The Plantar Response (L5, S1). With an object such as a key or the wooden end of an applicator stick, stroke the lateral aspect of the sole from the heel to the ball of the foot, curving medially across the ball. Use the lightest stimulus that will provoke a response, but be increasingly firm if necessary. Note movement of the toes, normally flexion.
Dorsiflexion of the big toe, often accompanied by fanning of the other toes, constitutes a Babinski response. It often indicates a central nervous system lesion in the corticospinal tract.
A Babinski response may also be seen in unconscious states due to drug or alcohol intoxication or in the postictal period following a seizure.
Some patients withdraw from this stimulus by flexing the hip and the knee. Hold the ankle, if necessary, to complete your observation. It is sometimes difficult to distinguish withdrawal from a Babinski response.
Clonus. If the reflexes seem hyperactive, test for ankle clonus. Support the knee in a partly flexed position. With your other hand, dorsiflex and plantar flex the foot a few times while encouraging the patient to relax, and then sharply dorsiflex the foot and maintain it in dorsiflexion. Look and feel for rhythmic oscillations between dorsiflexion and plantar flexion. In most normal people, the ankle does not react to this stimulus. A few clonic beats may be seen and felt, especially when the patient is tense or has exercised.
A marked Babinski response is occasionally accompanied by reflex flexion at hip and knee.
Sustained clonus indicates central nervous system disease. The ankle plantar flexes and dorsiflexes repetitively and rhythmically.
Clonus may also be elicited at other joints. A sharp downward displacement of the patella, for example, may elicit patellar clonus in the extended knee.
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