Assessing for pronator drift helps to detect mild upper limb weakness in a patient who's awake and able to follow directions.
Pronator drift indicates abnormal function of the corticospinal tract in the contralateral hemisphere.
[citation needed] The patient is asked to hold both arms at 90 degree flexion and elbows fully extended at shoulder level in front of them, with the palms upwards, and hold the position.
Closing the eyes accentuates the effect, because the brain is deprived of visual information about the position of the body and must rely on proprioception.
In the presence of an upper motor neuron lesion, the supinator muscles in the upper limb are weaker than the pronator muscles, and as a result, the arm drifts downward and the palm turns toward the floor.