What is the effect of heroin on pupils?

Effect of Heroin on Pupils

Heroin causes miosis (pupillary constriction or "pinpoint pupils") through activation of the pupillary sphincter muscle, which is one of the most sensitive and reliable objective indicators of opioid use. 1

Mechanism of Action

Heroin, like other opioids, produces its miotic effect through:

• Activation of μ-opioid receptors in the central nervous system 1
• Direct stimulation of the pupillary sphincter muscle 2
• Parasympathetic dominance over sympathetic influence on pupillary control 3

The pupillary constriction occurs because:

• Heroin acts as a pro-drug allowing rapid and complete central nervous system absorption 4
• Opioids inhibit the release of noradrenaline at synaptic terminals, affecting autonomic control of pupil size 1
• The effect is dose-dependent, with higher doses producing more pronounced miosis 5, 6

Clinical Characteristics of Heroin-Induced Miosis

• Pupil diameter typically decreases to 2-3 mm during opioid intoxication 3
• The pupillary light reflex remains present but is reduced in amplitude 3
• The velocity of both constriction and redilation to light stimulus is decreased 6
• Miosis is most easily detected under moderately dim lighting conditions (best observed at 4-16 foot-lamberts) 5
• Peak miosis typically occurs approximately 90 minutes after opioid administration 5

Diagnostic Significance

Miotic pupils are a key component of the heroin overdose syndrome, which consists of:

1. Abnormal mental status
2. Substantially decreased respiration
3. Miotic pupils

This triad has a sensitivity of 92% and specificity of 76% for diagnosing heroin overdose 4.

Important Clinical Considerations

• Even during significant opioid-induced respiratory depression with hypoxia and hypercarbia, the pupillary light reflex remains quantifiable, making it a valuable neurological assessment tool 3
• In severe overdose with significant hypoxia, sympathetic activation may partially counteract the miotic effect, but pupils typically remain between 2-3 mm 3
• Pupil size is affected by lighting conditions - increasing by approximately 0.35 mm when one eye is closed versus both eyes open 5
• Pupil diameter decreases approximately 1.0 mm with each log unit increase in lighting intensity 5

Clinical Implications

• Miosis is one of the most sensitive and frequently assessed objective indices of opioid effects 5
• The presence of miosis, along with respiratory depression and altered mental status, should prompt consideration of naloxone administration in suspected opioid overdose 4
• Monitoring pupillary response can help assess the effectiveness of naloxone treatment in opioid overdose 1, 4
• The pupillary examination remains useful for neurological assessment even during significant opioid toxicity 3

Understanding the effect of heroin on pupils is critical for recognizing opioid overdose, which is a common cause of preventable deaths in urban and suburban settings 4.