is an umbrella term for a wide range of symptoms and consists of three main types: Inattentive/Attention Deficit (AD), Hyperactive-Impulsive (H-I), and Combination (C) types [20,21]. While attention-deficit/hyperactivity (AD/H) symptoms in patients with FD have been shown to be associated with poorer adaptive functioning (AF) [22], no further exploration of AD/H symptoms in FD has been done. A pilot study specifically documenting and exploring patient reports of attention deficits will be beneficial as a prequel to more in-depth studies of attention deficits in patients with FD.

The present pilot study examines the self-reported symptoms of attention deficits/ hyperactivity in adults with FD in comparison with the existing normative control data, as well as potential differences in the frequency between symptoms of attention deficits and symptoms of hyperactivity. In addition, we explored the possible association between attention-deficit/hyperactivity symptoms and poorer adaptive functioning in patients with FD.

#### **2. Materials and Methods**

Data was derived from a subset of data in existence at the Emory Lysosomal Storage Disease Center. Specifically, data concerning Attention Deficit/Hyperactivity, Attention, Inattention, Hyperactivity-impulsivity, Somatic Symptoms, Depression, Anxiety, and Mean Adaptive Functioning were utilized from the Achenbach System of Empirically Based Assessment (ASEBA) Adult Self-Report (ASR) questionnaires completed by patients with FD between January 2005 and July 2013. Approval from the Institutional Review Board was granted through Emory University (IRB00068700).

The ASEBA ASR is a reliable, validated measure of social-adaptive and psychological functioning in adults aged 18–59 and the OASR for ages 60–90+ [23]. Norms represent the mix of ethnicities, socioeconomic status, urban–rural–suburban residency, and geography within the US. Raw scores are converted to T-scores to permit comparisons with the general population. Scale scores are normed by gender and age and categorized as normal (<93rd percentile), borderline-clinical (93rd–97th percentiles), or clinical (>97th percentile). The ASEBA is used with a wide variety of medical conditions, including cystic fibrosis, Fabry, Morquio, Turner, Williams, Angelman, and Prader-Willi syndromes [22–25].

#### *Data Analysis*

ASEBA ASR raw data was entered into assessment data manager (ADM, version 9.0) ASEBA scoring software (https://adm-assessment-data-manager.software.informer.com/ 9.0/, accessed on 1 June 2021), which produces detailed profiles on multiple aspects of psychological functioning. For this study, data from the DSM-Oriented Scale for AD/H, as well as the Attention Problem Syndrome scale, Depression scale, Somatic Complaints scale, and Mean Adaptive Functioning scale, were utilized. Subjects with T-scores in the borderline-clinical and clinical ranges were considered to have symptoms for the purposes of this study.

All data analysis was done using SAS 9.4 (SAS Institute, Cary, NC, USA). Demographic participant characteristics were summarized using frequencies and proportions. Chi-square and Fisher's exact tests were used to assess the associations between mean adaptive functioning and demographic variables of interest. Similarly, chi-square tests were used to assess the association between depressive symptoms and gender, AD/H symptoms, H-I symptoms, and AD symptoms. The prevalence of AD/H in our study sample was compared to the most recent estimated prevalence of AD/H among the US adult population [26] using Fisher's exact test. All statistical tests were assessed using an alpha = 0.05.

#### **3. Results**

Existing data from 69 adults with FD who completed the ASEBA ASR questionnaire was examined. The demographic information is presented in Table 1. The ages ranged from 18 to 61 years.


**Table 1.** Demographic characteristics of the subjects.

Of the 69 subjects who completed the ASEBA ASR, twenty (29%) endorsed symptoms within the borderline-clinical-to-clinical range on the AD/H problems scale (Figure 1). This represents a significantly higher prevalence of AD/H symptoms in our population of adults with FD than in the general population (*p* < 0.001), using the most recently estimated prevalence (4.4%) of adult ADHD in the United States [26].

