Paralytic shellfish poisoning (PSP) is the most common and most severe form of shellfish poisoning, caused by saxitoxins — potent neurotoxins that block voltage-gated sodium channels, preventing sodium ion influx and disrupting nerve conduction. [1-3] The worldwide mortality rate averages approximately 6%, higher in developing countries and in children, with death primarily from respiratory failure. [3] Toxins are heat-stable and cannot be destroyed by cooking, freezing, or any standard food preparation. [1]
1. History
- Key question: "What shellfish did you eat, when, and how much?" — clams and mussels are the most common vectors [1]
- Onset: Symptoms typically appear 30–60 minutes after ingestion (range 30 min to 3 h); rapid onset correlates with severity [1][3]
- Symptom progression: Perioral paresthesias → facial/extremity numbness → generalized weakness → ataxia → dysphagia/dysarthria → flaccid paralysis → respiratory failure [3-4]
- Associated symptoms: Nausea, vomiting, diarrhea, headache, hypersalivation, diaphoresis [3]
- Important negatives: Patients remain conscious and alert throughout the poisoning, even during paralysis — altered mental status suggests an alternative or additional diagnosis [3]
- Ask about: geographic location, recent red tide/algal bloom advisories, others who ate the same meal (cluster cases), source of shellfish (market vs. self-harvested) [5-6]
2. Alarm Features
- Dysphagia and dysarthria — may be stronger indicators of true PSP than paresthesia alone [7]
- Rapidly progressive weakness or paralysis (especially within the first 2–4 hours)
- Respiratory distress — dyspnea, accessory muscle use, declining SpO₂
- Ataxia, inability to stand or walk
- In most fatalities, death occurs within 12 hours of ingestion [3]
- Children are at particularly high risk for fatal outcomes [1]
3. Medications
- No antidote exists for saxitoxin poisoning [3][8]
- Antibodies to saxitoxin have been developed in animal models but are not available for human use [3]
- Avoid medications that may further depress respiratory drive (opioids, benzodiazepines, sedatives) unless intubated
- Activated charcoal may be considered if presentation is within 1 hour of ingestion and airway is protected, though evidence is limited
- Antiemetics (ondansetron) for symptomatic GI relief
- Toxins are not destroyed by gastric acid [1]
4. Diet
- Acute: NPO if any concern for aspiration (dysphagia is common in moderate-severe cases)
- Prevention: Avoid shellfish harvested during or shortly after harmful algal blooms ("red tides" or "brown tides") [1]
- Cooking, freezing, smoking, pickling, or canning does not eliminate saxitoxins [1]
- Bivalve mollusks (clams, mussels, oysters, scallops) are the highest-risk vectors; non-traditional vectors include puffer fish, lobster, and xanthid crabs [2-3]
- Advise patients in endemic coastal areas to heed local shellfish harvesting bans
5. Review of Systems
- Neurologic: Paresthesias (perioral, facial, extremity), weakness, ataxia, dizziness, "floating" sensation, dysphagia, dysarthria [3]
- Respiratory: Dyspnea, difficulty breathing, respiratory failure
- GI: Nausea, vomiting, diarrhea, abdominal pain
- Cardiovascular: Hypotension (rare); AV node conduction suppression has been described [4]
- Autonomic: Hypersalivation, diaphoresis [3]
- Mental status: Should be preserved — confusion or memory loss suggests amnesic shellfish poisoning (domoic acid) or alternative diagnosis [1][3]
6. Collateral History and Family History
- Critical: Identify co-ingestors — PSP often presents in clusters; others who shared the meal may be symptomatic or at risk [5]
- Source of shellfish: commercially purchased vs. recreationally harvested (recreational harvest carries higher risk, especially during red tide seasons) [6]
- Geographic and seasonal context: temperate waters, Atlantic/Pacific coasts of North America (especially Alaska), Chile, Philippines, Europe [1]
- Family history is not relevant to PSP (non-hereditary, purely toxin-mediated)
7. Risk Factors
- Consumption of bivalve mollusks (clams, mussels, oysters, scallops) from temperate coastal waters [1][3]
- Harvesting during or after harmful algal blooms (red tides) [1][6]
