Retinal and optic medication toxicity

Retinal and Optic Nerve Drug Toxicities

1. Hydroxychloroquine (HCQ) & Chloroquine (CQ) Retinopathy

Pathophysiological Mechanism

Hydroxychloroquine and chloroquine possess high structural affinity for melanin and concentrate avidly within the retinal pigment epithelium (RPE) of the macular region. Intracellularly, the drugs inhibit lysosomal enzymatic degradation and interfere with normal autophagy within RPE cells, impairing the digestion of shed photoreceptor outer segments. This leads to secondary metabolic dysfunction and apoptosis of the overlying parafoveal photoreceptor cells (ellipsoid zone loss), followed ultimately by secondary RPE cell death.

Current Hydroxychloroquine Screening

AAO’s 2025 revision, published in 2026, recommends daily HCQ dosing no more than 5 mg/kg actual weight, with additional caution and a 400-mg/day maximum starting approach in severe obesity. This reduces risk rather than defining a guaranteed safe dose. Dose, duration, renal disease, tamoxifen and older age at initiation affect risk.

Obtain baseline fundus examination, OCT and autofluorescence soon after starting. Annual OCT plus wide-pattern FAF are primary screening tools; low-risk patients may defer annual screening during the first five years. Visual fields and mfERG are confirmatory/secondary tests, with field patterns chosen to detect both parafoveal and pericentral disease rather than assigning an exclusive ethnic pattern. The 24-2C can sample both regions. Confirm borderline abnormalities and communicate with the prescriber before stopping an important systemic medicine. Mild toxicity often has limited progression after cessation; severe disease can progress for years.

This is AAO guidance; European/UK local pathways may differ. Use the current jurisdictional protocol and product information. See the AAO revision.

2. Ethambutol Toxic Optic Neuropathy

Ethambutol can cause bilateral central or cecocentral visual loss, dyschromatopsia and later pallor. Risk relates to dose, duration and renal function, without a guaranteed safe threshold. Establish baseline vision and arrange monitoring under the local TB protocol, especially in high-risk patients. New symptoms need urgent examination, central fields and OCT as appropriate, plus communication with the TB team. Withdrawal of the suspected agent should preserve an effective alternative infection regimen. Recovery varies and permanent loss can occur; normal early discs do not exclude toxicity.

3. Other High-Yield Drug Toxicities for the EBOD Exam

• Tamoxifen (Selective Estrogen Receptor Modulator):
  • High cumulative doses induce tamoxifen retinopathy, characterised by numerous refractile, glistening, yellow-white crystalline deposits in the inner retina (paramacular and peripapillary), focal pigmentary changes, and pseudocystic foveal cavitation / cystoid macular oedema.
• Phenothiazines (Thioridazine & Chlorpromazine):
  • Thioridazine: Causes severe, irreversible, dose-dependent pigmentary retinopathy (greater risk at high doses without an absolute safe threshold). Manifests as coarse retinal pigment clumping, "salt and pepper" fundus, geographic nummular chorioretinal atrophy, nyctalopia, and constriction of visual fields.
  • Chlorpromazine: Produces golden-brown, dust-like granular pigmentary deposits in the corneal endothelium, an anterior subcapsular axial stellate cataract, and violaceous or slate-gray discoloration of the sun-exposed skin and conjunctiva.
• Vigabatrin (Sabril - Irreversible GABA-Transaminase Inhibitor):
  • Used for selected refractory epilepsy and infantile spasms. It can cause irreversible concentric field loss that may be asymptomatic. Assess baseline and follow-up vision according to the current local label and developmental ability. Cooperative patients can undergo perimetry; ERG, OCT or other approaches can be considered when fields are impractical, with their limitations explained. They are not universally equivalent tests. The current UK Sabril SmPC recommends baseline assessment within four weeks, assessment every three to six months during treatment and after discontinuation, using an individualised diagnostic approach.
• Phosphodiesterase-5 (PDE5) Inhibitors (Sildenafil, Tadalafil):
  • Weakly cross-inhibits retinal PDE6 (located in photoreceptor outer segments), causing transient, reversible cyanopsia (blue-tinted vision), increased light sensitivity, and blurred vision at peak serum concentrations. NAION has been reported in association with PDE5 inhibitors, but shared vascular risks and observational evidence complicate causal attribution. Sudden visual loss requires urgent assessment and coordinated medication review.

Pharmacokinetic and Toxicity Limits

The Henderson–Hasselbalch ratio must specify the drug: for a weak base, ionised/un-ionised ratio is 10 raised to pKa − pH; a weak acid uses the opposite relationship for ionised/un-ionised. Solubility and epithelial integrity affect delivery rather than making every ionised molecule absolutely impenetrable. Nasolacrimal occlusion reduces but does not abolish systemic exposure.

Ethambutol toxicity is associated with dose, duration and renal dysfunction; dyschromatopsia can vary and may not be the first sign. Possible mitochondrial/metal-chelation mechanisms do not prove a single complete pathway. New bilateral central visual dysfunction requires urgent review with the TB team, stopping the suspected agent appropriately while maintaining effective infection treatment. Screening intervals depend on local TB risk/protocol rather than universal monthly testing for all. Vigabatrin monitoring follows the local label and the patient’s ability to perform fields; ERG is not a proven equivalent screening test in every infant. Topical brimonidine is contraindicated below two years, with further caution in older small children; prostaglandin and CAI precautions should not be described as universal contraindications for every uveitis history or sulfonamide-antibiotic allergy.