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Neuroradiology Case of the Week

Case 106

Ravinder Sidhu MD, Leena Ketonen, MD, PhD
and PL Westesson, MD, PhD, DDS

Clinical Presentation: A 29-year-old female presented with aphasia and altered mental status changes.

Radiological Findings: Axial T1-weighted MR images showed diffuse thickening of leptomeninges involving cerebral convexities, more marked on right side (Fig. 1A). A few hyperintense foci were also seen in posterior fossa (Fig. 1B). There was diffuse, marked enhancement of the meninges especially prominent at interpenduncular at basal cisterns (Figs. 2A&B). Dura also showed enhancement with nodularity well appreciated on right side (Fig. 2C). Axial FLAIR MR images showed hyperintense dura along with increased signals at interpeduncular region (Fig. 3 A&B).

Diagnosis: Leptomeningeal melanosis

Discussion: 
     Primary pigmented tumors of the leptomeninges are an uncommon and varied group of entities, and include pigmented meningioma, malignant melanoma, meningeal melanocytoma, melanotic schwannoma, and melanoblastosis [1]. Primary melanocytic neoplasms are rare lesions arising from normally occurring leptomeningeal melanocytes. Melanoblasts are derived from neural crest elements found within the basal layer of the epidermis and the leptomeninges covering the base of brain and the brain stem. Consequently, the areas most commonly involved are the pons, cerebellum, cerebral peduncles, medulla, interpeduncular fossa, and inferior surfaces of the frontal, temporal, and occipital lobes. These neoplasms are generally divided into three main types, including diffuse melanosis, meningeal melanocytoma, and primary malignant melanoma [2,3].
     Diffuse melanosis occurs more frequently in children with congenital intradermal benign pigmented nevi in a rare nonfamilial disorder known as neurocutaneous melanosis [4]. Meningeal melanocytoma and primary melanoma of the leptomeninges (leptomeningeal melanosis) are similar in their origin from leptomeningeal melanocytes, but actually represent both ends of the spectrum, ranging from a lesion that is benign in appearance and behavior to one that is malignant. However, neither of these entities is associated with pigmented lesions elsewhere, including benign congenital pigmented nevi or frank cutaneous malignant melanoma. Diffuse dural involvement is rarely seen in metastatic melanoma. A wide variety of clinical features may be seen with meningeal melanocytoma such as seizures, chronic basal meningitis, multiple cranial nerve palsies, chronic spinal arachnoiditis, psychiatric disturbances and radiculopathy [5,6].
     Radiologically, contrast enhanced head CT may demonstrate iso- to hyperattenuating lesions with variable contrast enhancement. Gadolinium enhanced MR shows more extensive involvement of leptomeninges in the form of nodular, irregular thickening of leptomeninges. The MR appearance is strongly influenced by the paramagnetic effects of melanin, which causes shortening of T1 and T2 relaxation times. Therefore, MR appearance of these lesions is generally that of high-signal intensity on T1-weighted images and diminished signal on T2-weighted images, with enhancement after contrast administration. The process may not be limited to the leptomeninges of the brain, but spinal cord and nerve root involvement may also be seen [5]. It is important to differentiate this kind of marked enhancement of meninges from inflammatory conditions such as meningitis and contrast leakage seen in end stage renal disease [7].
     The CSF analysis may show melanocytes or malignant cells. Histological examination of cells obtained from pigmented tumors is of limited value other than to demonstrate intracytoplasmic melanin. Immunohistochemical analysis is indispensable in differentiating meningeal melanocytoma from other pigmented lesions. Meningeal melanocytoma is characterized by a positive immunohistochemical reaction to antimelanoma antibodies (HMB-45), S-100 protein, and vimentin. Vimentin usually appears in meningeal melanocytoma but is only rarely present in malignant melanoma [1].
     Gadolinium enhanced MR plays an important role in the diagnosis of this condition and assists in the decision regarding the appropriate biopsy site.

References:

1. Litofsky NS, Zee CS, Breeze RE, Chandrasoma PT. Meningeal melanocytoma: diagnostic criteria for a rare lesion. Neurosurgery 1992; 31:945-947.
2. Vanzieleghem BD, Lemmerling MM, Van Coster RN. Neurocutaneous melanosis presenting with intracranial amelanotic melanoma. AJNR Am J Neuroradiol 1999; 20:457-460.
3. Faillace W, Okawara SH, McDonald JV. Neurocutaneous melanosis with extensive intracerebral and spinal cord involvement. J Neurosurg 1984; 61:782-785.
4. Leaney B, Rowe P, Klug G. Neurocutaneous melanosis with hydrocephalus and syringomyelia. J Neurosurg 1985; 62:148-152.
5. Painter TJ, Chaljub G, Sethi R, Singh H, Gelman B. Intracranial and intraspinal meningeal melanocytosis. AJNR Am J Neuroradiol 2000; 21:1349-1353.
6. Alsatian J, Tampieri D, Salzar A, Melancon D, Duong H. MR of leptomeningeal melanocytosis in a patient with neurofibromatosis. J Comput Assist Tomogr 1997; 21:38-40.
7. Demiri A, Kawamura Y, Sze G, Duncan C. MR of parenchymal neurocutaneous melanosis. AJNR Am J Neuroradiol 1995; 16:603-606.