Primary amelanotic leptomeningeal melanocytosis mimicking subarachnoid hemorrhage: a case report of the deceptive masquerader

BackgroundPrimary diffuse leptomeningeal melanocytosis (PDLM) is a rare central nervous system neoplasm. Diagnosis is challenging in amelanotic variants because they lack the characteristic T1 hyperintensity on magnetic resonance imaging (MRI), leading to confusion with infectious meningitis or hemorrhage.Case PresentationA 24-year-old male presented with severe headache and neck stiffness. Initial computed tomography suggested a subarachnoid hemorrhage (SAH); however, digital subtraction angiography was negative. MRI revealed diffuse leptomeningeal enhancement without T1 hyperintensity. Cerebrospinal fluid (CSF) analysis was non-diagnostic. Owing to progressive clinical deterioration despite CSF diversion, a biopsy was performed. Histopathology confirmed amelanotic PDLM with positive S-100, HMB-45, and Melan-A markers, but negative Fontana-Masson staining. Hematology-oncology consultation recommended MEK inhibitor therapy based on MAPK pathway rationale; however, molecular genetic profiling was pending and the patient’s rapid neurological deterioration precluded initiation of targeted therapy. The patient succumbed to the disease approximately 3 months after biopsy.ConclusionThis case highlights the diagnostic pitfalls of amelanotic leptomeningeal melanocytosis and the narrow therapeutic window in advanced PDLM. In young patients with “angiogram-negative SAH” or unexplained leptomeningeal enhancement, the absence of T1 hyperintensity should not exclude melanocytic neoplasms. Early biopsy and prompt molecular profiling are crucial for timely diagnosis and treatment planning.