What does it mean if the right leg extends when performing a patellar reflex on the left patellar tendon?

Contralateral Leg Extension During Patellar Reflex Testing: Clinical Significance and Neurological Implications

When the right leg extends during testing of the left patellar tendon reflex, this indicates a pathological crossed adductor reflex, which strongly suggests an upper motor neuron lesion affecting the spinal pathways and requires prompt neurological evaluation.

Understanding the Abnormal Reflex Response

The patellar tendon reflex is normally a monosynaptic reflex that should only produce contraction of the ipsilateral quadriceps muscle when the patellar tendon is tapped. This results in extension of only the tested leg.

When testing reveals contralateral leg extension (right leg extending when left patellar tendon is tapped), this represents a pathological finding called a crossed adductor reflex that warrants further investigation:

Neurological Significance

• This finding indicates hyperreflexia with spread of the reflex response beyond the normal anatomical boundaries
• It suggests disruption of normal inhibitory pathways in the spinal cord
• The presence of this crossed reflex strongly points to an upper motor neuron lesion

Diagnostic Algorithm

1. Confirm the abnormal finding:

   • Repeat the patellar reflex test on both sides
   • Ensure proper positioning with legs hanging freely
   • Use consistent tapping force

2. Assess for other upper motor neuron signs:

   • Check for increased tone/spasticity in lower extremities
   • Test for Babinski sign (upgoing plantar response)
   • Evaluate for clonus at the ankle
   • Assess for weakness with characteristic upper motor neuron pattern

3. Localize the lesion:

   • If bilateral hyperreflexia with crossed responses: Consider cervical or thoracic spinal cord pathology
   • If asymmetric findings: Consider lateralized lesion in brain or spinal cord

Potential Underlying Conditions

The crossed patellar reflex may indicate several serious neurological conditions:

• Cervical or thoracic myelopathy
• Multiple sclerosis
• Spinal cord compression
• Cerebrovascular accident affecting corticospinal tracts
• Space-occupying lesions affecting the spinal cord

Recommended Diagnostic Workup

Based on this abnormal reflex finding, the following diagnostic approach is recommended:

1. Complete neurological examination focusing on:

   • Motor strength in all extremities
   • Deep tendon reflexes throughout
   • Sensory examination including proprioception
   • Coordination testing

2. Imaging studies:

   • MRI of the cervical and thoracic spine should be prioritized to evaluate for spinal cord compression or myelopathy 1
   • Consider brain MRI if other signs suggest intracranial pathology

3. Electrophysiologic testing:

   • EMG and nerve conduction studies may help differentiate between upper and lower motor neuron disorders
   • Quantitative assessment of the patellar reflex using EMG can confirm hyperreflexia 2, 3

Clinical Pearls and Pitfalls

• Important distinction: A crossed adductor reflex is different from reinforcement techniques like the Jendrassik maneuver, which is used to enhance a normal reflex 3

• Avoid misinterpretation: Voluntary movement of the contralateral leg can sometimes be mistaken for a pathological reflex; ensure the patient is relaxed during testing

• Consider quantitative assessment: In cases where the clinical significance is unclear, objective measurement of reflex parameters using motion analysis or EMG can provide more definitive evidence of pathology 2, 4

• Don't delay evaluation: The presence of a crossed reflex should prompt urgent neurological consultation, as it may indicate serious underlying pathology requiring timely intervention

This abnormal reflex finding represents an important clinical sign that should not be dismissed as a normal variant, as it may be the earliest indication of significant neurological disease affecting the central nervous system.