WBR0497

Author	PageAuthor::William J Gibson (Reviewed by Serge Korjian and Yazan Daaboul)
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Anatomy
Sub Category	SubCategory::Musculoskeletal/Rheumatology, SubCategory::General Principles
Prompt	[[Prompt::A 45-year-old female scientist presents to her primary care physician because of pain and tingling in his right hand. Upon further questioning, the patient reports that the pain is worse at night. Over the past 3 months, the patient has hurried to submit two R01 applications and has been working often at his keyboard. When the skin overlying the patient’s right wrist is tapped lightly, the patient reports a feeling of “pins and needles” in his thumb and the index finger, middle finger, and lateral half of the ring finger on the dorsum of the right hand. The affected nerve courses between which two muscle groups in the forearm?]]
Answer A	AnswerA::Flexor digitorum profundus and flexor pollicis longus
Answer A Explanation	AnswerAExp::The flexor digitorum profundus flexes the wrist and the metacarpophalangeal and interphalangeal joints. The flexor pollicis longus flexes the thumb. The median nerve does not traverse between these 2 muscle groups.
Answer B	AnswerB::Flexor digitorum superficialis and flexor digitorum profundus
Answer B Explanation	[[AnswerBExp::The flexor digitorum superficialis flexes the middle phalanges at the proximal interphalangeal joints. The flexor digitorum profundus flexes the wrist and the metacarpophalangeal and interphalangeal joints. The median nerve traverses between the flexor digitorum superficialis and flexor digitorum profundus.]]
Answer C	AnswerC::Flexor digitorum superficialis and flexor pollicis longus
Answer C Explanation	AnswerCExp::The flexor digitorum superficialis flexes the middle phalanges at the proximal interphalangeal joints. The flexor pollicis longus flexes the thumb. The median nerve does not traverse between these 2 muscle groups.
Answer D	AnswerD::Opponens pollicis and flexor digitorum superficialis
Answer D Explanation	[[AnswerDExp::The opponens pollicis is responsible for the opposing activity of the thumb. It is innervated by the recurrent branch of the median nerve. It is one of the three muscles of the thenar muscles (along with the abductor pollicis brevis and the flexor pollicis brevis). The flexor digitorum superficialis flexes the middle phalanges at the proximal interphalangeal joints.]]
Answer E	AnswerE::Abductor pollicis brevis and opponens pollicis
Answer E Explanation	[[AnswerEExp::Along with the flexor pollicis brevis, both the abductor pollicis brevis and the opponens pollicis are components of the thenar muscles. They are both innervated by the recurrent branch of the median nerve. The abductor pollicis brevis is responsible for thumb abduction, while the opponens pollicis is responsible for thumb opposition.]]
Right Answer	RightAnswer::B
Explanation	[[Explanation::Carpal tunnel syndrome (CTS) is the most common entrapment mononeuropathy. It is characterized by sensation of pain and paresthesias in the distribution of the median nerve (C6-T1) of the affected hand. The median nerve lies deep within the flexor retinaculum between the flexor digitorum profundus and the flexor digitorum superficialis and provides sensory innervation to the thumb and the index finger, middle finger, and lateral half of the ring finger on the dorsum of the hand. Patients with CTS often report pain that is worsened at night with loss of sleep. CTS should be suspected among both male and female patients of any age. The most important risk factors for the development of CTS include occupational activities, such as bending of the wrists (such as working at a keyboard). Other risk factors include dislocation of the lunate bone, which causes acute CTS, and chronic dialysis, which results in CTS due to β2-microglobulin amyloidosis. On physical examination, patients may have decreased sensation and reduced strength in the regions innervated by the median nerve. More advanced cases may demonstrate loss of the thenar eminence suggestive of muscle atrophy. Phalen's (maximal wrist extension) and Tinel's tests (light tapping on the wrist) are two provocative tests that may be helpful in the diagnosis of CTS. However, they are both signs of low sensitivity and specificity; and the diagnosis of CTS should always be confirmed by electrodiagnostic studies. Electrophysiologic findings include prolonged distal latency and delayed conduction velocity of the sensory and/or motor components of the median nerve. Although patients often complain of unilateral wrist pain, electrodiagnostic studies in CTS often reveal entrapment in both wrists with varying degrees of severity. Educational Objective: Carpal tunnel syndrome (CTS) is a clinical syndrome caused by the entrapment of the median nerve at the level of the flexor retinaculum. Manifestations of CTS include sensations of pain and paresthesias in the distribution of the median nerve of the affected hand, which include the thumb and the index finger, middle finger, and lateral half of the ring finger on the dorsum of the hand. Diagnosis should always be confirmed by electodiagnostic studies, but Phalen's (maximal wrist extension) and Tinel's tests (light tapping on the wrist) are signs on physical examination that may suggest the diagnosis. References: Solomon DH, Katz JN, Bohn R, et al. Nonoccupational risk factors for carpal tunnel syndrome. J Gen Intern Med. 1999;14(5):310-4. First Aid 2014 page]]
Approved	Approved::Yes
Keyword	WBRKeyword::Carpal tunnel syndrome, WBRKeyword::Median nerve, WBRKeyword::Nerve, WBRKeyword::Palsy, WBRKeyword::Tinel's sign, WBRKeyword::Phalen's sign, WBRKeyword::Flexor digitorum superficialis, WBRKeyword::Flexor digitorum profundus
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