# Muscular Dystrophies: A Physiatric Perspective

## Introduction

The **muscular dystrophies** are a heterogeneous group of inherited disorders characterized by progressive skeletal muscle weakness and degeneration due to defects in structural proteins of the muscle fiber. Physiatrists play a central role in managing mobility, preventing contractures, optimizing respiratory and cardiac function, and maximizing quality of life across the lifespan. The most common and well-studied form is **Duchenne muscular dystrophy (DMD)**.

## Classification and Genetics

### Major Types

**Duchenne muscular dystrophy (DMD)**: X-linked recessive; dystrophin absent; most severe. **Becker muscular dystrophy (BMD)**: X-linked recessive; dystrophin reduced/dysfunctional; milder course. **Myotonic dystrophy type 1 (DM1)**: Autosomal dominant; CTG repeat expansion; multisystem. **Facioscapulohumeral dystrophy (FSHD)**: Autosomal dominant; D4Z4 contraction on chromosome 4.

**Limb-girdle muscular dystrophies (LGMD)**: Multiple subtypes; autosomal dominant or recessive. **Emery-Dreifuss muscular dystrophy**: X-linked or autosomal; early contractures and cardiac conduction defects. **Congenital muscular dystrophies**: Present at birth or early infancy; multiple subtypes.

| Type | Inheritance | Protein Defect | Onset | Key Features |
|------|-----------|---------------|-------|--------------|
| DMD | X-linked recessive | Dystrophin absent | 2-5 years | Proximal weakness, loss of ambulation ~10-13 yrs |
| BMD | X-linked recessive | Dystrophin reduced | 5-15 years | Milder course, ambulation preserved longer |
| DM1 (Myotonic) | AD; CTG repeat | DMPK | Variable | Myotonia, multisystem, cataracts, cardiac |
| FSHD | AD | DUX4 derepression | Teens-20s | Face, scapula, arm weakness |
| LGMD | AD or AR | Multiple | Variable | Proximal weakness, heterogeneous |
| Emery-Dreifuss | X-linked or AD/AR | Emerin/lamin A/C | Childhood | Early contractures, cardiac conduction defects |

### Dystrophin and the Dystrophin-Associated Glycoprotein Complex

**Dystrophin** links the intracellular cytoskeleton (actin) to the extracellular matrix (laminin). Absence (DMD) or dysfunction (BMD) leads to membrane instability during contraction. Repeated muscle injury overwhelms regenerative capacity, replaced by fibrosis and fat.

## Duchenne Muscular Dystrophy

### Clinical Course

Onset: **2-5 years** with delayed motor milestones, proximal weakness, and Gowers sign. **Gowers sign**: Uses hands to climb up legs when rising from floor (proximal weakness). **Pseudohypertrophy** of calves due to fatty and fibrous infiltration. Loss of ambulation typically by **age 10-13**.

Progressive scoliosis after loss of ambulation. Cardiomyopathy in virtually all patients by age 18. Respiratory failure: Leading cause of death; median survival **mid-20s** with modern care.

### Current Medical Treatments

**Corticosteroids** (prednisone/deflazacort): Mainstay; delays loss of ambulation by 2-3 years. **Exon-skipping therapies**: Eteplirsen (exon 51), golodirsen (exon 53), casimersen (exon 45). **Gene therapy**: Micro-dystrophin delivery (delandistrogene moxeparvovec, FDA-approved 2023). **Ataluren**: For nonsense mutations (approved in Europe). Cardiac management: ACE inhibitors or ARBs from age 10; beta-blockers as needed.

![Clinical progression timeline of Duchenne muscular dystrophy with rehabilitation intervention milestones](images/dmd-clinical-timeline.png)

## Rehabilitation Management

### Mobility and Ambulation

**Physical therapy**: Maintain strength, flexibility, and functional mobility. Stretching of heel cords, hip flexors, and iliotibial bands (contracture-prone areas). Night splints and **ankle-foot orthoses** to delay equinus contractures. **Knee-ankle-foot orthoses (KAFOs)** may extend ambulation in select patients.

Standing frames after loss of ambulation to maintain bone density and prevent contractures. **Power wheelchair** prescription when ambulation becomes unsafe or excessively fatiguing.

