Focal Neurogenic Muscle Hypertrophy and Focal Neurogenic Myositis

Area of Science:

• Neurology
• Muscle Physiology
• Pathology

Background:

• Focal neurogenic muscle hypertrophy (F-NMH) and focal myositis (F-NM) describe paradoxical skeletal muscle changes after peripheral motor axon injury.
• Over 100 cases reported since 1978, indicating a rare but documented phenomenon.

Purpose of the Study:

• To review clinicopathological data of F-NMH and F-NM patients.
• To analyze clinical presentation, electrodiagnostic findings, imaging, histopathology, and treatment outcomes.

Main Methods:

• Retrospective review of 6 patients (2007-2025) and an expanded literature review of 94 patients with F-NMH or F-NM.
• Analysis of medical records, electrodiagnostic studies, muscle histopathology, and magnetic resonance imaging (MRI).

Main Results:

• Unilateral calf hypertrophy was most common, followed by trapezius and biceps brachii.
• Electrodiagnostics revealed mixed denervation/reinnervation changes; 50% showed complex repetitive discharges.
• Muscle biopsies showed mixed myopathic/neurogenic changes; 39% had inflammation, which did not affect prognosis.

Conclusions:

• F-NMH is a rare, non-progressive, benign reaction to peripheral nerve injury with unknown multifactorial causes.
• Muscle inflammation is not consistently found and appears to be an epiphenomenon.
• Spontaneous improvement occurs, and treatments like botulinum toxin and prednisone are effective.