Invited Review: Focal Neurogenic Muscle Hypertrophy and Focal Neurogenic Myositis-Article

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This 2025 invited review in Muscle & Nerve by Felice et al. comprehensively examines focal neurogenic muscle hypertrophy (F-NMH) and focal neurogenic myositis (F-NM), rare conditions characterized by paradoxical localized muscle enlargement following peripheral motor axon injury. The authors present a series of six patients seen between 2007–2025 with unilateral muscle hypertrophy, mostly in the calf, tibialis anterior, or tensor fascia lata, associated with radiculopathy (mainly S1 or L5 root compression). Clinical features included firm focal muscle enlargement, pain, cramps, and sometimes neurologic deficits. Elevated creatine kinase levels were common.<br /><br />Electrodiagnostic studies revealed a mix of ongoing denervation and chronic neurogenic reinnervation changes; complex repetitive discharges were present in half the cases, supporting continuous muscle fiber activity as a key pathogenetic mechanism leading to hypertrophy. MRI confirmed focal muscle enlargement with variable signal alterations (edema, gadolinium enhancement). Muscle biopsies showed combined neurogenic and myopathic features with fiber hypertrophy and atrophy; inflammatory infiltrates were present in roughly 39% but were not correlated with clinical severity or prognosis, suggesting inflammation may be incidental.<br /><br />Expanding on literature review encompassing 94 patients, the review highlights the predominance of unilateral calf involvement due to chronic radiculopathy, with less frequent involvement of trapezius, biceps brachii, and tensor fascia lata. The condition is non-progressive, and about one-third improve spontaneously without treatment. Botulinum toxin injections and oral prednisone emerged as the most effective therapies, especially for symptom relief and moderating muscle bulk.<br /><br />The etiology remains poorly understood but likely multifactorial, linked to continuous muscle fiber overactivity from ephaptic discharges and compensatory mechanisms post-denervation. The presence of inflammation does not alter clinical course. Genetic factors have not been identified but warrant further study. Overall, F-NMH and F-NM represent benign, rare neurogenic muscle responses to focal nerve injury that clinicians should differentiate from other causes of focal limb enlargement, and management can often be conservative or involve botulinum toxin for symptomatic cases.

Keywords

focal neurogenic muscle hypertrophy

focal neurogenic myositis

peripheral motor axon injury

unilateral muscle hypertrophy

radiculopathy

electrodiagnostic studies

muscle biopsy

botulinum toxin therapy

muscle fiber overactivity

chronic denervation