Amelanotic Primary Diffuse Leptomeningeal Melanoma: A Case Report

Background Primary diffuse leptomeningeal melanoma (PDLM) is an exceptionally rare malignant tumor of the central nervous system (CNS). The amelanotic subtype presents additional diagnostic challenges because of its nonspecific neuroimaging features, which frequently result in diagnostic delays and poor clinical outcomes. Case presentation : A 25-year-old East Asian woman presented with a one-month history of persistent headache, dizziness, and vomiting. Lumbar puncture revealed markedly elevated intracranial pressure exceeding 500 mmH₂O. Cranial magnetic resonance imaging (MRI) demonstrated nonspecific signal abnormalities in the right dorsothalamic region, with no enhancement on gadolinium-contrast scans. Cytological examination of the cerebrospinal fluid (CSF) showed no evidence of malignant cells. One month after admission, positron emission tomography–magnetic resonance imaging (PET-MRI) revealed markedly increased radiotracer uptake in the right thalamic lesion. Repeat CSF cytology detected atypical cells, and immunohistochemical staining was positive for SOX-10, S100, Melan-A, and HMB-45, confirming the diagnosis of amelanotic PDLM. Despite aggressive therapeutic interventions, the patient’s condition deteriorated rapidly, and she died four months after symptom onset. Conclusions This case highlights the considerable challenges associated with the early diagnosis of amelanotic PDLM. For patients presenting with unexplained intracranial hypertension and nonspecific meningeal involvement, clinicians should maintain a high index of suspicion for this rare entity, even when characteristic imaging findings are absent. Repeated cerebrospinal fluid cytological and immunohistochemical examinations are essential for establishing a definitive diagnosis. The integration of molecular diagnostic approaches may further enhance early detection and facilitate individualized therapeutic strategies.