What are the necessary tests for diagnosing Juvenile Dermatomyositis, Muscular Dystrophy, Spinal Muscular Atrophy (SMA), and Chronic Inflammatory Demyelinating Polyneuropathy (CIDP)?

Diagnostic Tests for Juvenile Dermatomyositis, Muscular Dystrophy, Spinal Muscular Atrophy, and CIDP

Each of these neuromuscular disorders requires specific diagnostic tests, with muscle enzymes, genetic testing, electrophysiological studies, and imaging playing central roles in establishing definitive diagnoses.

Juvenile Dermatomyositis (JDM)

JDM diagnosis requires a comprehensive panel of tests to confirm the characteristic muscle inflammation and skin involvement:

Essential Tests:

• Muscle enzymes - CPK, LDH, AST, ALT, and aldolase (if available) 1
• Inflammatory markers - ESR (or plasma viscosity) and CRP 1
• Myositis-specific and myositis-associated antibodies - particularly anti-TIF 1-γ, anti-NXP2, anti-MDA5 1
• Muscle MRI - T2-weighted/STIR sequences to detect muscle inflammation; increasingly used as an alternative to muscle biopsy 1, 2
• Nailfold capillaroscopy - to assess for characteristic capillary changes 1
• Skin assessment - using formal Cutaneous Assessment Tool (CAT) 1

Additional Important Tests:

• Muscle biopsy - especially in atypical presentations without rash 1
• Complete blood count and blood film 1
• Renal and liver function tests 1
• Pulmonary function tests - including CO diffusion 1
• Echocardiogram and ECG - to assess cardiac involvement 1
• Swallow function assessment - may include speech therapy assessment or video fluoroscopy 1
• EMG/nerve conduction studies - when diagnosis is uncertain or to differentiate myopathy from neuropathy 1, 3

Muscular Dystrophy

Muscular dystrophy diagnosis focuses on genetic confirmation following initial screening:

Essential Tests:

• Creatine kinase (CK) - markedly elevated (>1000 U/L in DMD) 1
• Genetic testing 1:
  • Deletion/duplication testing - multiplex ligation-dependent probe amplification (MLPA) or multiplex PCR
  • Gene sequencing - if deletion/duplication testing is negative
• Muscle biopsy - if genetic testing is inconclusive, to assess dystrophin expression 1

Additional Important Tests:

• Thyroid function tests - to rule out endocrine causes of myopathy 1, 4
• Electromyography (EMG) - to confirm myopathic pattern 1
• Echocardiogram and ECG - to assess cardiac involvement 4
• Pulmonary function tests - to evaluate respiratory muscle involvement 4

Spinal Muscular Atrophy (SMA)

SMA diagnosis relies primarily on genetic confirmation:

Essential Tests:

• Genetic testing - SMN1 gene deletion/mutation analysis 1
• EMG/nerve conduction studies - shows denervation with normal nerve conduction 1
• Creatine kinase (CK) - normal to mildly elevated 1

Additional Important Tests:

• Muscle biopsy - if genetic testing is inconclusive, shows grouped atrophy 1
• Pulmonary function tests - to assess respiratory involvement 1

Chronic Inflammatory Demyelinating Polyneuropathy (CIDP)

CIDP diagnosis focuses on electrophysiological confirmation of demyelination:

Essential Tests:

• Nerve conduction studies/EMG - showing demyelination (slowed conduction velocities, conduction blocks, temporal dispersion) 1
• Cerebrospinal fluid analysis - elevated protein with normal cell count (albuminocytologic dissociation) 1
• Nerve biopsy - if diagnosis remains unclear, shows demyelination and remyelination 1

Additional Important Tests:

• MRI of nerve roots and plexuses - may show nerve root enhancement 1
• Blood tests - to exclude other causes:
  • Complete blood count
  • Metabolic panel
  • Thyroid function
  • Serum protein electrophoresis
  • Anti-ganglioside antibodies
  • Glucose tolerance test

Important Diagnostic Pitfalls

1. Misdiagnosis between inflammatory myopathies and muscular dystrophies - Inflammatory changes can occur in early muscular dystrophy, leading to misdiagnosis as polymyositis 5

2. Normal muscle enzymes despite active disease - In JDM, muscle enzymes may be normal despite active disease 1

3. Elevated liver enzymes (AST/ALT) from muscle damage - Often mistaken for liver disease in muscular dystrophies 4

4. Relying solely on clinical presentation - Genetic confirmation is essential for muscular dystrophy and SMA 1

5. Inadequate muscle biopsy sampling - Choosing the right muscle for biopsy is crucial; EMG can help identify affected muscles 1

6. Overlooking cardiac and pulmonary involvement - These systems require assessment in all these conditions, even without obvious symptoms 1, 4