Pupils in Organophosphate Poisoning
In organophosphate poisoning, pupils are characteristically constricted (miosis) as part of the cholinergic toxidrome. 1 This is a key diagnostic feature that helps differentiate organophosphate poisoning from other toxidromes.
Pathophysiology of Pupillary Changes
Organophosphate compounds inhibit acetylcholinesterase, leading to:
- Accumulation of acetylcholine at synapses 2
- Overstimulation of muscarinic and nicotinic receptors
- Miosis (pupillary constriction) due to stimulation of the muscarinic receptors in the iris sphincter muscle
Clinical Presentation
Pupillary constriction is part of the broader "SLUDGE" syndrome seen in organophosphate poisoning, which includes:
- Salivation
- Lacrimation
- Urination
- Defecation
- GI upset
- Emesis
Additional clinical features include:
- Bronchorrhea and bronchospasm
- Bradycardia (or sometimes tachycardia)
- Miosis (constricted pupils)
- Altered mental status
- Seizures
- Confusion
- Muscle fasciculations
- Weakness and paralysis 1
Diagnostic Considerations
- Pupillary constriction is one of the most frequent signs of organophosphate poisoning 2
- The diagnosis is primarily clinical, based on history of exposure and characteristic findings
- Laboratory confirmation through measurement of cholinesterase activity in blood should be performed but treatment should not be delayed while awaiting results 1
- Reduction of cholinesterase activity to below 50% of normal indicates significant poisoning 1
Special Considerations
- In infants and young children, presentation may differ with drowsiness being more prominent than muscle twitching 1
- Pupillary findings should be interpreted in context with other symptoms and exposure history
- Respiratory failure is the most common cause of mortality (74.4% of cases) and requires careful monitoring 2
Management Implications
The presence of miosis (constricted pupils) is an important clinical sign that:
- Confirms cholinergic toxicity
- Helps guide appropriate treatment with atropine and pralidoxime
- Can be used to monitor response to treatment (pupils should dilate with adequate atropinization)
Treatment Response
- Atropine administration should continue until reversal of cholinergic symptoms, including pupillary dilation
- Initial atropine dosage is 1-2 mg IV for adults (doubled every 5 minutes until reversal of symptoms)
- For children, the initial dose is 0.02 mg/kg 1
- Adequate atropinization is indicated by pupillary dilation, among other signs
Pitfalls to Avoid
- Do not rely solely on pupillary findings for diagnosis or treatment decisions
- Remember that some patients may have mixed exposures that can complicate the clinical picture
- Avoid medications that can worsen organophosphate poisoning, including succinylcholine, mivacurium, morphine, theophylline, aminophylline, reserpine, and phenothiazine-type tranquilizers 1
- Ensure proper decontamination to prevent secondary exposure to healthcare providers
In summary, miosis (constricted pupils) is a hallmark finding in organophosphate poisoning and an important clinical sign for diagnosis, treatment guidance, and monitoring of treatment response.