**Figure 1.** Prevalence of ASEBA symptoms in adults with Fabry disease.

Among the twenty subjects endorsing symptoms within the borderline-clinical-toclinical range on the AD/H scale, the source of those scores was almost equally balanced between the symptomatic endorsement of AD items, H-I items, and combined AD/H items, with a final three subjects whose endorsement of items was evenly split such that they fell within the normal ranges on the individual subscales while still falling within the symptomatic range on the overall combined AD/H scale (Table 2). All subjects endorsing the H-I symptoms within borderline-clinical-to-clinical range also scored in the borderline-clinical-to-clinical range on the AD/H scale; however, only 12/19 (63.2%) subjects endorsing AD symptoms also scored in the borderline-clinical-to-clinical range on the AD/H scale.

**Table 2.** Subscale breakdown among adults with FD endorsing AD/H symptoms (*n* = 20).


Almost half of the adults with FD (49%) were also noted to self-report depressive symptoms in the borderline-clinical-to-clinical range on the ASEBA ASR, with no significant differences between the male and female subjects (*p* = 0.537). A third of the adults with FD (33%) self-reported symptoms of anxiety, with no significant differences between the male and female subjects (*p* = 0.0870). Almost a third of adults with FD (29%) self-reported difficulties in adaptive functioning, with no significant differences between the male and female subjects (*p* = 0.060). Over a third of adults with FD (38%) self-reported somatic symptoms in the borderline-clinical-to-clinical range, with no significant differences between the male and female subjects (*p* = 0.2468).

Adults who scored in the borderline-clinical-to-clinical range on the AD/H scale, AD subscale, and H-I subscale were significantly more likely to self-report both depressive symptoms and somatic problems (Table 3). Adults scoring in the borderline-clinical-toclinical range on the AD/H scale and H-I scale were significantly more likely to self-report anxiety symptoms as well (Table 3). There were no differences between males and females in any of these categories.


**Table 3.** Association between the comorbid symptoms and symptoms of AD/H, AD, and H-I in adults with FD when using the ASEBA Adult Self-Report.

> There were no significant demographic differences between those with and without AF deficits; however, the adults with FD who self-reported AD problems, AD/H symptoms, depressive symptoms, and anxiety were also significantly more likely to report AF difficulties (Table 4).

> **Table 4.** Association between psychological symptoms and adaptive functioning in adults with FD when using the ASEBA Adult Self-Report.


*p*-values calculated using chi-sq or Fisher's exact test.

#### **4. Discussion**

The present study is a pilot exploration of self-reported attention deficit symptoms in adults with Fabry disease. The results demonstrate a higher prevalence of AD/H symptoms in adults with FD than in the general United States population, with a roughly equal numbers of adults with FD endorsing AD symptoms, H-I symptoms, and Combined symptoms. While ADHD is more common in men than women in the general population [26], and some studies have found greater evidence of cognitive impairment in men with FD than women [14,17], our study found no gender differences in the rate of AD/H, H-I, or AD symptoms amongst adults with FD.

While all subjects endorsing H-I symptoms fell within the symptomatic range on the AD/H scale, only two-thirds of subjects endorsing AD symptoms did so. The remaining third endorsing AD without AD/H symptoms suggests that attention difficulties within FD are not solely linked to AD/H and lends credence to prior research outlining cognitive difficulties in attention in FD [9,11,13,16]. However, the reverse is also true; the endorsement of an equally high rate of H-I symptoms among our FD population suggests a previously unrecognized prevalence of such symptoms among adults with FD separate from attention deficits.

Of note, almost half of adults with FD in the present study endorsed symptoms of depression (49%), with no significant differences between men and women. This replicates the previously reported high rates of depression among adults with FD, with prevalence estimates ranging from 15% to 62% [9,13,25,27–29]. The present study likewise supported research demonstrating that depression in FD does not follow gender norms, with males reporting equal or greater rates than females [14,27]. While the most common factor associated with depression in FD is chronic pain [13,27,30], economic status, relationship status, specific coping styles, and somatic symptoms such as anhidrosis and acroparaesthesia have also been associated with depression and a lower QOL [27,30,31].

While depression can have deleterious effects on attention [15], its interaction with hyperactivity-impulsivity goes in the opposite direction; it is more likely to be a consequence of ADHD than a cause. Thus, while adults with FD who reported symptoms of AD/H, AD and H-I were more likely to also report symptoms of depression, this is consistent with previous research demonstrating that people with ADHD are at risk for depression and anxiety as a result of living with ADHD [26,32–34].

