Prazosin in Scorpion Sting Management
Prazosin is highly effective for managing severe scorpion envenomation, particularly from Indian red scorpion (Mesobuthus tamulus) stings, and should be used as the primary treatment for cardiovascular manifestations including hypertension, pulmonary edema, and autonomic storm. However, current American guidelines do not mention prazosin as standard therapy, likely reflecting geographic differences in scorpion species and envenomation patterns 1.
Evidence Quality and Geographic Context
The evidence for prazosin comes primarily from Indian studies addressing Mesobuthus tamulus envenomation, which causes severe cardiovascular toxicity through massive catecholamine release 2, 3, 4. North American scorpion stings (primarily Centruroides species) typically cause different clinical manifestations—predominantly neurotoxic rather than cardiovascular effects—which explains why prazosin is not mentioned in American Heart Association guidelines 1.
When Prazosin Should Be Used
Indications for Prazosin Therapy
- Severe hypertension following scorpion sting, particularly when accompanied by signs of autonomic storm 2, 3
- Acute pulmonary edema developing after scorpion envenomation 2, 5
- Cardiovascular manifestations including impaired left ventricular function and myocardial failure 4, 5
- Prevention of pulmonary edema in severe cases when initiated early 2
Dosing Protocol
A standardized three-hourly dosing protocol is superior to six-hourly dosing for preventing pulmonary edema 2. The specific regimen includes:
- Adults: 0.5 mg orally every 3 hours during the acute phase 2, 5
- Children: 0.125 mg orally every 3 hours 5
- Severe cases with established pulmonary edema: Additional doses beyond the scheduled regimen may be required 2
Mechanism and Clinical Effects
Prazosin works as a post-synaptic alpha-1 receptor blocker that antagonizes the massive catecholamine surge caused by scorpion venom 4, 5. Clinical improvement typically occurs within 1 hour, with maximal benefit at 3 hours, manifested by resolution of pulmonary edema, normalization of heart rate, and disappearance of cardiac murmurs 5.
Comparative Effectiveness
Prazosin is significantly more effective than scorpion antivenom for managing cardiovascular complications 3. In a direct comparison:
- Prazosin group: 7.5% developed pulmonary edema, 0% mortality, mean recovery time 1.25 days 3
- Antivenom group: 80% developed pulmonary edema, 16% mortality, mean recovery time 2.26 days 3
This dramatic difference occurs because antivenom neutralizes circulating venom but does not reverse the autonomic storm already triggered at receptor sites, whereas prazosin directly blocks the pathophysiologic effects 3, 4.
Critical Pitfalls to Avoid
Do Not Combine with Calcium Channel Blockers
Adding nifedipine to prazosin increases the risk of pulmonary edema and should be avoided 6. In one study, 8 of 19 patients (42%) who received nifedipine plus prazosin developed acute pulmonary edema despite blood pressure control, whereas patients receiving prazosin alone did not develop this complication 6. Calcium channel blockers appear to contribute to cardiopulmonary instability and increase myocardial oxygen consumption in this setting 6.
Timing Is Critical
The three-hourly dosing protocol prevents pulmonary edema development, whereas six-hourly dosing allows breakthrough complications 2. Patients managed with six-hourly prazosin developed pulmonary edema in 4-8 hours despite blood pressure control, requiring rescue doses 2.
Application to Different Clinical Contexts
For Indian Red Scorpion (Mesobuthus tamulus) Stings
Prazosin should be first-line therapy for any severe envenomation with cardiovascular manifestations 2, 3, 4. The standardized three-hourly protocol should be initiated immediately upon presentation with severe symptoms 2.
For North American Scorpion Stings
Standard supportive care remains appropriate, with antivenom (Anascorp) for severe Centruroides envenomation 1. Prazosin is not mentioned in current American guidelines because the cardiovascular toxicity pattern differs significantly from Indian species 1. However, if severe hypertension and pulmonary edema develop (which is rare with North American species), prazosin could be considered based on the pathophysiologic rationale 4, 5.
Monitoring and Recovery
Clinical improvement manifests as resolution of vasoconstriction (warming of cool extremities), typically within 11.5 hours with the three-hourly protocol versus 18 hours with conventional management 2. ECG changes including Q-waves, T-wave abnormalities, and left anterior hemiblock may be present and typically resolve with treatment 5.