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Distension of the subarachnoid space of the optic nerve is not a common feature of optic neuritis. We describe a patient with optic neuritis with swelling of the optic nerve head of the right eye. On magnetic resonance imaging (MRI) there was marked distension of the optic nerve sheath due to an increase of fluid in the subarachnoid space. The location of the lesion in the optic nerve and concurrent inflammatory changes of the arachnoid trabecula and septae may have had a role in the pathophysiology of this condition.
A 38 year old man was admitted with pain on eye movements and loss of vision in the right eye. Best corrected visual acuities measured 20/40 on the right and 20/20 on the left. The patient identified 16 out of 18 Ishihara plates with the right eye and 18/18 with the left eye. There was a relative afferent pupillary deficit (RAPD) on the right. Funduscopy demonstrated a swollen optic disc on the right. The left optic disc was normal. Spontaneous venous pulsations (SVP) were detectable bilaterally. Laboratory examinations, including red and white blood counts, C reactive protein, sedimentation rate, serologies for syphilis, HIV, herpes, toxoplasmosis, Lyme disease and cytomegalovirus, as well as collagen vascular disorders and coagulopathies were all in normal range. The right visual field (Octopus program G2) demonstrated a wedge-shaped defect in the inferior nasal and temporal visual field pointing towards the macula. The left visual field was normal. Neurological examination was normal. MRI of the brain was normal but showed enhancement of the right optic nerve in the T1 weighted axial and coronal (not shown) image and hyperintense fluid in the expanded optic nerve sheath on the T-2 weighted image (Fig 1A and B). Two days after admission the visual acuity in the right eye decreased to 20/100 and only two out of 18 Ishihara plates were identified. SVP were no longer present on the right. The swelling of the right optic disc progressed and temporal peripapillary Patton folds appeared, suggesting the diagnosis of papilloedema (Fig 2). Within 2 weeks visual acuity improved to 20/25 right eye and colour vision returned to normal. A repeat MRI of the orbits 7 weeks later demonstrated normal diameters of both perioptic subarachnoid spaces (Fig 1C).
Distension of the perioptic subarachnoid space is a hallmark MRI feature of papilloedema due to an intracranial mass lesion, inflammatory disease, and pseudotumour cerebri.1 Unilateral distension of the of the optic nerve sheath due to increased fluid volume of the subarachnoid space of the optic nerve has previously been reported in some patients with optic hydrops, anterior ischaemic optic neuropathy, and anatomical anomalies such as arachnoid cysts.2–4 This patient with optic neuritis demonstrated marked distension of the subarachnoid space of the right optic nerve, presumed to be caused by an increase of total fluid following optic neuritis. As all cerebrospinal fluid compartments are thought to communicate, equalisation of fluid via the chiasmal cistern would have been expected to occur. The MRI scan of the brain and orbits, however, demonstrated localised and isolated stasis of fluid in the right optic nerve subarachnoid space only. The reason for this fluid congestion causing a optic nerve sheath compartment syndrome could not be identified by neuroimaging. The site of inflammation of the optic nerve and local anatomical variations and alterations of the subarachnoid space—for example, the amount and number of trabecula and septae in the subarachnoid space,5 may have a crucial role in the pathophysiology of unilateral papilloedema.
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