Focal Neurogenic Muscle Hypertrophy and Focal Neurogenic Myositis

Focal neurogenic muscle hypertrophy (F-NMH) and focal myositis from neurogenic causes (F-NM) are terms used to describe the paradoxical changes in skeletal muscle following injury to peripheral motor axons. Since the first clinicopathologic description in 1978, over 100 patients with F-NMH and F-NM have been reported. We reviewed the medical records and clinicopathological data on our series of 6 patients from 2007 to 2025, and then expanded the review to include all patients reported in case studies or series with well-defined F-NMH or F-NM based electrodiagnostic studies, muscle histopathology, or both. Inclusive of our 6 patients, 94 patients were reviewed. Most presented with unilateral calf muscle hypertrophy followed in frequency by involvement of the trapezius, biceps brachii, tibialis anterior, and tensor fascia lata. Radiculopathy was the most common injury site. Electrodiagnostic studies showed a mixture of ongoing and chronic changes of denervation and reinnervation. Complex repetitive discharges were observed in 50%. Magnetic resonance imaging showed focal muscle hypertrophy with variable signal changes. Muscle biopsies showed mixed myopathic and neurogenic changes with 39% having inflammation. During the period of observation, 37 improved including 7 without treatment, and none worsened. Botulinum toxin and oral prednisone were the most efficacious treatments. F-NMH is a rare, non-progressive, and seemingly benign paradoxical reaction to peripheral nerve injury. The cause is unknown but likely multifactorial based on clinical observations and experimental studies. Muscle inflammation is not a consistent finding, does not appear to alter the clinical course or prognosis, and may be an epiphenomenon.