Optic neuritis
Optic neuritis is inflammatory and demyelinating disorders of the optic nerve. It is vision threating disease of eye. patient has gradual or sudden painless loss of vision.
Etiology of Optic neuritis
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), tuberculosis, lyme disease and cryptococcal meningitis in patients with AIDS.
6. Autoimmune disorders associated with optic neuritis include sarcoidosis, systemic lupus erythematosus, polyarteritis nodosa, Guillain-Barre syndrome and Wegener’s granulomatosis.
7. Toxic optic neuritis
Clinical profile of optic neuritis
Anatomical types of 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.
Typical versus atypical optic neuritis
Traditionally, the term typical optic neuritis refers to the one associated with demyelination, particularly multiple sclerosis and the term atypical neuritis is labeled for the one associated with causes other than demyelination disorders.
Clinical features
Symptoms of optics neuritis
Optic neuritis may be asymptomatic or may be associated with following symptoms:
• Visual loss. Monocular 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 perceived 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. Patients 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 of optics neuritis
1. Visual acuity is usually reduced markedly.
2. Colour vision is often severely impaired (typically red desaturation).
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.
4. Ophthalmoscopic features.
Papillitis is characterised by hyperaemia of the disc and blurring of the margins. Disc becomes oedematous and physiological cup is obliterated (in papillitis disc oedema rarely exceeds 2–3 D, while in papilloedema it become 3–6 D).
Retinal veins are congested and tortuous. Splinter haemorrhages and fine exudates may be seen on the disc.
Slitlamp 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’.
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.
8. Fundus fluorescein angiography reveals mild to moderate leak in early phase which increases with the time.
Differential diagnosis
• Papillitis should be differentiated from papilloedema, ischaemic optic neuropathy, anterior orbital compressive neuropathy and pseudopapilloedema.
• Acute retrobulbar neuritis. It must be differentiatedfrom malingering, hysterical blindness, cortical blindness and indirect optic neuropathy.
Treatment of optic neuritis
1. Treatment of the causes. 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.
2. 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:
• 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.
• Intravenous methylprednisolone.
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. Then taper prednisolone over 4 days. This therapy will delay conversion to clinical MS over the next 2 years.
Note. Intravenous dexamethasone has been reported to be as effective as methylprednisolone in many studies.
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.
3. Interferon therapy has been reported to reduce recurrences in patients with multiple sclerosis. However, the treatment is very expensive and with unknown long term benefits.