A 36-year-old woman is evaluated in the clinic due to left hand clumsiness. The patient works as an office administrative assistant and has been having difficulty typing. Six months ago, she had an episode of vertigo and difficulty walking that completely resolved after several days. On physical examination, she is asked to close her eyes and stretch out her arms with the palms facing up. Several seconds later, her left arm involuntarily drifts downward, and the palm turns toward the floor. There is no evidence of ataxia, nystagmus, or tremor. Examination of the lower extremities is normal. Romberg test is negative. Which of the following best explains the findings observed in this patient's current physical examination?
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On examination, this patient with arm clumsiness has a normal Romberg test (indicating normal proprioception); no ataxia, nystagmus, or tremor (indicating normal cerebellar function); and a unilateral pronator drift. Pronator drift is a physical examination finding that, especially in the absence of demonstrable proprioceptive deficits, is virtually pathognomonic for upper motor neuron or pyramidal/corticospinal tract disease.
The pronator drift test can illuminate otherwise unnoticeable upper motor neuron weakness. It is performed by having the patient outstretch the arms with the palms up and eyes closed. Upper motor neuron lesions cause more weakness in the supinator muscles compared to the pronator muscles of the upper limb. As a result, the affected arm drifts downward and the palm turns (pronates) toward the floor.
Therefore, this young woman's current arm weakness is likely due to an upper motor neuron lesion. Given her history of neurologic deficits disseminated in space and time (eg, current arm weakness, previous ataxia, vertigo), she most likely has multiple sclerosis, an inflammatory demyelinating disorder characterized by white matter lesions in the CNS.
(Choice A) Basal ganglia dysfunction typically results in extrapyramidal signs, such as resting tremor, rigidity, bradykinesia, and choreiform movements. Pronator drift is not typically observed.
(Choice B) Cerebellar dysfunction usually causes ataxia, intention tremor, and impaired rapid alternating movements. Cerebellar lesions result in an ipsilateral upward arm drift, rather than the downward drift seen in pyramidal tract lesions, due to hypotonia (ie, cerebellar drift). Other pyramidal tract signs (eg, focal weakness, spasticity, hyperreflexia, Babinski sign) are not observed.
(Choice C) Proprioception is evaluated by passively moving the distal phalange of a digit up and down and having patients identify the direction of movement with their eyes closed. It is also assessed with the Romberg test in which patients are observed for unsteadiness as they stand with their feet together, arms to the sides, and eyes closed. Patients with impaired proprioception (eg, vitamin B12 deficiency, tabes dorsales) have difficulty determining joint position and may lose balance during Romberg testing.
Educational objective:
Pronator drift is a relatively sensitive and specific sign for upper motor neuron or pyramidal (corticospinal) tract disease affecting the upper extremities. It is performed by having the patient outstretch the arms with the palms up and eyes closed. In patients with upper motor neuron lesions, the affected arm drifts downward and the palm turns (pronates) toward the floor.