The crossed extensor reflex is a spinal cord-mediated motor response in which stimulation of one leg produces extension of the opposite leg.

• Normal in neonates: Observed in healthy newborns as part of primitive reflexes.

• Pathologic persistence: If present beyond 6 months of age, it suggests central nervous system (CNS) injury or anoxic brain damage.

• This reflex is part of protective motor mechanisms and demonstrates spinal cord integrity.

Pathophysiology

• Mediated at the spinal cord level via polysynaptic reflex arcs:

  1. Afferent input: Nociceptive or tactile stimulation of one leg (usually medial thigh or sole).

  2. Interneuronal processing: Signals cross to the contralateral side of the spinal cord.

  3. Efferent output: Contralateral leg extends and adducts; toes fan outward.

• Neonatal persistence is physiologic due to immature corticospinal inhibition.

• Post-infancy persistence reflects disinhibited reflexes secondary to upper motor neuron lesions or CNS damage.

Examination Technique

Patient Positioning

• Place the neonate or infant in a supine position with legs extended.

• Ensure the infant is calm and relaxed.

Stimulation and Observation

1. Tap or apply gentle pressure to the medial aspect of one thigh, just above the patella.

2. Observe the opposite leg for:

   • Extension

   • Adduction

   • Fanning of the toes

3. Repeat bilaterally to compare responses.

Assessment Notes

• In neonates (<6 months), this response is normal and expected.

• Persistence beyond 6 months is abnormal and warrants further neurological evaluation.

Clinical utility

• Neonatal neurological assessment: Confirms normal reflex development.

• Indicator of CNS lesion: Persistent reflex in older infants or children suggests:

  • Anoxic brain injury

  • Spinal cord lesions

  • Upper motor neuron dysfunction

• Early detection: Helps in identifying neurologic deficits before overt clinical signs appear.

Differential Diagnosis

Pediatric considerations

• Normal up to 6 months; beyond this age, persistence indicates neurological pathology.

• Useful in early detection of CNS injury or developmental abnormalities.

Geriatric considerations

• Not applicable; this reflex is specific to neonates and young infants.

Limitations

• In older children, voluntary movements or agitation may obscure the reflex.

• Interpretation requires experience to differentiate normal neonatal response from pathologic persistence.

• Must be assessed alongside other primitive and pathological reflexes.

Patient counseling

• Explain that this reflex is normal in newborns and part of standard neurological assessment.

• For abnormal persistence, emphasize the importance of neurological evaluation and early intervention.

• Reassure caregivers that early detection can guide management and improve outcomes.

Conclusion

The crossed extensor reflex is a normal neonatal reflex reflecting spinal cord integrity. Persistence beyond 6 months of age signals CNS dysfunction, including anoxic brain injury or upper motor neuron lesions. Careful observation and correlation with other neurological findings are essential for accurate interpretation and timely intervention.

References

1. Vinken PJ, Bruyn GW. Handbook of Clinical Neurology: Neonatal Reflexes and Motor Development. Amsterdam: Elsevier; 1983.

2. Nelson KB, Ellenberg JH. Neurological assessment of the newborn infant. Pediatrics. 1976;58(5):624–635.

3. Ropper AH, Samuels MA, Klein JP. Adams and Victor’s Principles of Neurology. 11th ed. New York: McGraw-Hill; 2021.

4. Paneth N. Neurological Examination in the Newborn Infant. Arch Dis Child Fetal Neonatal Ed. 1997;76:F83–F88.

5. Prechtl HF. The Neurological Examination of the Full-term Newborn Infant. 2nd ed. Oxford: Oxford University Press; 2001.