### Contracture Prevention

Daily stretching regimen is the cornerstone of contracture management. Prone positioning to stretch hip flexors. Serial casting if contractures progress despite stretching. Surgical release considered if contractures significantly impair function or seating. Post-surgical rehabilitation to maintain surgical gains.

### Scoliosis Management

Develops in **90%** of non-ambulatory DMD patients. Corticosteroid treatment delays but does not prevent scoliosis. **Spinal fusion** recommended when Cobb angle reaches **20-30 degrees** (while FVC >30%). Bracing does not prevent curve progression but may delay surgery. Post-operative rehabilitation focuses on seating, respiratory function, and ADLs.

### Respiratory Management

Serial **FVC monitoring** starting at diagnosis; decline begins around age 10-12. **Cough augmentation**: Manual assisted cough, mechanical insufflation-exsufflation. **Nocturnal NIV (BiPAP)** when FVC <50% or nocturnal hypoventilation symptoms develop. Daytime ventilation as FVC declines further.

Tracheostomy ventilation for patients who choose long-term ventilatory support. Respiratory infection prevention and aggressive treatment of respiratory illnesses.

### Cardiac Management

Echocardiography and ECG annually from diagnosis or age 6. **ACE inhibitors** started prophylactically at age 10 (or earlier if EF declines). Cardiomyopathy develops in virtually all DMD patients by late teens. Cardiac MRI for more sensitive detection of myocardial fibrosis. Arrhythmia monitoring; consideration of ICD in selected cases.

## Other Muscular Dystrophies

### Myotonic Dystrophy Type 1

**Myotonia** (delayed muscle relaxation) plus progressive weakness. Distal predominant weakness initially; facial weakness (hatchet face). Multisystem: Cataracts, cardiac conduction defects, endocrine dysfunction, cognitive impairment. Rehabilitation: Ankle-foot orthoses, adaptive equipment, cardiac monitoring, daytime somnolence management.

### Facioscapulohumeral Dystrophy

Facial weakness, **scapular winging**, and proximal arm weakness. Asymmetric involvement is characteristic. Rehabilitation: Scapular bracing, shoulder ROM exercises, leg orthoses for foot drop. Generally slower progression; life expectancy near normal.

### Limb-Girdle Muscular Dystrophies

Proximal weakness of hip and shoulder girdle muscles. Over 30 genetic subtypes identified. Rehabilitation approach similar to DMD but tailored to progression rate. Variable severity and rate of decline depending on subtype.

![Comparison of weakness distribution patterns across common muscular dystrophy types](images/muscular-dystrophy-weakness-patterns.png)

## Psychosocial Considerations

Depression and anxiety common in patients and caregivers. Social isolation increases after loss of ambulation. Educational accommodations and vocational planning. Transition planning from pediatric to adult care.

Palliative care and advance directive discussions. Peer support groups and advocacy organizations (MDA, Parent Project Muscular Dystrophy).

## Key Clinical Pearls

1. Corticosteroids remain the mainstay of DMD treatment, delaying loss of ambulation by 2-3 years and reducing the risk of scoliosis, but require monitoring for side effects including weight gain, osteoporosis, and behavioral changes. 2. Daily stretching of heel cords, hip flexors, and iliotibial bands is the single most important intervention for delaying contracture development in DMD. 3. Spinal fusion for scoliosis should be performed while FVC remains above 30% to minimize perioperative respiratory complications; delayed referral may result in a missed surgical window. 4. Cardiac surveillance with echocardiography should begin by age 6 in DMD, with prophylactic ACE inhibitor therapy initiated by age 10 to delay cardiomyopathy progression.

## References

1. Birnkrant DJ, et al. "Diagnosis and Management of Duchenne Muscular Dystrophy, Part 1-3: DMD Care Considerations Working Group." *Lancet Neurol*. 2018;17(3):251-267.
2. Bushby K, et al. "Diagnosis and Management of Duchenne Muscular Dystrophy: A Multicentre Study." *Lancet Neurol*. 2010;9(1):77-93.
3. McDonald CM, et al. "Longitudinal Pulmonary Function Testing in Duchenne Muscular Dystrophy." *Arch Phys Med Rehabil*. 1997;78(7):740-748.
4. Tawil R, et al. "Facioscapulohumeral Muscular Dystrophy: FSHD-Related Clinical Trial Preparedness." *Neuromuscul Disord*. 2016;26(9):523-528.