Toxic Amblyopia

Etiology

Toxic Causes

Nutritional Causes

Pathophysiology

• Axonal degeneration of the papillomacular bundle
• Demyelination of the optic nerve (especially around the optic disc)
• In methanol poisoning: direct retinal ganglion cell toxicity (formic acid accumulation)

Clinical Features

Symptoms

• Bilateral, symmetric, painless, progressive visual loss (weeks to months)
• Central or cecocentral scotoma (connecting the blind spot to fixation) — the hallmark
• Dyschromatopsia (impaired color vision), often the earliest symptom
• Decreased contrast sensitivity
• Visual acuity typically 20/50–20/200 at presentation

Signs

• Normal or mildly hyperemic optic discs early on (unlike ischemic optic neuropathy)
• Temporal pallor of the optic disc develops late as nerve fiber loss progresses
• No afferent pupillary defect early (bilateral symmetric disease masks it)
• Fundus otherwise normal initially

Diagnosis

Clinical

• Bilateral symmetric central/cecocentral scotoma on visual field testing (Humphrey, Goldmann)
• Dyschromatopsia on Ishihara or Farnsworth-Munsell 100-hue testing
• Careful history: tobacco use, alcohol, medications, diet, bariatric surgery

Investigations

Key Differential Diagnoses

• Leber hereditary optic neuropathy (LHON) — mitochondrial genetics, typically young males
• Dominant optic atrophy — autosomal dominant, childhood onset
• Compressive optic neuropathy — MRI essential
• Demyelinating optic neuritis — typically painful, unilateral, younger patients
• Glaucoma — peripheral field loss, cupped disc

Management

Remove the Cause

• Stop the offending drug or toxin (e.g., ethambutol, linezolid, amiodarone)
• Abstain from tobacco and alcohol
• Address underlying malnutrition

Nutritional Supplementation

• Vitamin B12 (cyanocobalamin or hydroxocobalamin) — IM injections if absorption is impaired
• Thiamine — especially post-bariatric surgery; parenteral initially (Harrison's, p. 12704)
• Multivitamin B complex (B1, B2, B6, folate)
• High-protein, well-balanced diet

Prognosis

• Recovery is possible if treated early, before irreversible optic atrophy sets in
• The duration and severity of deficits before treatment are key predictors of outcome (Harrison's, p. 12704)
• Visual recovery may take weeks to months
• Temporal optic disc pallor and established optic atrophy generally indicate poor visual prognosis

Special Situations

Tobacco-Alcohol Amblyopia

Methanol Poisoning

• Fomepizole or ethanol (alcohol dehydrogenase inhibitors)
• Hemodialysis
• Folinic acid (to enhance formate metabolism)
• Corticosteroids (controversial, sometimes used for optic nerve inflammation)

Post-Bariatric Surgery

• Cecocentral scotoma + bilateral painless visual loss = toxic/nutritional optic neuropathy until proven otherwise
• Treat empirically with B-vitamins while awaiting workup — low risk, high reward
• Early treatment before optic atrophy is critical for visual recovery

Infantile Esotropia

Definition

Epidemiology

• Most common form of strabismus in infants
• Incidence: ~1% of the general population
• More common in preterm infants and those with neurological impairment
• Familial predisposition (positive family history in ~20–30% of cases)
• Associated with cerebral palsy, hydrocephalus, Down syndrome

Etiology and Pathogenesis

1. Motor Imbalance Theory (Worth)

2. Sensory Fusion Defect Theory (Chavasse)

Contributing Factors

• Genetic: Autosomal dominant inheritance pattern in some families
• Neurological: High association with CNS abnormalities (up to 30% in some series)
• Accommodative element: Usually non-accommodative, though partial accommodative components can exist

Clinical Features

Core Characteristics (Esotropia and Exotropia, p. 10)

Cross-Fixation

• A hallmark feature: the child uses the right eye to look left and the left eye to look right
• Each eye is used in adduction (turned in)
• This can mask amblyopia since both eyes are being used — though risk of amblyopia remains (Esotropia and Exotropia, p. 10)

