WBR0550

Author	[[PageAuthor::William J Gibson (Reviewed by Alison Leibowitz [1] and Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Anatomy
Sub Category	SubCategory::Musculoskeletal/Rheumatology
Prompt	[[Prompt::A 42-year-old man presents for a follow-up visit after being treated for injuries following a motor vehicle collision a few weeks earlier. The patient suffered a fracture of the surgical neck of the left humerus and underwent surgical repair. One week prior, the arm was freed from immobilization, but the patient has not regained full motor function. On physical examination, the patient is unable to abduct the left arm beyond 30 degrees, but he retains full function of the intrinsic muscles of the hand and is able to move his wrist freely. Which of the following nerves of the brachial plexus (shown below) is most likely injured in this patient? ]]
Answer A	AnswerA::A
Answer A Explanation	AnswerAExp::Point A indicates Erb’s point, the union of the C5 and C6 roots of the brachial plexus.
Answer B	AnswerB::B
Answer B Explanation	[[AnswerBExp::Point B represents the posterior cord of the brachial plexus, which gives rise to the axillary and radial nerves. While posterior cord injury would cause weakness of the deltoid muscle through loss of axillary nerve fibers, the patient would most likely exhibit additional symptoms arising from loss of radial nerve function, such as wrist drop.]]
Answer C	AnswerC::C
Answer C Explanation	AnswerCExp::Point C represents the axillary nerve, which is classically injured in fractures near or at the surgical neck of the humerus. The axillary nerve innervates the deltoid muscle, which is responsible for abduction of the arm beyond 15 degrees.
Answer D	AnswerD::D
Answer D Explanation	AnswerDExp::Point D represents the radial nerve. The radial nerve can be injured in conditions such as “Saturday night palsy”, in which the patient typically exhibits weakness of the wrist extensors.
Answer E	AnswerE::E
Answer E Explanation	AnswerEExp::Point E represents the union of the C8-T1 roots of the brachial plexus. This area can be injured in Klumpke palsy, causing a total claw deformity of the hand due to lumbrical dysfunction.
Right Answer	RightAnswer::C
Explanation	[[Explanation::The patient in this vignette has suffered a lesion of the axillary nerve due to a fracture of the surgical neck of the humerus. Lesions of the axillary nerve should be considered when patients present with either fractured surgical neck of the humerus or anterior dislocation of the humerus. The axillary originates from the posterior cord of the brachial plexus (C5-C6 roots) before it traverses inferolaterally in front of the subscapularis muscle. The axillary nerve then passes by the lower edge of the subscapularis muscle in the quadrilateral space and innervates the deltoid muscle. The deltoid is responsible for arm abduction beyond 15 degrees (The supraspinatus muscle, innervated by suprascapular nerve C5-C6, is responsible for arm abduction of the first 15 degrees). Lesions of the axillary nerve cause a sensory deficit over the lateral arm and eventually will cause a flattening of the shoulder due to atrophy of the deltoid muscle. Educational Objective: Fractures of the surgical neck of the humerus may cause lesions of the axillary nerve, which innervates the deltoid muscle. The deltoid muscle is responsible for abducting the shoulder beyond 15 degrees. References: Rotari V, Mousallem CD, et al. Position of the anterior branch of the axillary nerve in relation to the humeral bone length. Am J Orthop. 2012;41(10):452-455. First Aid 2014 page 414.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Brachial, WBRKeyword::Brachial plexus, WBRKeyword::Arm, WBRKeyword::Deltoid, WBRKeyword::Nerve, WBRKeyword::Innervation, WBRKeyword::Muscular, WBRKeyword::Nervous system, WBRKeyword::Axillary nerve, WBRKeyword::Abduction, WBRKeyword::Abduct, WBRKeyword::Axillary
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::