Big toe sign: An important neurologic examination based upon what the big toe (and other toes) do when the sole of the foot is stimulated. If the big toe goes up, that may mean trouble.

The big toe response, also called the Babinski reflex, is obtained by stimulating the external portion (the outside) of the sole. The examiner begins the stimulation back at the heel and goes forward to the base of the toes. There are diverse ways to elicit the big toe sign. A useful way that requires no special equipment is with firm pressure from the examiner's thumb. Just stroke the sole firmly with the thumb from back to front along the outside edge.

Too vigorous stimulation may cause withdrawal of the foot or toe, which can be mistaken as a big toe sign.

Most newborn babies give a big toe response because their nervous system is so immature. Upon stimulation of the sole, they extend the great toe. Many young infants do this, too, and it is perfectly normal. However, in time during infancy the big toe response vanishes and, under normal circumstances, should never return.

A big toe response in an older child or adult is abnormal. It is a sign of a problem in the central nervous system (CNS), most likely in a part of the CNS called the pyramidal tract.

Asymmetry of the big toe response -- when it is present on one side but not the other -- is abnormal. It is a sign not merely of trouble but helps to lateralize that trouble (tell which side of the CNS is involved).

The big toe reflex is known by a number of other names: including the plantar response (because the sole is the plantar surface of the foot), the toe sign or phenomenon, and the Babinski phenomenon or sign.

It is quite common but entirely incorrect to say that the big toe sign is positive or negative. It is either present or absent.