Babinski sign

Formally a sign in neurology, the Babinski sign, also called the plantar reflex, is a basic part of a physical examination. It is simple, quick, and reasonably sensitive although nonspecific for motor neuron disorders.

Technique

The examiner scratches the sole of each foot, using increasingly firm touch and sharp probing. Initially, lightly stroke with a finger, then with a key or other blunt object, then with slightly pointed implement such as a wooden applicator stick broken in half. If the Babinski sign is positive, the big toe turns downward. The normal response is for it to turn upwards. Lack of movement of the toe does not have strong significance.

This mechanism may not be present in infants under a year of age.

Underlying mechanism

Increasingly noxious stimuli are used because the sign is the elicitation of a cutaneous nociceptive (i.e., pain) reflex. The exact mechanism is not completely understood, but electromyographic studies suggest that skin areas, in the distribution of spinal nerves, appear to have specific pain reflexes, primarily in the spinal cord but affected by the brain. "The area of skin from which the reflex can be obtained is known as the receptive field of the reflex. To be more specific, a noxious stimulus to the sole of the foot, which is the receptive field, causes immediate flexion of the toes, ankle, knee, and hip joints with attendant withdrawal of the foot from the stimulus...

The plantar reflex contrasts with the great toe reflex: "Stimulation of the ball of the toe, which is the receptive field, causes extension (dorsiflexion) of the toe with flexion at ankle, knee, and hip joints. The differences between these two reflexes are in the receptive fields and the fact that the great toe is flexed in one and extended in the other. The reason for the extension in the toe reflex is to remove the toe from the stimulus."

An abnormal plantar reflex produces extension of the great toe instead of the normal flexion response.

Interpretation

"...The abnormal response reliably indicates metabolic or structural abnormality in the corticospinal system upstream from the segmental reflex. Thus the extensor reflex has been observed in structural lesions such as hemorrhage, brain and spinal cord tumors, and multiple sclerosis, and in abnormal metabolic states such as hypoglycemia, hypoxia, and anesthesia."