Why do newborns have a positive Babinski (Babinski reflex) sign?

Why Newborns Have a Positive Babinski Reflex

The positive Babinski reflex in newborns is normal and represents an immature nervous system where the pyramidal tract has not yet fully developed control over spinal motoneurons.

Neurophysiological Basis

The Babinski reflex (also called plantar response) involves more than just toe movement—it's part of a broader flexion synergy that includes all muscles that shorten the leg. In newborns, this response manifests differently than in adults due to developmental factors:

• In newborns, the pyramidal tract (corticospinal tract) is still developing and has not yet established complete control over spinal reflexes 1
• The flexion synergy is brisk in newborns, involving all flexor muscles of the leg, including the toe "extensors" which physiologically function as leg flexors 1
• This produces the characteristic upgoing toe response (positive Babinski) when the lateral sole is stimulated

Normal Developmental Timeline

The positive Babinski reflex follows a predictable developmental course:

• Present from birth in most healthy term infants
• Gradually becomes suppressed as the nervous system matures
• Disappears in the majority of infants by the end of the fifth month of age 2
• Persistence beyond 5-6 months may suggest neurological abnormality

It's important to note that while extension of the great toe is the most frequent response to lateral sole stimulation in neonates, a small but significant number of flexor responses can be observed even in normal newborns 3.

Clinical Significance

The Babinski sign has specific clinical implications depending on age:

• In adults and older children: A positive Babinski sign indicates pyramidal tract dysfunction 4
• In newborns: A positive Babinski sign is physiologically normal and expected
• Transition period: As the nervous system matures, the response changes from extension to flexion of the toes

Mechanism of Transition

As the infant's nervous system develops:

1. The pyramidal tract gradually gains more control over spinal motoneurons
2. The primitive flexion synergy becomes less pronounced
3. The toe "extensors" are no longer recruited as part of this synergy
4. The toes begin to go down instead of up in response to stimulation
5. A segmental reflex involving small foot muscles develops (similar to abdominal reflexes) 1

Assessment Considerations

When assessing the Babinski reflex in newborns:

• The technique of examination significantly affects results (explaining widely differing percentages of extensor responses reported in studies) 3
• The response should be evaluated in the context of the entire leg's reaction, not just toe movement 1
• Absence of the reflex during the neonatal period is not necessarily abnormal 2

Historical Context

While the sign bears Babinski's name, the phenomenon had been observed as early as 1784. However, it was Joseph François Félix Babinski who first interpreted its clinical significance in 1896, recognizing that it indicated pyramidal dysfunction in adults but was normally present in infancy 4, 5.

This understanding transformed neurological examination, establishing physical findings as crucial indicators of underlying pathology rather than relying solely on patient history 5.

The Babinski reflex remains one of the most reliable and enduring neurological signs in clinical practice, helping distinguish between central and peripheral nervous system conditions.