Effect of Cocaine on Pupil Size
Cocaine causes pupillary dilation (mydriasis) through its sympathomimetic effects, blocking the reuptake of norepinephrine at sympathetic nerve terminals in the iris dilator muscle. This pharmacological action makes cocaine a useful diagnostic agent for Horner syndrome while also explaining one of its characteristic clinical manifestations in intoxication.
Mechanism of Action
Cocaine affects pupil size through several mechanisms:
- Sympathomimetic activity: Cocaine blocks the reuptake of norepinephrine and dopamine at presynaptic adrenergic terminals, causing accumulation of catecholamines at the postsynaptic receptor 1
- Iris dilator muscle stimulation: The increased norepinephrine concentration activates alpha1 receptors in the iris dilator muscle, leading to pupil dilation 2
- Dose-dependent response: The sympathomimetic effects of cocaine, including pupillary dilation, occur in a dose-dependent fashion 1
Clinical Applications
Diagnostic Use in Horner Syndrome
- Topical cocaine is traditionally used to confirm the diagnosis of Horner syndrome (ocular sympathetic denervation) 2
- In normal eyes, cocaine causes pupil dilation by blocking norepinephrine reuptake
- In Horner syndrome, the affected pupil fails to dilate as much as the unaffected pupil due to the absence of functioning sympathetic nerve terminals 2
- Newer alternatives like apraclonidine may be replacing cocaine for this diagnostic purpose due to cocaine's controlled substance status and relatively weak dilating effect 2
Clinical Manifestations in Cocaine Intoxication
Pupillary dilation is one of the characteristic signs of cocaine intoxication, occurring alongside other sympathomimetic effects:
- Tachycardia and hypertension
- Hyperthermia
- Diaphoresis
- Agitation 3
Age-Related Considerations
The pupillary effects of cocaine are influenced by age:
- The mydriatic effect of topical cocaine significantly increases with age 4
- This age-related augmentation likely results from increased corneal permeability in older individuals 4
- Clinicians should consider this age-dependent variation when interpreting pupillary responses to cocaine
Clinical Complications
The mydriatic effect of cocaine can precipitate serious ocular complications:
- Acute angle-closure glaucoma: Cocaine-induced mydriasis can trigger acute angle-closure glaucoma in predisposed individuals with shallow anterior chambers 5
- This complication, while rare, can lead to sudden painful vision loss and requires immediate treatment 5
Chronic Cocaine Use Effects
Long-term cocaine use may alter pupillary responses:
- Chronic cocaine administration can attenuate alpha2-adrenoceptor-mediated mydriasis 6
- This suggests that chronic cocaine exposure may lead to adaptations in the adrenergic system controlling pupil size
Clinical Pitfalls and Considerations
- Diagnostic interpretation: When using cocaine for Horner syndrome diagnosis, weak or equivocal results may occur due to its indirect mechanism of action 2
- Medical emergencies: Recognize that mydriasis in cocaine users may be part of a constellation of symptoms indicating serious intoxication requiring immediate intervention 3
- Ocular risk assessment: Consider screening for narrow angles in patients receiving intranasal cocaine for medical procedures, especially in middle-aged or older adults 5
In summary, cocaine reliably causes pupillary dilation through its sympathomimetic effects, making it both a useful diagnostic tool and a characteristic sign of intoxication that clinicians should recognize.