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Introduction: Serotonin syndrome (toxicity) describes adverse drug effects from toxic amounts of intra-synaptic central nervous system serotonin. A wide range of drugs have been implicated to cause serotonin toxicity, not all justifiably. The plausible agents all have a final common pathway resulting in a substantial increase in central nervous system serotonergic neurotransmission. Serotonin toxicity is characterized by neuromuscular excitation, mental status changes, and autonomic dysregulation. Signs and symptoms represent a spectrum of toxicity (mild to life-threatening) related to increasing serotonin concentrations. As there is no consensus on the threshold for "toxicity" or diagnostic criteria, the true incidence of serotonin toxicity is unknown. The incidence in overdose is easier to quantify and is reasonably common in serotonergic antidepressant overdoses. In a large case series of overdoses, moderate serotonin toxicity occurred in 14% of poisonings with a selective serotonin reuptake inhibitor. While half those ingesting a monoamine oxidase inhibitor in combination with a serotonergic agent in overdose exhibit at least moderately severe serotonin toxicity. In contrast, the incidence of serotonin toxicity in those on therapeutic serotonergic agents appears to be very low., Objectives: To provide a narrative review of the current diagnostic criteria, utilizing case reports of fatalities to evaluate how many meet the various diagnostic criteria and propose practical solutions to resolve controversies in diagnosis., Methods: A review of serotonin toxicity diagnostic criteria in the English literature was completed by searching Embase and PubMed from January 1990 to July 2021 for the keywords "serotonin syndrome/toxicity" paired with "diagnostic criteria" or "diagnosis." Also, fatal cases of serotonin toxicity identified from a recent systematic review were independently examined to determine what diagnostic criteria were met and whether serotonin toxicity or another cause was most likely., Review of Diagnosis Criteria: Serotonin toxicity is a clinical diagnosis, four diagnostic criteria (Sternbach, Serotonin Syndrome Scale, Radomski, and Hunter) have been proposed. However, the Serotonin Syndrome Scale has not been validated in patients with serotonin toxicity and only utilized in those on a serotonergic agent. The remaining three criteria are utilized more widely but have undergone little refinement or validation., Review of Fatal Cases: Shortfalls with diagnostic criteria can be illustrated by examining case fatalities. Of 55 fatal cases reviewed, 12 (22%) were unlikely to be serotonin toxicity. Sternbach and Radomski criteria were met by 25 (45%), 20 (36%) had insufficient data reported and 10 (18%) met an exclusion criterion. Few had sufficient information reported to determine whether Hunter Criteria were met, with only 13 (24%) documented as meeting the criteria, the remaining 42 (76%) had insufficient data., Resolving Shortfalls in Current Diagnostic Criteria: As serotonin toxicity is a clinical diagnosis, issues arise when basing the diagnosis on symptom criteria alone, without considering whether the drug/s ingested increase central nervous system serotonin or whether there is an alternative diagnosis. This has resulted in case reports and government warnings for drugs that cannot plausibly cause significant serotonin toxicity (e.g., ondansetron and antipsychotics). We propose when assessing for a serotonin toxidrome, both the causative agent(s) and clinical scenario is considered to determine the likelihood of serotonin toxicity. Then the clinical features assessed, those with a moderate to high prior probability (e.g., serotonergic drug-drug interaction, overdose, recent initiation or increase in dose of serotonergic agent/s) could be diagnosed based on the Hunter criteria. However, those with a low probability (e.g., stable therapeutic doses of a serotonergic agent) require more specific and stringent criteria. Finally, we propose a minimum dataset for case reports/series of serotonin toxicity., Conclusions: More complete and accurate reporting of serotonin toxicity cases is required in the future, to avoid further misleading associations that are physiologically implausible.

Background: Vilazodone is an antidepressant with selective serotonin reuptake inhibition and partial 5HT1A agonism. Serotonin syndrome is believed to be due to excessive stimulation of 5-HT2A and 5-HT1A receptors, resulting in the clinical triad of altered mentation, autonomic instability and neuromuscular abnormalities. The goal of this study is to define serotonergic effects after vilazodone exposure., Methods: A retrospective review of two databases: the American Association of Poison Controls Centers' National Poison Data System (NPDS) and the American College of Medical Toxicology's Toxicology Investigators Consortium (ToxIC Registry). A case series of four patients from one medical toxicology service is also presented., Results: During the 52-month study period, a total of 3192 vilazodone human exposures were reported to NPDS. Of these, 1734 (54%) were isolated vilazodone cases. The clinical effects of vilazodone toxicity included drowsiness (20%), vomiting (14%), tachycardia (11%) and agitation (10%). Most patients (71%) had symptoms for between 2 and 24 h, though some (14%) remained symptomatic for more than 24 h. The most common treatment was intravenous fluids (15%) and the most serious intubation (2%). From the ToxIC Registry, a total of 23 cases of vilazodone exposures were identified. Of these, 17 (74%) had vilazodone listed as the first (primary) agent and 10 (43%) involved vilazodone-only ingestions. Nine (39%) cases documented serotonin syndrome; and most (8/9; 89%) listed vilazodone as the primary agent. All (n = 4) subjects in the case series with acute vilazodone toxicity had serotonin syndrome., Conclusions: Vilazodone overdose, including vilazodone-only ingestions, are associated with serotonin syndrome. Serotonergic toxicity and appropriate treatments should be considered when caring for patients with vilazodone ingestions.