Finally, the present study found adults with FD-endorsing AD symptoms, AD/H symptoms, depressive symptoms, and anxiety were also significantly more likely to endorse adaptive functioning (AF) difficulties. An indication of the effectiveness with which individuals cope with the demands of everyday tasks and responsibilities as parents, students, employees, etc., AF is measured via evaluations such as the ASEBA focused on individuals' relationships, jobs, education, substance use, psychological issues, and coping skills. These findings corroborate earlier research in which FD patients had a higher rate of mean AF deficits compared to population norms, with poorer AF associated with greater rates of AD/H, depression, and anxiety [22].

All of the above findings make clear the need to pay attention to the psychological symptoms associated with FD, including the possibility of symptoms of AD/H, and expand our standard of care to include mental health treatments, if necessary. Of note, of the 20 people who self-reported AD/H symptoms in our sample, four had been prescribed medication typically used for ADHD at some point in their life, though only one had undergone a clinical diagnosis for their symptoms. As symptoms of ADHD are more heterogeneous and subtle in adults than children [35,36], with only 25% of adults with ADHD receiving treatment [26], it is possible that ADHD symptoms in adults with FD are being overlooked amidst the urgency of the other symptoms of FD.

The limitations of this study include the use of self-reported symptoms at a single point in time; however, adults with ADHD have been found to be quite reliable in identifying their own symptoms via self-reported measures [35], and an earlier study found that adults with FD were, if anything, more likely to underreport than overreport neurocognitive complaints [16]. Another limitation is the comparison between self-reported symptoms (FD population) and diagnosis (US population). To our knowledge, there is no nationally representative database of self-reported symptoms of AD/H, as compared to the frequency of diagnosis. Previous research has likewise used self-reported ADHD symptoms rather than diagnoses and presented evidence for the use of such as an effective tool [37]. Finally, this study included data primarily from Caucasian adults with FD and may not be generalizable to adults with FD of other ethnicities.

The implications of this study include the need for greater attention to cognitive and psychological health in people with FD, particularly in the areas of attention, AD/H-like symptoms, depression, anxiety, and adaptive functioning. Genetic counselors and other healthcare providers should address such issues in their annual clinic appointments and make referrals as needed to maximize overall treatment for patients with FD.

The recommendations for future research include a more objective assessment of AD/H symptoms in patients with FD, as well as further in-depth neurocognitive studying of attention/concentration in FD. Such research should utilize objective neuropsychological tests with the existing normative data with the general population.

#### **5. Conclusions**

In conclusion, the present study suggests that adults with FD are at a higher risk than the general population for attention deficits, as well as symptoms of ADHD, with equal rates among men and women. When serving adults with FD clinically, genetic counselors and other healthcare professionals should address multiple areas of care, including the physical, psychological, and cognitive issues that may accompany the disease.

**Author Contributions:** Conceptualization, N.A; methodology, N.A., A.C. and D.L.; software, E.W.H.; formal analysis, E.W.H.; writing—original draft preparation, N.A.; writing review and editing, N.A., A.C., E.W.H. and D.L.; and funding acquisition, N.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Pfizer Inc., grant number WI194299.

**Institutional Review Board Statement:** This study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Institutional Review Board of Emory University (IRB00068700).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data availability Statement:** The data for this study are available by contacting the corresponding author.

**Acknowledgments:** The authors wish to thank all patients with Fabry who participate so generously in this research regarding Fabry disease and their lived experiences.

**Conflicts of Interest:** Nadia Ali, Ph.D. received research support from Sanofi Genzyme, Shire Takeda, BioMarin, Amicus, and Pfizer, as well as lecturers' honoraria from Sanofi Genzyme, BioMarin, Amicus, and Vitaflo. These activities were monitored and in compliance with the conflicts of interest policies at Emory University. Amanda Caceres, M.MSc, CGC received research support from Genzyme and Pfizer. Eric W Hall, Ph.D. has no conflicts of interest to report. Dawn Laney, M.S., CGC consults for Genzyme, Amicus, and Shire and is a study coordinator in clinical trials sponsored by Genzyme, Amicus, and Protalix. She is a co-founder of ThinkGenetic, Inc. She also received research funding from Alexion, Amicus, Genzyme, Pfizer, Retrophin, Shire, and Synageva. These activities are monitored and are in compliance with the conflicts of interest policies at Emory University. The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; or in the decision to publish the results.

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