A 22-year-old man comes to the emergency department due to sudden-onset pain and stiffness on one side of his neck. He was diagnosed with a psychiatric disorder 2 days ago and has been adherent with his newly prescribed medication. For the past few months, the patient has heard voices telling him to quit his job, and he believes that his girlfriend's parents are monitoring his sexual thoughts with a special device. He has been socially withdrawn and has difficulty showering regularly. Vital signs are within normal limits. The head is tilted to one side and cannot be straightened without considerable pain. The most likely cause of this patient's symptoms is antagonism of which of the following receptors?

Question

A 22-year-old man comes to the emergency department due to sudden-onset pain and stiffness on one side of his neck.  He was diagnosed with a psychiatric disorder 2 days ago and has been adherent with his newly prescribed medication.  For the past few months, the patient has heard voices telling him to quit his job, and he believes that his girlfriend's parents are monitoring his sexual thoughts with a special device.  He has been socially withdrawn and has difficulty showering regularly.  Vital signs are within normal limits.  The head is tilted to one side and cannot be straightened without considerable pain.  The most likely cause of this patient's symptoms is antagonism of which of the following receptors?

Quizplus · Accepted Answer

This patient is experiencing an acute dystonic reaction (ie, spasmodic torticollis), a type of extrapyramidal symptom (EPS) most likely due to recent initiation of antipsychotic medication. Acute dystonia can develop abruptly anytime between 4 hours and 4 days after receiving an antipsychotic medication. The condition is characterized by a sudden, involuntary contraction of a major muscle group. Other examples of acute dystonia include oculogyric crisis (forced, sustained elevation of the eyes in an upward position), opisthotonus (arching of the back with the head thrown backward), and, rarely, laryngospasm. The risk is highest with high-potency, first-generation antipsychotics (eg, haloperidol, fluphenazine). Acute dystonic reactions are theorized to result from the antipsychotic property of D2 antagonism in the nigrostriatal pathway. In the striatum, the inhibitory effects of dopaminergic neurons (D2) are normally balanced by the excitatory actions of muscarinic cholinergic neurons (M1). Strong dopaminergic blockade causes an excess of cholinergic activity, resulting in extrapyramidal side effects (eg, acute dystonic reactions, akathisia, parkinsonism). Medications with M1 receptor antagonist properties, such as benztropine or the antihistamine diphenhydramine, help re-establish the dopaminergic-cholinergic balance and effectively treat acute dystonia. (Choice A) Blockade of alpha-1 adrenergic receptors causes orthostatic hypotension and dizziness. (Choice C) Flumazenil, a benzodiazepine GABA-A receptor antagonist, can be used to treat benzodiazepine overdose. However, it may precipitate withdrawal seizures in patients with chronic benzodiazepine use. (Choice D) Antagonism of central histamine H1 receptors causes sedation and weight gain. (Choice E) Antagonism of muscarinic receptors causes anticholinergic side effects (eg, confusion, dry mouth, urinary retention). Low-potency antipsychotics (eg, chlorpromazine, thioridazine) are more likely to cause these effects due to their strong blockade of central and peripheral muscarinic cholinergic receptors. (Choice F) Second-generation antipsychotics antagonize serotonergic 5-HT2A, but this does not typically cause dystonia. Educational objective: Acute extrapyramidal symptoms (eg, dystonia, akathisia, parkinsonism) are due to D2 blockade in the nigrostriatal pathway. First-generation, high-potency antipsychotics (eg, haloperidol, fluphenazine) strongly block D2 receptors and are most likely to cause extrapyramidal symptoms. __________ References: Second-generation antipsychotics and extrapyramidal adverse effects. (http://www.ncbi.nlm.nih.gov/pubmed/24995318)

A 22-Year-Old Man Comes to the Emergency Department Due to Sudden-Onset