Sir,

Optic nerve sheath dilatation or meningocele is a rare condition describing an ‘enlargement and dilation of primarily the optic nerve sheath’.1 There is an expansion of the cerebrospinal fluid space around the optic nerve with no associated inflammation, orbital or cerebral neoplasm at the apex of the orbit. We describe a case of bilateral meningoceles with an associated unilateral cystoid macular oedema (CMO).

Case report

A 59-year-old Caucasian male presented with a 10-month history of blurring of vision in the right eye. He had experienced a 6-month period of retro-orbital and facial pain. His past medical history included presumed tuberculoma of a cervical gland at the age of 15 years, hypertension, chronic obstructive airways disease and angina.

His best corrected visual acuity was 6/9 in the right eye and 6/6 in the left on presentation. Colour vision was tested with Ishihara colour plates and was normal on both sides. On examination he had bilateral symmetrical proptosis but the patient indicated that his appearance had been stable for many years. His pupil reactions were normal. His ocular motility was unrestricted and his anterior segment was normal. Contact lens fundoscopy revealed CMO in the right eye. There was no disc swelling. The left eye showed no fundal abnormalities.

Investigations including thyroid function tests, full blood count, serum B12, folate, urea and electrolytes, erythrocyte sedimentation rate and a treponemal screen were normal. Anti-nuclear antibodies were negative. Humphrey visual field testing showed non-specific depression in the periphery of both eyes which was not consistent with any neurological defect. A lumbar puncture was performed with an opening pressure of 24 cm of water.

Magnetic resonance imaging (MRI) scans revealed bilateral optic nerve sheath dilatation and enlargement but with normal sized optic nerves (Figure 1).

Figure 1
figure 1

T2-weighted MR image (axial view) showing bilateral optic nerve meningoceles. Note the dilated optic nerve sheath with normal sized optic nerves.

Full size image

The patient was followed up for the next 5 years and demonstrated a gradual deterioration in visual acuity in the right eye. Colour vision was also reduced in the right eye with a score of 10 correct responses out of the first 21 Ishihara plates. Visual evoked responses at this stage revealed no evidence of pathway delay. There was no progression in his proptosis. There was no evidence of a hyperopic shift in his refraction. The initial CMO that had been noted had resolved leaving focal areas of chorioretinal atrophy and scattered areas of hyper- and hypopigmentation. At no stage was there any evidence of choroidal folds. His best corrected visual acuity was 2/36 in the right eye and remained 6/6 on the left.

Discussion

The term ‘optic nerve head meningocele’ (ONHM) was first coined in Garrity et al’s landmark paper1 in which 13 patients were described with this condition. Expansion of the optic nerve sheath without any obvious underlying cause such as inflammation or neoplasia has been described but the terminology has differed. The terms used have included arachnoid cysts, optic hydrops and patulous subarachnoid space.2,3,4 To date there have been over 30 cases described in the literature.5

On reviewing the literature two subsets of patients with idiopathic ONHM are seen. The first is a group in which there is concurrent evidence of progressive hyperopia and choroidal folds. This typically affects middle-aged males. It is possible to have this as an isolated entity without evidence of an associated meningocele. In a case series by Dailey et al6 four out of the seven patients reported with hyperopia and choroidal folds had optic nerve meningoceles. Several of the patients had associated changes at the macula which included a retinal pigment epitheliopathy and central serous retinopathy. The second subgroup includes all the others with no underlying pathology.

In this particular subgroup macular pathology has not been noted. Our case is unique in that there was associated macular pathology in the form of a transient cystoid macular oedema followed by a retinal pigment epithelopathy. We postulate that the mechanism behind this may be a tractional force of the meningocele on the globe.

The role of raised intracranial pressure is unclear in the aetiology as only a few of the reported cases have had raised intracranial pressure as measured by lumbar puncture. No significant evidence is available to suggest that some cases of optic nerve meningocele are variants of benign intracranial hypertension (BIH) although some parallels can be drawn. Helmke et al7 demonstated optic nerve sheath dilatation in patients with raised intracranial pressure. Additionally associated macular pathology may be seen in BIH.8

The symptoms with which people present are either a change in refractive error, visual disturbance (either a drop in visual acuity or a field disturbance), pain or a combination of these. Typically the pain is a retro-orbital pain. The patient in the current report did have facial and orbital pain but these symptoms were transient and only lasted for several months. The pain may be due to referred pain from trigeminal nerve stimulation from the dura.

The natural history of this condition is not clearly known and it is difficult to prognosticate in the early stages. On the evidence available patients with hyperopia and choroidal folds tend to follow a more benign course. However, there are patients in both groups who develop progressive visual loss. In these cases performing surgical decompression of the optic nerve sheath should be considered. There is evidence that this may improve vision or halt the progression in some cases.1 However we believe that all cases with visual loss should have a thorough examination of the macula and electrophysiological testing to determine the contribution of optic nerve pathology to visual loss before surgery is contemplated.