Arachnoid cyst causing an optic neuropathy in neurofibromatosis 1

Abstract

A 9-year-old girl with neurofibromatosis type 1 (NF1) (Fig. 1A) and a 1-year history of controlled left unilateral juvenile glaucoma, related to congenital ectropion uveae, presented with a rapid, unexplained, painless visual loss in the left eye. Eight months after the onset of visual loss, best corrected visual acuity was 20/20 in the right and 20/200 in the left eye. A relative afferent pupillary defect was observed in the left eye. Right eye examination was normal. There was a left minimal proptosis and a mild congenital ptosis. Slit-lamp examination disclosed ectropion uveae (Fig. 1B) and fundoscopy showed a glaucomatous cupped optic disc with a normally coloured neuro–retinal rim (Fig. 1C, D). Ishihara colour vision and eye motility were normal. Intraocular pressure was 14/14 mmHg on topical prostaglandin analogue treatment. Automated visual fields disclosed an unexplained left central scotoma, in addition to glaucoma-related arcuate defects (Fig. 1E, F). Cerebral and orbital magnetic resonance imaging (MRI) revealed a lesion surrounding the left intracranial portion of the optic nerve, extending into the posterior part of the optic canal (Fig. 1G).The lesion was isointense to cerebrospinal fluid (CSF) on T1- and T2-weighted images without enhancement after gadolinium injection, and an arachnoid cyst was suspected. The patient underwent decompression surgery with removal of thin, lobulated cysts around the chiasm and optic nerve. Vision did not recover postoperatively. Histopathological examination by routine paraffin technique and light microscopy demonstrated cyst walls composed of a collagen-rich, fibrillar tissue, surrounding spaces partially lined by an epithelium-like cell layer (Fig. 1H). A few psammoma bodies were found (Fig. 1I). Prussian blue stain depicted large areas of previous spontaneous bleedings within the lesion (Fig. 1J). Immunohistochemistry demonstrated that the cells of the walls reacted with vimentin antibodies (clone 3B4,), but the epithelium-like cells did not (Fig. 1H). All cells were negative for cytokeratins, (clone AE1/AE3 + 8/18, detecting cytokeratins no.: 1, 4, 5, 7, 8, 10, 13, 14, 18 and 19), glial fibrillar acidic protein (polyclonal antibody) and S-100 protein (polyclonal antibody). In summary, the tissue morphology indicated thickened dura mater and leptomeninges with a few meningothelial cells, consistent with an arachnoid cyst. (A) 9-year-old girl with neurofibromatosis type 1, showing typical café-au-lait skin pigmentations. (B) Congenital ectropion uveae of the left iris. (C) Normal right optic disc. (D) Left cupped optic disc. (E) Left visual field disclosing a paracentral scotoma in addition to glaucoma-related arcuate defects. (F) Normal right visual field (Octopus©) shown for comparison. (G) T2-weighted coronal MRI scan disclosing an arachnoid cyst surrounding the left optic nerve (asterisk). (H) Micrograph of the arachnoid cyst. The cyst wall consists of fibrillar material lined by a few epithelial-like cells. (Haematoxylin and eosin stain [H&E]; original magnification × 100.) (I) Cyst wall in higher magnification. A psammoma body is seen in purple. (H&E; original magnification × 250.) (J) Part of cyst wall. The blue areas indicate iron deposition from previous bleedings. (Prussian blue stain; original magnification × 50). Optic neuropathies are common findings in NF1 and are usually related to gliomas. Our patient had stable juvenile glaucoma caused by congenital ectropion uveae, a neural-crest-derived anterior segment dysgenesis syndrome which can be associated with NF1 (Trovó-Marqui et al. 2004). The occurrence of unexplained central visual loss, despite controlled glaucoma, prompted neuroimaging, which disclosed an intracranial arachnoid cyst extending into the optic canal, possibly precipitating the central visual loss by compression. Radiologically, orbital arachnoid cysts may mimic gliomas because the high water content of gliomatous tissue produces MRI signals identical to those of CSF (Brodsky 1993; Akor et al. 2003). In our case the lack of contrast enhancement supported the arachnoid cyst diagnosis (Cincu et al. 2007), but did not rule out the glioma. Arachnoid cysts are intracranial, space-occupying lesions of unknown pathogenesis. They are typically stable in size over time and mass symptoms are exceedingly rare (Cincu et al. 2007). Symptomatic cyst expansion can, among other causes, be related to intracystic haemorrhage (Cincu et al. 2007) and histopathology in the present case indicated previous bleeding in the cyst wall. Benign distension of normal fibrovascular tissue surrounding the visual pathways has been reported as a possible cause of compressive optic neuropathies (Akor et al. 2003). Although, as in our case, vision may not improve after surgery, biopsy for diagnosis and decompression may be warranted in atypical cases. In conclusion, in NF1, an intracranial arachnoid cyst can precipitate visual loss in a glaucomatous optic nerve, related to ectropion uveae. Glaucoma does not cause reduced central vision before the end stage of the disease, whereas central visual loss can be the presenting feature of a compressive optic neuropathy (Akor et al. 2003; Claes et al. 2006). Early detection by neuroimaging is prompted when unexplained, central visual loss occurs in glaucoma.