What does an absent patellar reflex indicate?

Understanding Absent Patellar Reflex

An absent patellar reflex most commonly indicates a neurological abnormality affecting the L3-L4 spinal nerve roots or segments, which may suggest radiculopathy, neuropathy, or other neurological conditions requiring further evaluation.

Neuroanatomical Basis

The patellar reflex (knee-jerk reflex) is a deep tendon reflex that primarily involves:

• Afferent pathway: L3-L4 spinal nerve roots (primarily L4)
• Efferent pathway: Femoral nerve innervating the quadriceps muscle
• Spinal cord integration: L3-L4 spinal cord segments

Clinical Significance of Absent Patellar Reflex

Common Causes

1. Radiculopathy

   • L3-L4 nerve root compression (most common)
   • May present with associated pain, sensory changes, or weakness
   • Often caused by disc herniation, spinal stenosis, or epidural masses 1

2. Peripheral Neuropathy

   • Diabetic neuropathy
   • Alcohol-related neuropathy
   • Vitamin B12 deficiency
   • Guillain-Barré syndrome

3. Age-Related Changes

   • Elderly patients may have diminished or absent reflexes without pathology
   • Studies show significantly higher proportion of absent patellar reflexes in patients over 10 years old (in canine studies) 2

4. Spinal Cord Disorders

   • Myelopathy
   • Cauda equina syndrome
   • Spinal cord injury

Associated Signs to Evaluate

When encountering an absent patellar reflex, assess for:

• Sensory changes in L3-L4 dermatomes (anterior thigh and medial leg)
• Motor weakness in quadriceps (knee extension)
• Other reflex abnormalities (compare with contralateral side)
• Toe flexor response upon patellar percussion (may indicate radiculopathy) 3
• Adductor reflex abnormalities (can help differentiate causes of decreased patellar reflexes) 4

Diagnostic Approach

History Elements to Focus On

• Onset (sudden vs. gradual)
• Associated symptoms (pain, numbness, weakness)
• Trauma history
• Medical conditions (diabetes, alcohol use)
• Medication use (especially those affecting neurological function)

Physical Examination

1. Proper Technique for Testing

   • Patient seated with legs dangling freely
   • Patellar tendon struck with reflex hammer
   • Compare bilaterally for symmetry

2. Additional Neurological Testing

   • Test other deep tendon reflexes
   • Assess muscle strength in lower extremities
   • Evaluate sensation in relevant dermatomes
   • Check for Babinski sign and clonus

Imaging and Diagnostic Studies

For patients with absent patellar reflexes and concerning symptoms:

• MRI lumbar spine without contrast is the preferred initial study for suspected radiculopathy after 6 weeks of conservative management 1
• Electromyography (EMG) and nerve conduction studies to differentiate between radiculopathy and peripheral neuropathy
• Laboratory studies (glucose, HbA1c, vitamin B12, TSH) to evaluate for metabolic causes

Special Considerations

Unilateral vs. Bilateral Absence

• Unilateral: More suggestive of focal pathology (radiculopathy)
• Bilateral: More suggestive of systemic process (neuropathy)

Inverted Patellar Reflex

An absent quadriceps reflex with distant toe flexor response (when tapping the patellar tendon) is a sign similar to the inverted brachioradialis reflex and may localize to L3-L4 levels 3.

Age Considerations

• In elderly patients, absent reflexes may be a normal finding
• In pediatric patients, absent reflexes are more likely to be pathological

When to Be Concerned

Urgent evaluation is needed when an absent patellar reflex is accompanied by:

• Progressive neurological deficits
• Bladder or bowel dysfunction (suggesting cauda equina syndrome)
• Recent trauma
• Severe or worsening pain
• Multiple absent reflexes

Conclusion

An absent patellar reflex is an important neurological finding that requires careful evaluation within the clinical context. While it may represent a benign finding in some cases (particularly in elderly patients), it often indicates underlying neurological pathology affecting the L3-L4 nerve roots or peripheral nerves that warrants further investigation.