Isolated Pectoralis Major and Tricep Atrophy Secondary to Neuralgic Amyotrophy.

Neuralgic amyotrophy (NA), or Parsonage-Turner syndrome, describes a spectrum of relatively rare peripheral neuropathies characterized by acute pain followed by patchy, multifocal sensory disturbances, weakness, and atrophy, predominantly in the upper extremity. While typically affecting the upper brachial plexus, NA can also present with selective involvement of distal nerve branches. This case report details a unique presentation of NA in a 39-year-old male bodybuilder who developed isolated weakness and atrophy of the left pectoralis major and a single head of the left triceps following a painful prodrome without preceding trauma. Electrodiagnostic studies confirmed active denervation in these muscles, while imaging revealed no cervical pathology or brachial plexus abnormalities. The patient was diagnosed with NA, specifically affecting the lateral pectoral nerve, and a branch of the radial nerve. Our case is compared to existing literature, including isolated reports of pectoralis major and branch-specific radial nerve involvement in NA. While these less common deficits have been described as separate entities, our case uniquely demonstrates them co-occurring. It highlights the potential for NA to affect less common nerves (lateral pectoral and motor branches of the radial nerve) and specific nerve branches. Differential diagnoses included mononeuritis multiplex, compressive neuropathy, and infectious or inflammatory neuropathies. Mononeuritis multiplex was less likely due to the absence of sensory deficits and systemic symptoms. Infectious and inflammatory neuropathies were ruled out based on clinical presentation and lack of relevant markers. Compressive neuropathy was considered, however the acute painful prodrome, electrodiagnostic findings, and natural history favored NA. This case contributes to understanding the variable clinical spectrum of NA. It also underscores the importance of clinical history and comprehensive work-up in atypical presentations for prompt diagnosis and effective management. While the prognosis for NA is generally favorable, the long-term outcome for highly selective presentations requires further investigation.