*8.2. CRT and Implantable Cardioverter Defibrillator*

It is known that DMD patients are at risk of arrhythmias (such as atrial fibrillation, atrial flutter and ventricular tachycardia) but, in the absence of dedicated studies, the DMD Care Considerations Working Group suggests to apply the standard antiarrhythmic medications and device management recommendations. At present, also the indication for the ICD is based on the established adult HF guidelines [49] but should be individualized according to clinical status, nutritional state, and respiratory function.

Cardiac resyncronization therapy (CRT) implantation improved symptoms and heart function in two DMD patients with HF and left bundle branch block. Mortality remains higher in similar DMD patients without CRT [98]. Other authors reported limited benefits with the implantation of ICD and CRT in dystrophinopathic cardiomyopathies, with no increase of EF, no change or worsening of EDV [106]. However, some patients (a quarter) had subjective improvements in their daily activities. Causes of this poor response in DMD patients could be the normality of QRS complex and the extensive postero-lateral fibrosis [107].

#### **9. DMD Target Therapy**

Glucocorticoid treatment has been the standard of care for patients with DMD. Prednisone and deflazacort are the most commonly recommended steroids. The introduction of steroid therapy has changed the natural history of the disease, prolonging the autonomous ambulation period, delaying the cardiorespiratory insufficiency occurrence, and increasing children life-expectancy [4,108].

The understanding of molecular basis and knowledge of DMD has led to the advent of several experimental therapeutic approaches. The therapeutic approaches for DMD have focused on restoring dystrophin expression or mitigating the processes downstream of dystrophin deficiency.

The strategies that have been embraced for dystrophin protein restoration include (1) nonsense readthrough, (2) antisense oligonucleotides for exon skipping, and (3) gene therapy. To mitigate the dystrophic processes the approaches used are (1) inhibiting inflammation, (2) promoting muscle regeneration, (3) reducing fibrosis, and (4) facilitating mitochondrial function.

This translational research has led to the approval of first treatments for DMD and several other agent are under clinical investigation. Ataluren (Translarna™, PTC Therap.) is the first approved drug for DMD in Europe. Ataluren is an oral molecule that binds ribosomal RNA subunits and enables ribosomal readthrough of mRNA containing a premature stop codon. It is indicated for the treatment of DMD resulting from a nonsense mutation in the dystrophin gene.

More recently FDA conditionally approved AONs targeting exon 51 (eteplirsen) and 53 (golodirsen) for the treatment of DMD [61].

#### **10. Female DMD Carriers**

DMD is an X-linked condition and it usually affects males, with the majority of females having mutation in a single allele being asymptomatic carriers. About one-third of all DMD cases are caused by de novo mutations, with the other two-thirds due to inheritance from the mother. This means that every mother of an isolated male DMD case has a two-thirds chance of being a carrier. Most females with a pathogenic DMD gene variant present as asymptomatic carriers because of the presence of a second normally functioning allele. However, some female carriers can present symptoms from mild to more severe clinical courses, as muscle weakness, abnormal gait, fatigue, and cardiac involvement [109]. These patients are classified as "manifesting carriers." The "skewed inactivation of the normal X-chromosome" hypothesis was widely used to explain the mosaic pattern of dystrophin expression in skeletal as well as heart muscle in female DMD carriers. According to this hypothesis, a higher percentage of skewed inactivation of the normal X-chromosome was responsible for the occurrence of cardiomyopathy in some female MD carriers. However, in the study of Brioschi et al. [110], there was no relationship between the dystrophic muscular phenotype and either the X-inactivation pattern or the dystrophin transcriptional behavior, suggesting that the major cause of disease manifestation is simply the total dystrophin protein amount. Each male child of a carrier female has a 50% chance of being clinically affected with DMD. Carrier testing can indicate if a woman is at risk of having affected sons. Although female DMD carriers are mostly free of skeletal muscle symptoms, cardiac symptoms affect about 8% of this population with DCM as a common presentation. They may develop cardiomyopathy ranging from asymptomatic forms with mild abnormalities to progressive HF, even requiring heart transplantation [36]. The onset of clinical manifestations for symptomatic female carriers is variable, ranging from early childhood to late adulthood [111]. The incidence of cardiomyopathy increases with age, even in patients with normal electrocardiograms and no skeletal muscle symptoms. Therefore, in the clinical guidelines in Europe and the United States [15], adult dystrophinopathy carriers are recommended to undergo echocardiography every 5 years. Other cardiac manifestations include conduction defects and arrhythmias, but these could be consequences of long-term DCM. Acute HF and non-sustained ventricular tachycardia have been reported as initial presentations in late adulthood,

although these are not as common. Disease severity is variable and genotype–phenotype correlations are not well established in this group of patients, and cardiac involvement may be present without concomitant skeletal muscle manifestations [111–113]. Cardiac manifestations in female carriers may be subclinical under normal physiological conditions. They can worsen and become symptomatic during major events such as pregnancy. Approximately two-thirds of all patients with limbic-type muscular dystrophy experience muscle weakness during pregnancy and these are probably related with weight gain and diaphragm elevation.

Besides cardiac manifestations, female DMD carriers can present with other systemic features: limb girdle weakness, gait disturbance, exercise intolerance, calf hypertrophy, and scoliosis have all been recognized as skeletal muscle manifestations in these patients. Elevated serum creatinine kinase (CK) is often found in patients with skeletal muscle presentations. Additionally, neurocognitive problems can present as learning disabilities or behavioral problems in this patient population [112,113].

**Author Contributions:** Conceptualization, R.A. and A.A.; writing—original draft preparation, E.M., N.C., C.C., L.A., M.C., M.S., A.D., M.G.; writing—review and editing, R.A., F.D., A.A.; supervision, R.A., A.A.; project administration, R.A.; funding acquisition, R.A., A.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by Ministery of Health, grant number RRC-2019-2366867.

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