WBR0573

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Neurology
Prompt	[[Prompt::A 68 year old man with a history of chronic stable angina and hypertension presents to the ED for 3 hours of violent involuntary movements of his right extremities. The patient explains that these attacks started suddenly and have since been repetitive and uncontrollable. On examination, the resident observes non-rhythmic, continuous, violent flinging motions of the right arm and leg. A brain CT scan is negative, but the resident still suspects an underlying ischemic stroke. Which of the following receptors is an important component of the pathway affected in this patient?]]
Answer A	AnswerA::α-adrenergic receptors
Answer A Explanation	[[AnswerAExp::α-adrenergic receptors are 2 types: α1 and α2. α1-receptors have several functions including vasoconstriction, pupillary dilation, and bladder sphincter muscle contraction. α2-receptors are responsible for decreasing sympathetic outflow and decreasing insulin release and lipolysis. α-adrenergic receptors are not involved in modulation of skeletal muscle function.]]
Answer B	AnswerB::5-HT2 receptors
Answer B Explanation	AnswerBExp::5-HT2 receptors mediate many of the central and peripheral physiologic functions of serotonin. They are not part of the inhibitory pathway of the basal ganglia.
Answer C	AnswerC::Nicotinic receptors
Answer C Explanation	[[AnswerCExp::Nicotinic acetylcholine receptors are transmitter gated ion channels than bind acetylcholine and are responsible for the postsynaptic action potential transmission at the neuromuscular junction. Although muscarinic acetylcholine receptors may be involved in the basal ganglia motor circuit, nicotinic receptors have no documented role.]]
Answer D	AnswerD::Dopamine D1 receptors
Answer D Explanation	[[AnswerDExp::Dopamine D1 receptors are involved in the excitatory or direct pathway in the basal ganglia. A decrease in D1 receptor activity or a defect in the excitatory pathway would lead to a decrease in motion. On the other hand, an imbalance favoring the D1 excitatory pathway over the inhibitory D2 pathway such as in this patient would lead to increased movement i.e. chorea, athetosis, or hemiballismus.]]
Answer E	AnswerE::Dopamine D2 receptors
Answer E Explanation	[[AnswerEExp::Dopamine D2 receptors are involved in the inhibitory or indirect pathway leading to a net decrease in motion. Any defect in the inhibitory pathway, such as a subthalamic nucleus lesion, would lead to a shift of the balance toward the excitatory pathway, leading to increase movement ie. hemiballismus, chorea, athetosis.]]
Right Answer	RightAnswer::E
Explanation	[[Explanation::Hemiballismus is rare hyperkinetic disorder characterized by involuntary movements of large amplitude involving 2 ipsilateral limbs. The movements are often repetitive, irregular, and poorly-patterned. They arise from an insult to the contralateral subthalamic nucleus resulting in an imbalance between the excitatory and inhibitory pathways of the basal ganglia. The subthalamic nuclei are an important part of the inhibitory pathway for modulating movement activated by dopamine binding to D2 receptors. The inhibitory pathway involves disinhibition of the subthalamic nucleus by the globus pallidus externus (GPe) allowing the subthalamic nucleus to activate the globus pallidus internus (GPi) that inhibits the thalamus and decreases movement. Any process focal to the subthalamic nuclei or its connections may cause hemiballismus, however the most common cause in elderly patients is a vascular lesion such as an infarct. Young patient usually have an underlying infection or inflammatory process. Educational objective: Hemiballismus is a rare disorder leading to involuntary wild flinging of the extremities contralateral to a subthalamic nucleus lesion in the basal ganglia. The subthalamic nucleus is part of the indirect or inhibitory pathway that involved dopamine binding to D2 receptors in the putamen. References: Shannon KM. Hemiballismus. Curr Treat Options Neurol. 2005;7(3):203-210. Ristic A, Marinkovic J, Dragasevic N, Stanisavljevic D, Kostic V. Long-term prognosis of vascular hemiballismus. Stroke. 2002;33(8):2109-11. Educational Objective: References: ]]
Approved	Approved::Yes
Keyword	WBRKeyword::Hemiballismus, WBRKeyword::Basal ganglia, WBRKeyword::Dopamine, WBRKeyword::Dopamine receptors, WBRKeyword::Inhibitory pathway, WBRKeyword::Excitatory pathway
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