Associated Features (may develop later)

Clinical Image

Diagnosis

History

• Age of onset, family history of strabismus, birth history (prematurity), developmental milestones, neurological symptoms

Examination

• Age-appropriate methods: preferential looking (Teller cards), Cardiff cards, or fixation behavior (central, steady, maintained — CSM)
• Assess fixation preference: unequal fixation indicates amblyopia

• Cover-uncover test: reveals manifest deviation (tropia)
• Alternate cover test + prism neutralization (APCT): measures the total angle of deviation
• Krimsky test / Hirschberg test: useful in preverbal infants who cannot cooperate with prism cover test

• Prism and Alternate Cover Test (PACT): gold standard for measuring angle — measured at distance (6 m) and near (33 cm)
• Typical angle in infantile esotropia: ≥30 PD at distance

• Mandatory — with cyclopentolate 1% or atropine
• Identifies significant hyperopia (>+2.00 D) that may have accommodative component
• Spectacles prescribed first if significant refractive error found

• Assess ductions and versions in 9 cardinal positions
• Identify IOOA, superior oblique underaction, A/V patterns

• Rule out structural pathology (retinoblastoma, optic nerve hypoplasia, retinal dystrophy)
• Identify optic nerve pallor, macular abnormalities

• MRI brain if neurological signs, atypical features, or nystagmus suggesting CNS pathology

Differential Diagnosis

Management

Step 1: Correct Refractive Error

• Cycloplegic refraction performed
• Spectacles prescribed if hyperopia ≥ +2.00 D or significant astigmatism
• Re-examine after 6–8 weeks of spectacle wear — if deviation fully corrected, it is accommodative

Step 2: Treat Amblyopia

• Occlusion therapy (patching): patch the preferred (fixing) eye to stimulate the amblyopic eye
• Part-time patching (2–6 hours/day) depending on age and depth of amblyopia
• Pharmacological penalization: atropine drops to preferred eye (blurs near vision, forces use of amblyopic eye)
• Must treat amblyopia before surgery if possible, as amblyopia treatment alone will not correct the angle

Step 3: Surgical Correction

Timing of Surgery

• Ideal: before 2 years of age (some advocate before 12–18 months) to maximize potential for binocular vision development
• Sensory critical period: earlier correction improves prospects for bifoveal fusion and stereopsis
• Surgery delayed until amblyopia is controlled and angle is stable

Surgical Options

Surgical Goals

• Achieve orthotropia or small-angle residual esotropia (within 10 PD of straight)
• Surgical success defined as alignment within 8–10 PD of orthotropia
• Gross stereopsis (peripheral fusion) is realistic; fine stereopsis is usually not achieved if surgery is delayed

Botulinum Toxin (Botox)

• Chemodenervation of medial rectus as an alternative or adjunct to surgery
• More useful in acquired or small-angle esotropias
• Less predictable for large-angle infantile esotropia; not first-line

Step 4: Postoperative Management

• Continue amblyopia therapy postoperatively
• Monitor for DVD — may require subsequent surgery (inferior rectus recession or superior rectus posterior fixation / Faden procedure)
• Monitor for IOOA — inferior oblique recession/myectomy
• Regular follow-up for refractive changes and re-development of amblyopia

Prognosis and Outcomes

Key Points Summary

1. Infantile esotropia = constant esotropia presenting before 6 months without resolution
2. Large angle (≥30 PD), cross-fixation, alternating — key hallmarks
3. Non-accommodative; cycloplegic refraction essential to rule out accommodative component
4. Amblyopia, DVD, and IOOA are major associated features
5. Management is stepwise: refraction → amblyopia treatment → surgery
6. Surgery before age 2 (bilateral medial rectus recession) optimizes sensory outcomes
7. Fine stereopsis is rarely achieved; motor alignment and peripheral fusion are realistic goals
8. Long-term follow-up mandatory for amblyopia recurrence, DVD, and IOOA