Babinski sign

**Definition / technique:** Stroke the lateral plantar aspect of the foot with a blunt object, from heel toward the toes and curving across the metatarsal heads. A positive (abnormal) response is extension (dorsiflexion) of the hallux, often with fanning of the other toes. The normal adult response is plantar flexion of the toes. This is the extensor plantar response, a primitive flexor-withdrawal reflex.

**Pathophysiology:** The corticospinal (pyramidal) tract normally inhibits the primitive withdrawal reflex. When upper motor neurons are damaged, this inhibition is lost and the reflex is released, causing great toe extension. In infants, the corticospinal tract is not fully myelinated, so the sign is physiologic until about age 1-2 years.

**Causes (UMN lesions):** - Stroke (internal capsule, cortex, brainstem) - Amyotrophic lateral sclerosis (UMN plus LMN signs) - Multiple sclerosis - Spinal cord compression or injury - Brain tumor - Subacute combined degeneration (vitamin B12 deficiency) - Friedreich ataxia - Brown-Séquard syndrome (ipsilateral to the lesion, below its level) - Anterior spinal artery syndrome - Neurosyphilis (tabes dorsalis, in some cases) - Post-ictal state, severe metabolic derangement, or deep sedation or anesthesia (a transient, bilateral false-positive)

**Associated UMN findings:** spasticity (clasp-knife), hyperreflexia, clonus, weakness (pyramidal pattern), and loss of superficial abdominal reflexes. In contrast, lower motor neuron lesions cause flaccid paralysis, hyporeflexia or areflexia, atrophy, and fasciculations, with no Babinski sign.

**Clinical pearls and high-yield points:** - Babinski is the most specific bedside sign of corticospinal tract dysfunction. - Adult presence = pathologic. Infant presence = normal. - Equivalent "Babinski-like" responses include Chaddock (stroking lateral malleolus), Oppenheim (stroking down the tibial crest), and Gordon (squeezing the calf). - In ALS, the combination of a Babinski sign with fasciculations and atrophy is classic. - In Friedreich ataxia, an extensor plantar response occurs along with areflexia (dorsal column and spinocerebellar involvement together with corticospinal involvement). - In subacute combined degeneration, there is both dorsal column loss (vibration and proprioception) and a Babinski sign. - Absence of the sign does not rule out a UMN lesion (for example, acute spinal shock may produce flaccidity and absent reflexes at first). - In coma, a bilateral Babinski sign suggests diffuse or structural brain dysfunction.