OPTIC NEURITIS

Optic neuritis includes inflammatory and demyelinating
disorders of the optic nerve.
Etiology
1. Idiopathic. In a large proportion of cases the
underlying cause is unidentifiable.
2. Hereditary optic neuritis (Leber’s disease)
3. Demyelinating disorders are by far the most
common cause of optic neuritis. These include
multiple sclerosis, neuromyelitis optica (Devic’s
disease) and diffuse periaxial encephalitis of
Schilder. About 70% cases of established multiple
sclerosis may develop optic neuritis.
4. Parainfectious optic neuritis is associated with
various viral infections such as measles, mumps,
chickenpox, whooping cough and glandular fever.
It may also occur following immunization.
5. Infectious optic neuritis may be sinus related
(with acute ethmoiditis) or associated with cat
scratch fever, syphilis (during primary or
secondary stage), lyme disease and cryptococcal
meningitis in patients with AIDS.
6. Toxic optic neuritis (see toxic amblyopias).
Clinical profile
Anatomical types. Optic neuritis can be classified
into three anatomical types:
Papillitis. It refers to involvement of the optic
disc in inflammatory and demyelinating disorders.
This condition is usually unilateral but sometimes
may be bilateral.
Neuroretinitis refers to combined involvement of
optic disc and surrounding retina in the macular
area.
Retrobulbar neuritis is characterized by
involvement of optic nerve behind the eyeball.
Clinical features of acute retrobulbar neuritis are
essentially similar to that of acute papillitis except
for the fundus changes and ocular changes
described below.
Symptoms. Optic neuritis may be asymptomatic or
may be associated with following symptoms:
Visual loss. Sudden, progressive and profound
visual loss is the hallmark of acute optic neuritis.
Dark adaptation may be lowered.
Visual obscuration in bright light is a typical
symptom of acute optic neuritis.
Impairment of colour vision is always present in
optic neuritis. Typically the patients observe
reduced vividness of saturated colours.
Movement phosphenes and sound induced
phosphenes may be percieved by patients with
optic neuritis. Phosphenes refer to glowing
sensations produced by nonphotic or the so
called inadequate stimuli.
Episodic transient obscuration of vision on
exertion and on exposure to heat, which recovers
on resting or moving away from the heat
(Uhthoff’s symptom) occurs in patient with
isolated optic neuritis.
Depth perception, particularly for the moving
object may be impaired (Pulfrich’s phenomenon).
Pain. Patient may complain of mild dull eyeache.
It is more marked in patients with retrobulbar
neuritis than with papillitis. Pain is usually
aggravated by ocular movements, especially in
upward or downward directions due to
attachment of some fibres of superior rectus to
the dura mater.
Signs are as follows:
1. Visual acuity is usually reduced markedly.
2. Colour vision is often severely impaired.
3. Pupil shows ill-sustained constriction to light.
Marcus Gunn pupil which indicates relative
afferent pupillary defect (RAPD) is a diagnostic
sign. It is detected by the swinging flash light
test (see page 474).
4. Ophthalmoscopic features. Papillitis is
characterised by hyperaemia of the disc and
blurring of the margins. Disc becomes oedematous
and physiological cup is obliterated. Retinal veins
are congested and tortuous. Splinter haemorrhages
and fine exudates may be seen on the disc.
Slit-lamp examination may reveal inflammatory
cells in the vitreous. Inflammatory signs may also
be present in the surrounding retina when papillitis
is associated with macular star formation and the
condition is labelled as ‘neuroretinitis’ (Fig. 12.8).
In majority of the cases with retrobulbar neuritis
fundus appears normal and the condition is
typically defined as a disease where neither the
ophthalmologist nor the patient sees anything.
Occasionally temporal pallor of the disc may be
seen.
5. Visual field changes. The most common field
defect in optic neuritis is a relative central or
centrocaecal scotoma. Other field defects noted
rarely include: paracentral nerve fibre bundle
defect, a nerve fibre bundle defect extending up
to periphery and a nerve fibre bundle defect
involving fixation point and periphery. The field
defects are more marked to red colour than the
white.
6. Contrast sensitivity is impaired.
7. Visually evoked response (VER) shows reduced
amplitude and delay in the transmission time.
Differential diagnosis
Papillitis should be differentiated from
papilloedema and pseudo-papilloedema (see
Table 12.2).
Acute retrobulbar neuritis. It must be differentiated
from malingering, hysterical blindness,
cortical blindness and indirect optic neuropathy.
Evolution, recovery and complications
In optic neuritis, typically, the visual acuity and colour
vision is lost progressively over 2-5 days. The rate of
visual recovery is slower than the rate of visual loss
and usually takes between 4 and 6 weeks. About 75
to 90 percent cases get good visual recovery.
However, recurrent attacks of acute retrobulbar
neuritis are followed by primary optic atrophy and
recurrent attack of papillitis are followed by
postneuritic optic atrophy leading to complete
blindness.
Treatment
Efforts should be made to find out and treat the
underlying cause. There is no effective treatment for
idiopathic and hereditary optic neuritis and that
associated with demyelinating disorders.
Corticosteroid therapy may shorten the period of
visual loss, but will not influence the ultimate level of
visual recovery in patients with optic neuritis. Optic
neuritis treatment trial (ONTT) group has made
following recommendations for the use of
corticosteroids:
1. Oral prednisolone therapy alone is contraindicated
in the treatment of acute optic neuritis, since, it
did not improve visual outcome and was
associated with a significant increase in the risk
of new attacks of optic neuritis.
2. A patient presenting with acute optic neuritis
should have brain MRI scan. If the brain shows
lesions supportive of multiple sclerosis (MS),
regardless of the severity of visual loss, each
patient should receive immediate intravenous
methylprednisolone (1 gm daily) for 3 days
followed by oral prednisolone (1 mg/kg/day) for
11 days. This therapy will delay conversion to
clinical MS over the next 2 years.
3. Indications for intravenous methylprednisolone
in acute optic neuritis patients with a normal
brain MRI scan are:
Visual loss in both eyes simultaneously or
subsequently within hours or days of each
other.
When the only good eye is affected.
When the slow progressive visual loss
continues to occur.