Sir,

Unilateral optic disc swelling is most frequently caused by a local optic nerve or intraocular pathology. We present a case of unilateral disc swelling secondary to raised intracranial pressure and associated with bilateral visual obscurations.

Case report

A 49-year-old female presented with a 1-month history of generalised headaches and transient visual obscurations. The episodes of visual loss were bilateral, lasted a few seconds and occurred several times a day. She was otherwise well. Past ophthalmic history included left amblyopia with a definite history of occlusion treatment in childhood. She was a nonsmoker and had controlled essential hypertension.

Ocular examination revealed corrected visual acuity of 6/5 right and 6/24 left. Anterior segment examination, intraocular pressures, and colour vision were normal. There was no afferent pupillary defect. The right optic disc was swollen and hyperaemic with superficial haemorrhages (Figure 1a). The left optic disc appeared normal (Figure 1b). Ocular movements were full and there was no proptosis.

Figure 1
figure 1

Fundus photograph of right (a) and left (b) optic discs.

Full size image

Magnetic resonance imaging (MRI) demonstrated a large left-sided medial sphenoid wing mass involving the lateral wall of the cavernous sinus and partially encasing the cavernous portion of the left internal carotid artery. No lesions were identified close to the right optic nerve, orbit, or orbital apex. There was evidence of midline shift suggesting raised intracranial pressure (Figure 2).

Figure 2
figure 2

Gadolinium enhanced coronal MRI scan demonstrating large intracranial mass.

Full size image

The patient underwent a craniotomy and excision of the tumour. At surgery the tumour was found to be adherent to the optic and occulomotor nerves. It was dissected from these structures but a residual layer was densely adherent to the lateral wall of the cavernous sinus. A small amount of residual tumour was cauterised. Histologically, the tumour was confirmed as a meningioma with a slightly raised mitotic count, it was therefore classified as atypical meningioma WHO Grade 2.

Comment

The term papilloedema refers specifically to optic disc swelling in the presence of raised intracranial pressure. Truly unilateral papilloedema is rare and poses a diagnostic dilemma. An intact optic nerve sheath is apparently necessary for the development of disc oedema in the presence of raised intracranial pressure.1 Acquired and congenital anomalies in the optic nerve sheath which prevent transmission of increased cerebrospinal fluid pressure to the optic nerve, have been hypothesised as a reason for unilateral papilloedema. Other postulated mechanisms include abnormalities in the venous sinuses and a difference in the lamina cribrosa between the two discs resulting in decreased transmission of the raised intracranial to the optic nerve in the scleral canal.2

In the case presented, the potential mechanism for the unilateral papilloedema is the presence of a tumour adherent to the intracranial optic nerve preventing transmission of the increased pressure down the perioptic space to the optic nerve head. High resolution CT or MRI imaging, not performed in this case, may demonstrate asymmetrical optic nerve sheath swelling, but this is not universal. Huna-Baron et al2 and To and Warren3 were unable to demonstrate a difference in size of the optic nerve sheaths using orbital CT or MRI in patients with unilateral papilloedema. This would support the conclusion that development of a single swollen disc in the presence of increased intracranial pressure does not result from gross difference in the optic nerve sheath between the two sides. The possibility of minute differences in the size of the optic nerve sheaths, beyond the sensitivity of CT or MRI cannot be eliminated.

Transient visual obscurations, usually lasting seconds, are a well-known accompaniment of raised intracranial pressure. They may occur with or without the presence of optic disc swelling. The pathogenesis remains something of an enigma. Hayreh4 explained the phenomenon as being due to ischaemia of the optic nerve head and downward herniation of the parahippocampal gyrus in the tentorial notch. Another theory is of intermittent occipital lobe ischaemia related to compression of the posterior cerebral artery usually from uncal herniation.5 Sadun et al6 suggested that transient ishaemia of the optic nerve head consequent to increased tissue pressure could explain the presence of unilateral visual obscurations occurring with or without raised intracranial pressure. This theory would, however, not explain the presence of bilateral visual obscurations in the presence of unilateral papilloedema.

This case serves to remind us that unilateral papilloedema, though rare, does occur and that transient visual obscurations can occur without or prior to development of optic disc swelling.