Context: Tramadol is a commonly used centrally acting analgesic associated with seizures and suspected to cause serotonin toxicity in overdose., Objective: This study sought to investigate the effects of tramadol overdose, and included evaluation for serotonin toxicity based on the Hunter Serotonin Toxicity Criteria where the seven clinical features of spontaneous clonus, inducible clonus, ocular clonus, agitation, diaphoresis, tremor and hyperreflexia are examined for in all patients taking serotonergic medications; seizures and central nervous system depression., Materials and Methods: This was an observational cases series based on a retrospective review of tramadol overdoses (> 400 mg) admitted to a tertiary toxicology unit from November 2000 to June 2013. Demographic details, information on ingestion (dose and co-ingestants), clinical effects, complications (seizures, serotonin toxicity and cardiovascular effects) and intensive care unit (ICU) admission were extracted from a clinical database., Results: There were 71 cases of tramadol overdose (median age: 41 years, range: 17-69 years; and median ingested dose: 1000 mg, interquartile range [IQR]: 800-2000 mg). Seizures were dose related and occurred in 8 patients, one of them co-ingested a benzodiazepine compared with 16 patients without seizures. There were no cases of serotonin toxicity meeting the Hunter Serotonin Toxicity Criteria. Tachycardia occurred in 27 and mild hypertension occurred in 32. The Glasgow Coma Score was < 15 in 29 and < 9 in 5 patients; three co-ingested tricyclic antidepressants and two tramadol alone (3000 mg and 900 mg). Respiratory depression occurred in 13, median dose: 2500 (IQR: 1600-3000) mg which was significantly different (p = 0.003) to patients without respiratory depression, median dose: 1000 (IQR: 750-1475) mg. Eight patients were admitted to ICU, five due to co-ingestant toxicity and three for respiratory depression., Discussion: Tramadol overdose was associated with a significant risk of seizures and respiratory depression in more severe cases, both which appear to be related to the ingested dose. There were no cases of serotonin toxicity, while opioid-like effects and adrenergic effects were prominent., Conclusion: Tramadol overdose is associated with seizures and respiratory depression, but is unlikely to cause serotonin toxicity.

Unlabelled: Metaxalone has only recently been associated with serotonin syndrome. The mechanism of action of this centrally acting muscle relaxant is unknown; however, the observation of serotonin syndrome in patients with metaxalone overdose suggests a role in the serotonergic pathway., Case Report: (Case 1) A 29-year-old woman with overdose of metaxalone presented to the emergency department with altered mental status, seizure-like activity, hyperthermia, rigidity in the lower extremities, myoclonus, and hyperreflexia. Vital signs on arrival include blood pressure of 168/80 mmHg, heart rate of 208 beats per minute (bpm), respirations of 20/min, a temperature of 41.6° C rectally, and room air oxygen saturation of 97%. She was intubated and sedated with benzodiazepines, and actively cooled. Serum paroxetine concentration was 23 (therapeutic range: 20-200) ng/mL, and serum metaxalone concentration was 31 mcg/mL (peak plasma concentrations average 0.9 mcg/mL at 3.3 h following a single oral dose of 400 mg). (Case 2) A 27-year-old man presented to the emergency department with altered mental status, rigidity in his lower extremities, myoclonus, and hyperreflexia. Vital signs on arrival include blood pressure of 158/131 mmHg, heart rate of 126 bpm, respiratory rate of 20 breaths per minute, and temperature of 37.2°C, with oxygen saturation of 98% on room air. His medication list included metaxalone and escitalopram. He was managed aggressively with IV boluses of diazepam, in total 80 mg, in the emergency department. Serum escitalopram concentration was 24 ng/mL with a therapeutic range of 21-64 ng/mL, and serum metaxalone concentration was 58 mcg/mL., Conclusion: These two cases suggest that at supratherapeutic concentrations metaxalone has serotonergic effects. Severe serotonin toxicity may result from metaxalone abuse in individuals using a selective serotonin reuptake inhibitor therapeutically.

We report the case of a 39-year-old woman who presented with serotonin syndrome and hypoglycaemia likely due to intoxication with a very high dose of venlafaxine. This case of venlafaxine-associated hypoglycaemia was treated first by glucose perfusion, but despite large doses, hypoglycaemia recurred. Blood glucose normalized after injection of octreotide, eliminating the need for hypertonic glucose. Octreotide has been shown to decrease glucose requirements and the number of hypoglycaemic episodes in patients with sulfonylurea-induced hypoglycaemia but, to our knowledge, its ability to resolve hypoglycaemic episodes due to massive venlafaxine overdose has not yet been described.

The symptoms and signs of carisoprodol intoxications do not resemble those caused by its metabolite meprobamate. Meprobamate most probably produces its effects through the GABAergic neurotransmitter system. The signs and symptoms of carisoprodol intoxications, however, are not easily explained by interaction with this neurotransmitter system. In the present study, four cases of carisoprodol intoxications are presented with emphasis on the presence of serotonergic signs and symptoms. All four cases fulfilled three different sets of criteria for the diagnosis of serotonin syndrome. These findings could indicate that an increased serotonin level in the central nervous system could explain some of the symptoms and signs of carisoprodol intoxications. This may have implications for the clinical evaluation and treatment of such intoxications. Since few laboratories routinely screen for carisoprodol it is important to keep this drug in mind when encountering intoxications displaying serotonergic symptoms.