The flexor withdrawal reflex is the flexion of the knee (and sometimes hip) in response to noxious stimulation of the sole of the foot. It is a normal spinal reflex in neonates, mediated at the level of the spinal cord. Persistence beyond 6 months or recurrence later in childhood may indicate central nervous system pathology.

Pathophysiology

• In neonates, nociceptive stimuli (such as pinching the sole) activate afferent sensory fibers.

• These fibers transmit impulses to the spinal cord, triggering flexor motor neurons and producing knee (and hip) flexion.

• Normally, inhibitory descending pathways mature by ~6 months, causing the reflex to disappear.

• Persistence or recurrence suggests:

  • Anoxic brain injury

  • Spinal cord lesions

  • Upper motor neuron disorders

Examination Technique

1. Patient Positioning: Place the neonate supine with legs extended.

2. Stimulation: Pinch the sole of the foot gently but firmly.

3. Observation:

   • Normally, infants <6 months show slow, uncontrolled flexion of the knee.

   • Flexion may extend to the hip and ankle.

4. Documentation: Note strength, speed, and symmetry. Record if the reflex is present beyond 6 months.

Clinical Utility

• Neurological assessment in neonates: Confirms integrity of spinal cord reflex arcs.

• Early detection of CNS lesions: Persistent or asymmetric responses indicate possible anoxic damage or central lesions.

• Developmental monitoring: Assesses neurological maturation in preterm and term infants.

Differential Diagnosis

Pediatric considerations

• Premature neonates: Reflex may be weak; careful observation is required.

• Full-term neonates: Reflex should be robust but slow and uncontrolled.

• Beyond 6 months: Persistence is abnormal and warrants neurological evaluation.

Limitations

• Reflex intensity varies with alertness, handling, or fatigue.

• False negatives may occur in sedated or critically ill infants.

• Requires skilled examiner for accurate interpretation.

Patient counseling

• Explain that the reflex is a normal protective response in neonates.

• Reassure parents that presence in infants <6 months is expected.

• Advise prompt evaluation if the reflex is persistent, asymmetric, or abnormal beyond expected age.

Conclusion

The flexor withdrawal reflex is a key neonatal spinal reflex that assesses the integrity of sensory and motor pathways. Proper elicitation and interpretation are essential for detecting neurological maturation and potential CNS injury.

References

1. O’Shea TM. Reflexes in the neonate: Clinical significance. Clin Perinatol. 2007;34:1–16.

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

3. Dhand R, et al. Neurologic examination of the newborn. Pediatr Clin North Am. 2015;62:1291–1313.

4. Gartner LM, et al. Neonatal Physical Assessment. 5th ed. Philadelphia: Lippincott Williams & Wilkins; 2012.