What does an equivocal Babinski sign indicate?

What Does an Equivocal Babinski Sign Indicate?

An equivocal Babinski sign indicates an uncertain or ambiguous plantar reflex response that cannot be definitively classified as either normal (downgoing toes) or pathological (upgoing hallux), and it warrants careful clinical correlation with other neurological findings and consideration of neuroimaging if upper motor neuron dysfunction is suspected based on the overall clinical picture. 1, 2

Clinical Significance of an Equivocal Response

An equivocal Babinski sign represents a diagnostic challenge because:

• The response may be neither clearly flexor (normal) nor clearly extensor (pathological), making interpretation difficult at the bedside 3
• Babinski himself recognized "formes frustes" (incomplete forms) of his reflex and acknowledged its occasional absence even in patients with other clinical signs of pyramidal lesions 3
• The sign has limited interobserver reliability (kappa 0.30) with only 56% agreement with known upper motor neuron weakness, meaning different examiners may interpret the same response differently 4
• The sign may be transiently absent in acute lesions due to temporary inexcitability of spinal motoneurons immediately after transverse spinal cord lesions or acute brain lesions 3

What to Look for When the Babinski is Equivocal

When faced with an equivocal plantar response, systematically assess for other upper motor neuron signs:

• Hyperreflexia in the lower extremities (increased deep tendon reflexes at knees and ankles) 1, 2
• Spasticity (velocity-dependent increase in muscle tone with passive movement) 1, 2
• Weakness with a pyramidal pattern (greater in extensors of upper limbs and flexors of lower limbs) 1
• Decreased speed of foot tapping, which has substantially better reliability (kappa 0.73) and accuracy (85% agreement) than the Babinski sign itself 4
• Sensory level on the trunk indicating spinal cord pathology 2
• Clonus at the ankle (sustained rhythmic contractions with dorsiflexion) 1

Common Pitfalls in Interpretation

Several factors can produce equivocal or misleading responses:

• Electrical stimulation may fail to activate the extensor hallucis longus in patients with true pyramidal dysfunction, while conversely evoking extensor responses in normal subjects 5
• Mechanical and electrical stimuli are not freely interchangeable for eliciting the reflex 5
• The sign is mediated specifically by the extensor hallucis longus, not the extensor hallucis brevis, so observing the wrong muscle can lead to misinterpretation 5
• Voluntary withdrawal or ticklishness can obscure the true reflex response 3

When to Pursue Neuroimaging

MRI of the brain and/or spinal cord is indicated when an equivocal Babinski sign is accompanied by: 1, 2

• Recent onset of cognitive symptoms or unexplained neurological manifestations 1
• Progressive weakness, sensory changes, or bowel/bladder dysfunction suggesting myelopathy 2
• Significant vascular risk factors or symptoms suggesting stroke 1
• Headaches, vision changes, or signs of increased intracranial pressure 2
• Multiple other upper motor neuron signs (hyperreflexia, spasticity, weakness) 1
• Recent trauma or infection history 2

The American College of Radiology designates MRI as the preferred imaging modality, with CT as an alternative only if MRI is contraindicated. 1, 2

Practical Approach

In the absence of other clear upper motor neuron signs or concerning symptoms:

• Document the equivocal nature of the response carefully and consider having another examiner assess the reflex 4
• Test foot tapping speed bilaterally as a more reliable alternative sign of pyramidal dysfunction 4
• Perform serial examinations since acute lesions may not immediately manifest a clear Babinski sign 3
• Consider the clinical context: an isolated equivocal Babinski in an otherwise neurologically normal patient without symptoms likely does not require immediate imaging 1

Conditions Associated with Positive Babinski Signs

When upper motor neuron dysfunction is confirmed, consider:

• Stroke or cerebral infarction with brainstem compression causing contralateral signs 1
• Thoracic myelopathy or cord compression requiring surgical decompression 1
• Hepatic encephalopathy as part of motor system abnormalities 1
• Demyelinating diseases in younger patients 2
• Spinal cord tumors particularly in pediatric populations 2