The Babinski sign, also called the plantar reflex, is a neurological test that observes the foot’s reaction to a specific type of stimulation. It serves as a standard component of neurological exams for individuals of all ages, from newborns to adults. The interpretation of this reflex, however, changes significantly depending on the age of the person being tested.
How the Plantar Reflex is Tested
To perform the test, a healthcare provider uses a dull instrument, such as the handle of a reflex hammer or a tongue depressor. The practitioner will stroke the outer edge of the sole, starting at the heel. This stroking motion continues up the lateral side of the foot toward the toes. The path then curves across the ball of the foot, moving medially toward the base of the big toe. This specific pathway is designed to elicit a clear and observable reflex from the toes.
The Babinski Sign in Infants
In infants, a response where the big toe extends upward and backward toward the top of the foot while the other toes fan out is considered normal. This reaction is known as a positive Babinski sign. It is a typical primitive reflex found in babies. The presence of this reflex is a healthy sign of neurological function in this age group.
This distinct reflex occurs because an infant’s central nervous system is still maturing. Specifically, the corticospinal tract, a major nerve pathway from the brain to the spinal cord, is not yet fully myelinated. Myelination is the process of forming a protective sheath around nerve fibers, which allows nerve impulses to move more quickly.
The Babinski sign is expected to disappear as the child’s nervous system develops. This change typically happens between the ages of 12 and 24 months. As myelination of the corticospinal tract completes, the infantile reflex is replaced by the mature response.
The Babinski Sign in Adults and Older Children
In adults and children over the age of two, the expected response to the plantar reflex test involves all the toes curling downward, or flexing. The foot might also pull away slightly from the stimulus. This downward curling is referred to as a negative Babinski sign and indicates that the central nervous system pathways are functioning as expected.
When an adult or older child exhibits a positive Babinski sign—the big toe extending upward and the other toes fanning out—it suggests a potential underlying issue within the central nervous system. This abnormal response points specifically to possible damage to the upper motor neurons, which are part of the corticospinal tract that controls voluntary movement. It signals a disruption in the communication between the brain and the spinal cord.
A positive Babinski sign in this age group is a diagnostic clue, not a diagnosis itself. It can be an indicator for a range of neurological conditions, including:
- A stroke
- Spinal cord injury
- Multiple sclerosis (MS)
- A brain tumor
- Meningitis
Observing this sign prompts further medical evaluation, which may involve imaging like CT scans or an MRI to identify the root cause of the abnormal reflex.