- Recreational/subsistence harvesting in unmonitored areas [6-7]
- Larger shellfish specimens may contain higher toxin concentrations [6]
- Travel to endemic regions (Alaska, Pacific Northwest, Chile, Philippines, parts of Europe) [1]
- Children — higher case-fatality ratio [1]
- Ingestion of large quantities of shellfish
8. Differential Diagnosis
- Tetrodotoxin poisoning (puffer fish) — clinically nearly indistinguishable from PSP; both block sodium channels; history of puffer fish ingestion is key [3]
- Neurotoxic shellfish poisoning (brevetoxins) — milder neurologic symptoms, more GI-predominant, associated with Gulf Coast red tides [1]
- Amnesic shellfish poisoning (domoic acid) — prominent memory loss, confusion, seizures; distinguishing feature is cognitive impairment [1]
- Ciguatera fish poisoning — pathognomonic hot-cold temperature reversal; associated with reef fish, not shellfish [8]
- Botulism — descending paralysis, but slower onset (12–36 h), cranial nerve involvement, pupillary dilation
- Guillain-Barré syndrome — ascending paralysis over days, not minutes to hours
- Stroke/brainstem event — asymmetric findings, focal deficits
- Anaphylaxis to shellfish — urticaria, angioedema, bronchospasm, hypotension
- Organophosphate poisoning — SLUDGE symptoms, miosis
- Myasthenia gravis crisis — fatigable weakness, known history
9. Past Medical History
- Asthma/COPD — increased risk of respiratory compromise
- Prior episodes of shellfish poisoning (does not confer immunity)
- Neuromuscular disorders (e.g., myasthenia gravis) — may worsen with sodium channel blockade
- Cardiac conduction disease — saxitoxin can suppress AV node conduction [4]
- Renal impairment — may affect toxin clearance
10. Physical Exam
- Vital signs: Monitor respiratory rate, SpO₂, and blood pressure closely; tachypnea or declining SpO₂ is ominous
- Neurologic:
- Perioral and facial numbness/tingling (earliest finding)
- Diminished sensation in extremities
- Decreased deep tendon reflexes
- Flaccid paralysis in severe cases
- Ataxia, dysmetria
- Dysarthria, dysphagia
- Preserved consciousness and pupillary responses (distinguishes from botulism) [3]
- Respiratory: Assess tidal volume, accessory muscle use, ability to count to 20 in one breath, negative inspiratory force
- GI: May have mild abdominal tenderness; usually non-specific
- Autonomic: Hypersalivation, diaphoresis [3]
11. Lab Studies
- No routine clinical lab test confirms PSP — diagnosis is clinical [1][8]
- Urine saxitoxin levels (HPLC-MS/MS) can confirm diagnosis retrospectively but are not widely available or rapid [7]
- Obtain to rule out mimics:
- CBC, BMP (electrolytes, glucose, renal function)
- Calcium, magnesium, phosphorus (neuromuscular causes)
- ABG/VBG — monitor for respiratory acidosis/CO₂ retention
- Lactate if hemodynamically unstable
- Save leftover shellfish for toxin testing (mouse bioassay or HPLC-MS/MS) — critical for public health confirmation [7]
- Notify poison control and public health authorities
12. Imaging
- Imaging is generally unnecessary if the clinical picture is consistent with PSP and history is clear
- CT head/CTA if stroke or brainstem pathology cannot be excluded (asymmetric findings, altered consciousness)
- Chest X-ray if intubated or concern for aspiration pneumonia
13. Special Tests
- Bedside pulmonary function: Negative inspiratory force (NIF) and forced vital capacity (FVC) — serial measurements to monitor for impending respiratory failure (similar to monitoring in GBS/myasthenia)
- Mouse bioassay on leftover food — the traditional reference method for PSP toxin detection [7][9]
- HPLC-MS/MS on urine or food samples — more specific, increasingly used [7]
- Poison Control Center consultation — recommended for all suspected cases [10]
14. ECG
- Obtain ECG in all patients — saxitoxin can suppress AV node conduction and inhibit the respiratory center [4]
- Monitor for:
- Prolonged PR interval / AV block
- Bradycardia
- QT prolongation (less well-characterized)
- Continuous cardiac monitoring recommended for moderate-severe cases
15. Assessment
Severity stratification (based on rate of progression and clinical features): [3]
- Mild: Perioral/facial paresthesias, mild GI symptoms; onset >1 hour post-ingestion; symptoms resolve in hours
