Benztropine Overdose: Ocular Side Effects
Primary Ocular Manifestations
Benztropine overdose causes mydriasis (dilated pupils), blurred vision, and cycloplegia (paralysis of accommodation) as direct anticholinergic effects on the eye, with potential progression to acute angle-closure glaucoma in susceptible individuals. 1
The FDA drug label explicitly lists the following ocular manifestations in benztropine overdose 1:
- Mydriasis (dilated pupils) - occurs due to paralysis of the pupillary sphincter muscle
- Blurred vision - results from both mydriasis and cycloplegia
- Cycloplegia - paralysis of the ciliary muscle preventing accommodation
- Photophobia - secondary to the dilated pupils and inability to constrict in bright light
Risk of Acute Angle-Closure Glaucoma
Patients with narrow anterior chamber angles are at significant risk for developing acute angle-closure glaucoma during benztropine overdose, which constitutes an ophthalmologic emergency. 2
The anticholinergic effects of benztropine can precipitate acute angle-closure crisis through 2:
- Mid-dilated pupil position causing maximum iris-lens contact
- Blockage of aqueous humor outflow through the trabecular meshwork
- Rapid elevation of intraocular pressure (IOP)
- Potential for permanent vision loss if untreated
Clinical presentation of acute angle-closure includes pain, nausea, redness, decreased vision, corneal edema, and very high IOP 2. This represents a medical emergency requiring immediate ophthalmologic intervention, as 18% of eyes may become blind following an acute angle-closure event, with 50% of blindness attributable to glaucoma 2.
Duration and Severity of Ocular Effects
Anticholinergic ocular toxicity from benztropine overdose can persist for prolonged periods, with documented cases showing symptoms lasting up to 9 days. 3
Serial serum concentrations demonstrate that prolonged intoxication may result from intermittent absorption rather than slow clearance, with fluctuating drug levels maintaining anticholinergic effects 3. One case report documented delirium and anticholinergic manifestations persisting for 5 days, necessitating prolonged hospitalization 4.
Management of Ocular Complications
The FDA recommends using a local miotic agent to reverse mydriasis and cycloplegia, along with darkening the room for photophobia management. 1
Specific treatment measures include 1:
- Physostigmine salicylate (1-2 mg SC or IV) to reverse anticholinergic intoxication systemically, with repeat dosing after 2 hours if needed
- Local miotic drops to constrict the pupil and restore accommodation
- Room darkening to reduce photophobia and light sensitivity
- Immediate ophthalmology consultation if acute angle-closure glaucoma is suspected
High-Risk Populations
Patients with pre-existing narrow anterior chamber angles, hyperopia, or anatomically small eyes face substantially elevated risk for vision-threatening complications. 2
Risk factors requiring heightened vigilance include 2:
- Hyperopic refractive error and short axial length eyes
- Older phakic patients with narrower anterior chamber angles
- Small corneal diameter (indicative of smaller eye)
- Family history of acute angle-closure
- Asian ethnicity (higher prevalence of narrow angles)
Critical Clinical Pitfalls
The most dangerous pitfall is failing to recognize acute angle-closure glaucoma in an overdose patient, as the anticholinergic symptoms may mask or be attributed to the systemic toxicity rather than a separate ophthalmologic emergency. 2
Additional considerations 1, 5:
- Anticholinergic agents like benztropine are frequently associated with angle-closure glaucoma in predisposed eyes 5
- The mid-dilated pupil position is particularly dangerous, creating maximum iris-lens contact 2
- Symptoms of nausea and confusion from systemic toxicity may obscure recognition of concurrent angle-closure symptoms 1
- Visual complaints should never be dismissed as merely "expected" anticholinergic effects without ruling out elevated IOP 2