Describe optic neuritis with its features, investigation and management.

Definition and Types

• The typical demyelinating presentation — a young adult, more often female, aged 20 to 45, with:
• Subacute monocular visual loss developing over hours TO days and progressing for up to two weeks
• Pain ON eye movement in 90%, often preceding the visual loss by a day or two — caused by traction of the superior and medial recti on the inflamed nerve sheath. Its absence should raise doubt about the diagnosis
• Dyschromatopsia, with red desaturation that is out OF proportion to the acuity. Asking the patient to compare a red object between the two eyes is a simple and sensitive bedside test
• A relative afferent pupillary defect, which is always present in unilateral disease; its absence in a patient with reduced vision in one eye should prompt reconsideration
• A central or centrocaecal scotoma, though any field defect may occur
• Reduced contrast sensitivity, which persists after the acuity recovers
• Uhthoff’S phenomenon — transient worsening of vision with a rise IN body temperature: a hot bath, exercise, fever or a hot meal. It reflects conduction block in partly demyelinated fibres, and is characteristic of demyelination
• The pulfrich phenomenon — a swinging object appears to move in an ellipse.

Investigation and the Optic Neuritis Treatment Trial

• Natural history — vision reaches its worst at 1 to 2 weeks, then recovery begins at 2 to 4 weeks, and most patients regain 6/12 or better by 6 to 12 months. Failure to begin recovering by 4 weeks is a red flag and demands reconsideration of the diagnosis
• The optic neuritis treatment trial (ONTT) governs management, and its three conclusions are examinable:
• 1. Intravenous methylprednisolone speeds recovery but does not alter the final visual outcome. It is therefore given for a functional reason — a patient who needs to see quickly — rather than to save vision
• 2. Oral prednisolone alone (in standard dose) is contraindicated, because it was associated with an increased rate of recurrence. This is the most important practical point and is frequently got wrong
• 3. MRI predicts the risk of multiple sclerosis. With one or more white-matter lesions the 15-year risk is 72%; with a normal MRI it is 25%. This is why MRI is done — not to diagnose the optic neuritis.
• Investigations — MRI brain and orbits with gadolinium and fat suppression, showing nerve enhancement and periventricular lesions; OCT, which shows retinal nerve fibre layer thinning after 3 to 6 months and is used to quantify axonal loss; visually evoked potentials, showing a delayed latency with preserved amplitude, which is the signature of demyelination and persists for life, so it can demonstrate a past subclinical episode; and, where atypical, aquaporin-4 and MOG antibodies, syphilis and Lyme serology, ACE and chest imaging, and vasculitic screening
• Red flags suggesting the neuritis is not typical demyelination — and each should prompt wider investigation:
• Age over 50 or under 12
• Bilateral simultaneous involvement
• Absence of pain
• Very severe visual loss (no perception of light), or progressive loss beyond 2 weeks
• NO recovery by 4 weeks
• Disc or retinal haemorrhage, or marked disc swelling
• Systemic symptoms — fever, weight loss, rash, arthralgia