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Optic Atrophy (6 Marks)
Definition
Optic atrophy refers to the late-stage degenerative changes in the optic nerve resulting from axonal degeneration anywhere in the pathway between the retina and the lateral geniculate body, manifesting with disturbance in visual function and changes in the appearance of the optic nerve head.
(Kanski's Clinical Ophthalmology, 10th ed.)
Classification
Optic atrophy is classified into four types:
1. Primary Optic Atrophy
- Occurs without antecedent swelling of the optic nerve head.
- Caused by lesions affecting the visual pathways from the retrolaminar portion of the optic nerve to the lateral geniculate body.
- Lesions anterior to the chiasm - unilateral atrophy; lesions at the chiasm/optic tract - bilateral atrophy.
Signs:
- Flat, chalk-white disc with clearly delineated margins
- Reduction in the number of small blood vessels on the disc surface
- Attenuation of peripapillary blood vessels and thinning of the retinal nerve fibre layer (RNFL)
- Atrophy may be diffuse or sectoral depending on cause
- Temporal pallor - seen in papillomacular bundle involvement (e.g., demyelinating disease, toxic/nutritional neuropathy)
Causes:
- Optic neuritis (demyelination)
- Compression by tumours and aneurysms
- Hereditary optic neuropathies (e.g., LHON, dominant optic atrophy)
- Toxic and nutritional optic neuropathy
- Trauma
2. Secondary Optic Atrophy
- Preceded by longstanding swelling of the optic nerve head (e.g., chronic papilloedema, papillitis, anterior ischaemic optic neuropathy).
Signs:
- Slightly or moderately raised white/grey disc with poorly delineated margins due to gliosis
- Obscuration of the lamina cribrosa
- Reduction in disc surface blood vessels
- Peripapillary circumferential retinochoroidal folds - Paton lines (especially temporal to disc)
- Sheathing of arterioles and venous tortuosity
3. Consecutive Optic Atrophy
- Caused by disease of the inner retina or its blood supply (i.e., ascending atrophy).
- Disc appears waxy with reasonably preserved architecture.
- Cause usually obvious on fundus examination.
- Causes: retinitis pigmentosa, central retinal artery occlusion, extensive retinal photocoagulation.
4. Glaucomatous Optic Atrophy
- Characterized by disc cupping with progressive neuroretinal rim thinning.
- Distinct from other types due to characteristic cup-to-disc ratio enlargement.
Hereditary Optic Atrophies
| Condition | Inheritance | Gene | Key Features |
|---|---|---|---|
| Dominant Optic Atrophy (Kjer type / OA1) | Autosomal dominant | OPA1 (Chr 3) | Most common hereditary optic neuropathy (1:50,000); insidious visual loss in childhood; temporal or diffuse pallor; 20% develop sensorineural hearing loss; VA 6/12-6/60; very slow progression over decades |
| Behr Syndrome | Autosomal recessive | - | Early childhood onset; diffuse atrophy; spastic gait, ataxia, mental handicap |
| Wolfram Syndrome (DIDMOAD) | AR / AD / Mitochondrial | WFS1, CISD2 | Diabetes insipidus, Diabetes mellitus, Optic Atrophy, Deafness; onset 5-21 years; diffuse severe atrophy with disc cupping |
| LHON | Mitochondrial | mtDNA mutations (ND4, ND1, ND6) | Acute/subacute painless visual loss; males >> females; bilateral (sequential); peripapillary telangiectasia in acute phase |
Visual Field Defects in Optic Atrophy
The pattern of field loss depends on the level of the lesion:
- Optic nerve lesion - central scotoma, arcuate defects
- Chiasmal lesion - bitemporal hemianopia
- Optic tract lesion - homonymous hemianopia (incongruous)
Investigations
- OCT - thinning of RNFL, loss of ganglion cell layer
- Visual field testing - Humphrey/Goldmann perimetry
- VEP - prolonged latency and reduced amplitude
- MRI brain and orbits - to identify compressive/demyelinating cause
- Genetic testing - for hereditary forms (OPA1 mutation, mtDNA)
Management
- No treatment reverses established optic atrophy.
- Treatment is directed at the underlying cause (e.g., surgical decompression for tumour, steroids for optic neuritis).
- Idebenone - used in LHON (mitochondrial antioxidant).
- Low vision aids and rehabilitation.
- Genetic counselling for hereditary forms.
Key Point for Exam: The hallmark fundus difference between primary and secondary optic atrophy is disc margins - clearly defined in primary, blurred/gliotic in secondary. Consecutive atrophy gives a waxy pale disc. These distinctions are classic exam discriminators.
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