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Meningeal Melanocytosis

Summary

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  • Rare pigmented neoplasm of the leptomeninges characterised by diffuse proliferation of melanocytes
  • Presents with neurological symptoms due to mass effect and hydrocephalus
  • Imaging shows diffuse leptomeningeal enhancement and T1 hyperintensity due to melanin content1

Pathophysiology

  • Originates from melanocytes in the leptomeninges, which are derived from neural crest cells
  • Diffuse proliferation of benign melanocytes within the pia mater and arachnoid membrane
  • Can occur in isolation or as part of neurocutaneous melanosis syndrome
  • May undergo malignant transformation to meningeal melanoma in rare cases

Demographics

  • Rare condition, with fewer than 200 cases reported in literature
  • More common in paediatric population, but can occur at any age
  • No significant gender predilection
  • Associated with neurocutaneous melanosis in approximately 50% of cases

Diagnosis

  • Clinical presentation:
    • Headache
    • Seizures
    • Cranial nerve palsies
    • Hydrocephalus
    • Spinal cord compression symptoms
  • CSF analysis:
    • Elevated protein
    • Presence of melanin-containing cells
  • Histopathology:
    • Diffuse infiltration of melanocytes in leptomeninges
    • Absence of cellular atypia or mitotic activity

Imaging

  • CT:
    • Diffuse leptomeningeal hyperdensity
    • Hydrocephalus may be present
  • MRI:
    • T1-weighted: Diffuse leptomeningeal hyperintensity due to melanin content
    • T2-weighted: Variable signal intensity, often hypointense
    • T1 post-contrast: Diffuse leptomeningeal enhancement
    • Susceptibility-weighted imaging (SWI): Blooming artefact due to melanin
  • Differential diagnosis:
    • Leptomeningeal metastases
    • Neurosarcoidosis
    • Meningitis (infectious or carcinomatous)

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  • A 20-year-old patient presented 5 years prior with headache. On that scan, there was acute-on-chronic hydrocephalus and a ventriculoperitoneal shunt was inserted.
  • On follow-up imaging over the next 5 years, there was no change in the diffuse leptomeningeal T1-hyperintensity.
  • There was no leptomeningeal enhancement but there was fluctuating diffuse dural thickening/enhancement that was likely to be related to the shunt.

Treatment

  • CSF diversion for hydrocephalus; surveillance for malignant transformation. The T1-hyperintense, SWI-blooming leptomeningeal signal from melanin is the key clue, and prompts a search for neurocutaneous melanosis

Differential diagnosis

Differential Diagnosis Differentiating Feature
Leptomeningeal metastases Multiple nodular lesions along cranial nerves and sulci; associated parenchymal metastases
Neurosarcoidosis Cranial nerve enhancement; hypothalamic/infundibular involvement; associated parenchymal sarcoid lesions
Meningioma Single extra-axial mass with dural tail sign; adjacent hyperostosis on CT; no diffuse leptomeningeal spread
Meningitis (infectious) Diffuse leptomeningeal enhancement without T1 hyperintensity; associated hydrocephalus and infarcts
Primary CNS lymphoma Periventricular location; homogeneous enhancement; restricted DWI; no T1 melanin signal
Dural metastases Multiple nodular dural-based enhancing lesions; irregular margins; adjacent bone destruction
Melanoma metastases Discrete parenchymal or dural nodules rather than diffuse leptomeningeal thickening; T1 hyperintensity in melanotic lesions
Meningeal carcinomatosis Diffuse smooth leptomeningeal enhancement without T1 hyperintensity; associated parenchymal lesions
Idiopathic hypertrophic pachymeningitis Typically involves dura mater rather than leptomeninges

  1. Painter et al. Intracranial and intraspinal meningeal melanocytosis. 2000. AJNR. American journal of neuroradiology - Open in new tab. ↩