A 38-year-old woman is being evaluated for intermittent tingling and numbness of the left hand for the past several months. Her symptoms predominantly involve the fourth and fifth digits and are often brought on by driving a car. Physical examination shows no abnormalities. An electrodiagnostic study is performed, during which a nerve stimulator is placed on the left fifth digit and a surface recorder is placed over the ulnar nerve above the elbow. Electrical stimulus is applied, and the graph obtained is shown below. Results from the asymptomatic contralateral extremity are also shown. Which of the following is primarily affected in this patient?

Question

A 38-year-old woman is being evaluated for intermittent tingling and numbness of the left hand for the past several months.  Her symptoms predominantly involve the fourth and fifth digits and are often brought on by driving a car.  Physical examination shows no abnormalities.  An electrodiagnostic study is performed, during which a nerve stimulator is placed on the left fifth digit and a surface recorder is placed over the ulnar nerve above the elbow.  Electrical stimulus is applied, and the graph obtained is shown below.  Results from the asymptomatic contralateral extremity are also shown.  Which of the following is primarily affected in this patient?

Quizplus · Accepted Answer

11ec160b_3b0e_592e_9249_3390d6f1cbff_MD0001_00 A nerve conduction study (NCS) measures the amplitude, velocity, and latency of an electrical stimulus applied to an isolated nerve and can help differentiate between demyelinating and axonal neuropathies: Demyelinating neuropathies are caused by damage to the myelin sheath. Loss of insulation results in delayed (or blocked) nerve conduction velocity. Axonal neuropathies are caused by damage to the nerve axon. Loss of axon fibers results in reduced signal amplitude. This patient has a delay in left-sided ulnar conduction velocity, indicating a demyelinating ulnar mononeuropathy. Focal nerve compression (eg, compression of the ulnar nerve at the elbow) is the most common cause of mononeuropathy and results in nerve ischemia, leading to apoptosis of Schwann cells and localized demyelination. In addition to sensory symptoms, patients with ulnar neuropathy may also have weakness of the fourth and fifth digits, elbow pain, and worsening of symptoms with elbow or wrist flexion. Severe disease can lead to wasting of the hand muscles and an ulnar claw deformity. (Choice A) An axonal neuropathy would primarily show diminished amplitude, rather than reduced velocity of nerve conduction. Common causes of axonal neuropathy include diabetes mellitus, toxins (eg, chemotherapeutic drugs), hypothyroidism, vitamin B₁₂ deficiency, and vasculitis. (Choice B) Neuronal cell bodies are located in the dorsal root ganglia and the anterior horn of the spinal cord. Neuronal cell body damage could potentially result in reduced signal amplitude if there is secondary axonal degeneration. (Choice C) Myasthenia gravis is due to autoimmune attack on the acetylcholine receptors of the postsynaptic membrane. This causes fatigable weakness of the ocular, facial, and proximal muscles. NCS is typically normal, but muscle stimulation studies demonstrate a reduction in motor signal strength with repetitive stimuli. (Choice E) Botulinum toxin blocks acetylcholine release from presynaptic membranes of the neuromuscular junction by impairing fusion of secretory vesicles with the synaptic membrane. Patients typically develop cranial neuropathies with symmetric, descending flaccid paralysis. NCS is typically normal; muscle stimulation studies may show a reduction in motor signal strength that improves with repetitive stimuli. Educational objective: Ulnar neuropathy is characterized by numbness of the fourth and fifth digits, pain, and weakness. It is most commonly caused by compressive injury, leading to tissue ischemia and demyelination (Schwann cell injury). A nerve conduction study can help differentiate demyelinating versus axonal neuropathies; demyelination causes slowed or blocked nerve conduction velocity whereas axonal injury leads to a reduction in signal strength.

A 38-Year-Old Woman Is Being Evaluated for Intermittent Tingling and Numbness