What is the likely diagnosis and recommended diagnostic and management approach for an adult-onset distal myopathy affecting the lower‑limb distal muscles, particularly the plantar‑flexor and hamstring groups?

Adult-Onset Distal Myopathy with Plantar Flexor and Hamstring Selectivity

This presentation is most consistent with a genetic distal myopathy, likely myotilin myopathy or a dysferlinopathy, requiring muscle MRI to define the precise pattern of involvement followed by targeted genetic testing.

Diagnostic Approach

Initial Clinical Assessment

• Age of onset and progression pattern: Adult-onset distal myopathies typically present from late adolescence to early adulthood with insidious progression over months to years 1, 2.
• Specific muscle involvement: Plantar flexor (gastrocnemius and soleus) predominance with hamstring involvement (biceps femoris and semimembranosus) is characteristic of myotilin myopathy 3.
• Distribution pattern: Lower limb distal weakness at onset distinguishes this from proximal myopathies and limb-girdle dystrophies 2, 4.

Laboratory Testing

• Creatine kinase (CK): May be normal or mildly elevated in distal myopathies; normal CK does not exclude muscular dystrophy 5, 2.
• Myositis-specific antibodies: Obtain anti-Jo-1, anti-SRP, anti-HMGCR, anti-Mi-2, and anti-MDA5 to exclude inflammatory myopathy, though the distal distribution makes this less likely 5.
• Thyroid function and parathyroid hormone: Essential to exclude endocrine myopathies, which typically present with normal CK 5.

Muscle Imaging (Critical Step)

• MRI with T1-weighted, T2-weighted, and STIR sequences: This is the most important diagnostic tool to define the precise pattern of muscle involvement and guide genetic testing 5, 6, 7.
• Expected pattern in myotilin myopathy: Proximally shows involvement of vastii, biceps femoris, and semimembranosus with sparing of gracilis and sartorius; distally shows involvement of soleus, gastrocnemius, tibialis anterior, extensor hallucis, and extensor digitorum 3.
• T1-weighted sequences: Demonstrate fatty replacement and chronic muscle damage 5.
• T2-weighted/STIR sequences: Detect muscle edema if inflammatory component present 5, 7.

Electromyography

• Confirm myopathic process: Look for polyphasic, short-duration, low-amplitude motor unit potentials with increased insertional activity, fibrillation potentials, and sharp waves 5.
• Identify optimal biopsy site: Target the most affected muscle on the contralateral side for biopsy 5.

Muscle Biopsy

• Perform after imaging: Biopsy the contralateral side of a weak muscle identified by EMG abnormalities 5.
• Expected findings in distal myopathies: Chronic myopathic changes, reduction or absence of specific proteins (dystrophin in dystrophinopathies), degenerating/regenerating fibers, myofibrillar disarrays, or rimmed vacuoles depending on etiology 5, 2, 4.
• Exclude inflammatory myopathy: Look for inflammatory infiltrates, which would be absent in genetic distal myopathies 5.

Genetic Testing (Definitive Diagnosis)

Based on the clinical presentation of adult-onset distal myopathy with plantar flexor and hamstring selectivity, prioritize testing for:

• MYOT (myotilin): Most likely given the specific pattern of plantar flexor and hamstring involvement 3.
• DYSF (dysferlin): Causes autosomal recessive distal myopathy (Miyoshi myopathy) with calf involvement 2.
• KLHL9: Recently described autosomal dominant distal myopathy with gastrocnemius and soleus involvement 1.
• Additional genes if initial testing negative: Consider ACTN2, CAV3, CRYAB, DNAJB6, DNM2, FLNC, LDB3, VCP, ANO5, GNE, DES, MYH7, NEB, RYR1, TTN, and PLEC 2, 8.

Key Differential Diagnoses to Exclude

Inflammatory Myopathies (Less Likely)

• Inclusion body myositis: Typically presents with finger flexor weakness and quadriceps involvement, not isolated plantar flexor/hamstring pattern 9.
• Immune-mediated necrotizing myopathy: Usually has acute onset, elevated CK, and proximal weakness 5.
• Dermatomyositis: May have normal CK but presents with skin manifestations and proximal weakness 5.

Non-Inflammatory Myopathies

• Mitochondrial myopathy: Presents with proximal weakness, ragged red fibers on biopsy, and multisystem involvement 5.
• Endocrine myopathy: Reversible with treatment of underlying disorder, typically proximal distribution 5.
• Drug-induced myopathy: Corticosteroid-induced myopathy has normal CK but proximal distribution 5.

Critical Pitfalls to Avoid

• Do not assume normal CK excludes muscular dystrophy: Late-onset muscular dystrophies and dysferlinopathies can present with normal or minimally elevated CK 5, 2.
• Do not biopsy without imaging guidance: MRI significantly reduces the number of candidate genes and improves biopsy yield 6, 7.
• Do not diagnose based solely on clinical pattern: No definite diagnosis can be made without identification of the molecular genetic defect 6.
• Recognize phenotypic variability: The same genetic variant can cause variable severity, as demonstrated in KLHL9-linked myopathy where family members showed extremely mild to severe phenotypes 1.

Management Considerations

Once genetic diagnosis is established:

• Genetic counseling: Autosomal dominant inheritance (MYOT, KLHL9) versus autosomal recessive (DYSF, ANO5, GNE) has implications for family screening 2.
• Supportive care: Physical therapy, orthotic devices for foot drop, and mobility aids as disease progresses 2, 4.
• Cardiac surveillance: Some distal myopathies (particularly desminopathies and certain FLNC variants) can have cardiac involvement requiring monitoring 2.
• No specific disease-modifying therapy: Unlike inflammatory myopathies, genetic distal myopathies do not respond to immunosuppression 5, 2.