- Moderate: Generalized paresthesias, extremity weakness, ataxia, dysarthria; onset 30–60 minutes
- Severe: Flaccid paralysis, dysphagia, respiratory failure; rapid onset (<30 minutes); most fatalities occur within 12 hours [3]
The rate of symptom progression in the first few hours is the best predictor of severity. Patients who survive the first 12–24 hours generally recover fully, with mild-moderate cases resolving in 2–3 days and severe weakness potentially persisting up to 1 week. [3]
16. Treatment Plan
- No antidote — treatment is entirely supportive [1][3][8]
- Airway management is the priority:
- Early intubation and mechanical ventilation for respiratory failure, declining NIF/FVC, or inability to protect airway [2]
- Have RSI equipment at bedside for any moderate-severe case
- GI decontamination: Consider activated charcoal (1 g/kg, max 50 g) if within 1 hour of ingestion and airway is protected
- IV fluids for hydration, especially if significant vomiting/diarrhea
- Antiemetics (ondansetron 4 mg IV) for nausea/vomiting
- Continuous monitoring: Cardiac telemetry, pulse oximetry, serial respiratory assessments
- Do not administer neuromuscular blocking agents unnecessarily — patient is already at risk for paralysis
- Notify public health authorities immediately to prevent additional cases [8]
17. Disposition
- Admit to ICU: Any patient with respiratory compromise, progressive weakness, dysphagia, dysarthria, or rapid symptom progression [3][8]
- Admit for observation (≥24 hours): All patients with neurologic symptoms beyond mild perioral paresthesias; the critical window for respiratory failure is the first 12 hours [3]
- Discharge considerations: Patients with isolated mild perioral paresthesias and GI symptoms who remain stable after 12 hours of observation with no progression
- Consult: Toxicology/Poison Control for all cases; critical care if any respiratory concern; public health for epidemiologic investigation
18. Follow Up / Return Precautions
- Return immediately for: any difficulty breathing, swallowing, or speaking; worsening numbness or weakness; lightheadedness or fainting
- Mild-moderate cases typically resolve fully within 2–3 days; severe weakness may persist up to 1 week [3]
- No known long-term sequelae in survivors (unlike amnesic shellfish poisoning, which can cause permanent memory deficits) [1]
- Patient counseling:
- Avoid shellfish from the same source
- Heed all local shellfish harvesting advisories and red tide warnings
- Cooking does not make contaminated shellfish safe [1]
- Report illness to local health department
- Follow-up with PCP within 1 week if discharged; sooner if any residual neurologic symptoms
References
1. Food Poisoning from Marine Toxins. — Vernon E. Ansdell CDC Yellow Book. 2025.
2. Paralytic Shellfish Poisoning: Seafood Safety and Human Health Perspectives. — Etheridge SM. Toxicon : Official Journal of the International Society on Toxinology. 2010.
3. Neurotoxic Marine Poisoning. — Isbister GK, Kiernan MC. The Lancet. Neurology. 2005.
4. Paralytic Shellfish Poisoning. — Acres J, Gray J. Canadian Medical Association Journal. 1978.
5. Case Report: Paralytic Shellfish Poisoning in Sabah, Malaysia. — Suleiman M, Jelip J, Rundi C, Chua TH. The American Journal of Tropical Medicine and Hygiene. 2017.
6. Shellfish Contamination With Marine Biotoxins in Portugal and Spring Tides: A Dangerous Health Coincidence. — Vale P. Environmental Science and Pollution Research International. 2020.
7. Case Diagnosis and Characterization of Suspected Paralytic Shellfish Poisoning in Alaska. — Knaack JS, Porter KA, Jacob JT, et al. Harmful Algae. 2016.
8. Illnesses Caused by Marine Toxins. — Sobel J, Painter J. Clinical Infectious Diseases : An Official Publication of the Infectious Diseases Society of America. 2005.
9. Marine Biotoxins: Occurrence, Toxicity, Regulatory Limits and Reference Methods. — Visciano P, Schirone M, Berti M, et al. Frontiers in Microbiology. 2016.
10. Human Poisonings by Neurotoxic Phycotoxins Related to the Consumption of Shellfish: Study of Cases Registered by the French Poison Control Centres From 2012 to 2019. — Sinno-Tellier S, Abadie E, de Haro L, et al. Clinical Toxicology. 2022.