Which antitubercular drug can cause optic neuropathy
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ethambutol AND optic neuropathy
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ethambutol optic neuropathy CDC tuberculosis treatment
Ethambutol is the classic antitubercular drug associated with toxic optic neuropathy, often presenting with reduced visual acuity, impaired red-green color vision, and central scotomas. Risk increases with higher doses and prolonged treatment. Lippincott Pharmacology and a recent systematic review support this association.
Isoniazid can rarely also cause optic neuropathy, but ethambutol is the standard exam answer.
Adr of hcq drug why does it need ophthalmology consultation
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hydroxychloroquine retinopathy screening
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American Academy Ophthalmology hydroxychloroquine retinopathy screening recommendations
Hydroxychloroquine (HCQ) requires ophthalmology consultation because it can cause toxic retinopathy, involving the macula and photoreceptors.
- Toxicity is related mainly to daily dose and duration of use.
- Early toxicity may be asymptomatic, yet detectable on retinal testing.
- Established retinal damage is generally irreversible and, if advanced, can continue to progress even after HCQ is stopped, causing central visual loss or a bull's-eye maculopathy.
- An ophthalmologist obtains a baseline retinal assessment to identify pre-existing macular disease and provides surveillance, commonly with OCT and visual-field testing.
Risk rises with excessive dosing, long duration, renal impairment, and concurrent tamoxifen. Keeping the dose at or below 5 mg/kg/day based on actual body weight reduces risk. The AAO joint statement recommends baseline retinal evaluation within the first few months; for low-risk patients, regular annual screening can generally begin after 5 years.
Pathophysiology
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hydroxychloroquine retinopathy pathophysiology
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hydroxychloroquine retinopathy pathophysiology retinal pigment epithelium photoreceptors review
Pathophysiology of HCQ retinal toxicity
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Drug accumulation
- Hydroxychloroquine has high affinity for melanin.
- It accumulates over time in melanin-rich ocular tissues, particularly the retinal pigment epithelium (RPE) and choroid.
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Lysosomal dysfunction in RPE
- HCQ accumulates in lysosomes and raises their pH.
- This interferes with normal lysosomal enzyme function and impairs the RPE's metabolism.
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Failure of photoreceptor outer-segment clearance
- Normally, the RPE phagocytoses and degrades shed outer segments of rods and cones.
- HCQ-related RPE dysfunction reduces this clearance, leading to accumulation of debris and progressive RPE injury.
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Secondary photoreceptor loss
- Dysfunction and atrophy of the RPE disrupt support to adjacent photoreceptors, particularly parafoveal cones and rods.
- This produces bilateral, symmetric outer-retinal thinning and loss of visual function.
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Late stage: bull's-eye maculopathy
- Parafoveal photoreceptor and RPE atrophy surrounds a relatively spared fovea, producing the characteristic bull's-eye maculopathy.
- Damage may progress even after stopping HCQ when toxicity is advanced, because the drug persists in pigment-containing tissues and injured retinal cells cannot regenerate.
In one line:
HCQ binds melanin and accumulates in RPE lysosomes -> lysosomal/RPE dysfunction -> impaired photoreceptor outer-segment phagocytosis -> photoreceptor death and irreversible maculopathy.
HCQ binds melanin and accumulates in RPE lysosomes -> lysosomal/RPE dysfunction -> impaired photoreceptor outer-segment phagocytosis -> photoreceptor death and irreversible maculopathy.
The exact mechanism is not fully settled, but this RPE-photoreceptor toxicity model is the accepted explanation. Kanski's Clinical Ophthalmology and this recent clinical review discuss the mechanism and screening implications.
Rapd
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relative afferent pupillary defect review
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relative afferent pupillary defect swinging flashlight test ophthalmology
RAPD = Relative Afferent Pupillary Defect, also called a Marcus Gunn pupil.
It indicates reduced light input through one optic nerve or retina compared with the other eye.
Test: Swinging flashlight test
- Shine light in one eye: normally both pupils constrict.
- Quickly swing the light to the other eye.
- If that eye has an RAPD, both pupils paradoxically dilate or constrict less because the brain perceives less light entering that eye.
It suggests
- Unilateral or asymmetric optic neuropathy: optic neuritis, ischemic optic neuropathy, compressive optic neuropathy, traumatic optic neuropathy.
- Severe asymmetric retinal disease: e.g., large retinal detachment or central retinal artery occlusion.
It is not usually present in symmetric bilateral disease, and it is not a feature of isolated cataract, refractive error, or most macular disease.
For HCQ toxicity: HCQ retinopathy is usually bilateral and symmetric, so an RAPD is generally absent unless toxicity or another optic nerve/retinal condition is markedly asymmetric.
Difference between uniocular and biocular doplopia
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monocular binocular diplopia
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monocular versus binocular diplopia causes cover test ophthalmology
| Feature | Monocular diplopia | Binocular diplopia |
|---|---|---|
| Meaning | Double vision from one eye alone | Double vision due to both eyes not aligning |
| Cover test | Persists when the normal eye is covered and disappears when the affected eye is covered | Disappears when either eye is covered |
| Main problem | Optical problem within the affected eye | Ocular misalignment |
| Common causes | Refractive error/astigmatism, dry eye or irregular cornea, cataract, lens subluxation, macular disease | Strabismus, CN III/IV/VI palsy, myasthenia gravis, thyroid eye disease, orbital disease, brainstem lesions |
| Pinhole test | Often improves if due to refractive/media cause | Does not correct with pinhole |
| Clinical importance | Usually ophthalmic and less often neurologic | May indicate a neurologic, neuromuscular, orbital, or extraocular-muscle disorder |
Quick rule:
- Diplopia gone by covering either eye = binocular diplopia.
- Diplopia persists in one eye despite covering the other = monocular diplopia.
New binocular diplopia with ptosis, unequal pupils, severe headache, weakness, or other neurologic symptoms needs urgent assessment. EyeWiki's diagnostic approach gives the same cover-test distinction.
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