A 19-year-old man is brought to the emergency department by his friends due to severe agit… — USMLE Step 2 CK

A $19$ -year-old man is brought to the emergency department by his friends due to severe agitation, confusion, and muscle twitching. The friends report they were at an electronic music festival where the patient consumed an unknown pill approximately $4$ hours ago. His medical history is significant for major depressive disorder, for which he has been taking sertraline for the past $6$ months.

Temperature is $39.6^\circ\text{C}$ ( $103.3^\circ\text{F}$ ), blood pressure is $168/102 \text{ mm Hg}$ , pulse is $132/\text{min}$ , and respirations are $24/\text{min}$ .

On examination, the patient is disoriented, markedly diaphoretic, and has mydriasis. Neurologic examination reveals shivering, hyperreflexia in the lower extremities, and sustained bilateral ankle clonus.

Laboratory studies reveal:

Laboratory Parameter	Result	Reference Range
Sodium	$122 \text{ mEq/L}$	$136-145 \text{ mEq/L}$
Potassium	$4.0 \text{ mEq/L}$	$3.5-5.0 \text{ mEq/L}$
Bicarbonate	$24 \text{ mEq/L}$	$22-28 \text{ mEq/L}$
Blood urea nitrogen	$10 \text{ mg/dL}$	$7-18 \text{ mg/dL}$
Creatinine	$0.8 \text{ mg/dL}$	$0.6-1.2 \text{ mg/dL}$
Thyroid-stimulating hormone (TSH)	$2.1 \ \mu\text{U/mL}$	$0.5-5.0 \ \mu\text{U/mL}$

Urine toxicology screen is positive for amphetamines.

Which of the following is the most likely underlying mechanism for this patient's acute presentation?

A
Noncompetitive antagonism of glutamatergic N-methyl-D-aspartate receptors
B
Accumulation of toxic acidic metabolites and unmeasured osmoles
Synergistic elevation of intrasynaptic serotonin levelsAnswer
D
Acute onset of schizophreniform disorder
E
Impaired free water excretion secondary to primary hypothyroidism

Answer

Synergistic elevation of intrasynaptic serotonin levels

The correct answer identifies the pathogenesis of Serotonin Syndrome. This patient's triad of altered mental status, autonomic instability (hyperthermia, tachycardia, diaphoresis), and neuromuscular excitability (hyperreflexia, clonus) is classic for Serotonin Syndrome. He ingested an unknown pill at a music festival, which, given the clinical picture and positive amphetamine screen, is highly likely to be MDMA (ecstasy). MDMA is a synthetic amphetamine derivative that promotes the massive presynaptic release of serotonin, dopamine, and norepinephrine. His home medication, sertraline, is a selective serotonin reuptake inhibitor (SSRI). The combination of a serotonin-releasing agent (MDMA) and a reuptake inhibitor (sertraline) causes a massive synergistic accumulation of serotonin in the synaptic cleft, precipitating this crisis. Furthermore, MDMA frequently causes profound hyponatremia (as seen with this patient's sodium of

122 \text{ mEq/L}

) via drug-induced inappropriate antidiuretic hormone secretion (SIADH) and increased free water intake, which confirms the diagnosis.

Step-by-Step Solution

Evaluate the clinical presentation and vital signs.

The patient exhibits hyperthermia, tachycardia, diaphoresis, and altered mental status.

These signs indicate an acute hypermetabolic toxidrome, narrowing the differential to serotonergic, sympathomimetic, or anticholinergic toxicity.

Analyze the specific neurologic findings.

The presence of shivering, hyperreflexia, and sustained bilateral ankle clonus is noted.

Clonus and hyperreflexia are the hallmark distinguishing features of Serotonin Syndrome, separating it from neuroleptic malignant syndrome (which presents with 'lead-pipe' rigidity) and pure sympathomimetic toxicity.

Correlate the toxidrome with the patient's history and lab findings.

The patient took an unknown pill (MDMA/ecstasy) while on home sertraline, and labs show severe hyponatremia without an acid-base disturbance.

MDMA promotes massive serotonin release and uniquely causes SIADH-like hyponatremia. Combining MDMA with an SSRI (sertraline) prevents serotonin clearance, leading to a life-threatening synergistic serotonergic crisis.

Key Concept

MDMA (ecstasy) intoxication combined with selective serotonin reuptake inhibitors (SSRIs) can precipitate severe serotonin syndrome. Additionally, MDMA is unique among common drugs of abuse for causing profound hyponatremia.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic