What causes a diminished right patellar reflex?

Causes of Diminished Right Patellar Reflex

The most common causes of a diminished right patellar reflex include L2-L4 radiculopathy, peripheral neuropathy, and total knee arthroplasty, with radiculopathy being the most frequent etiology requiring prompt evaluation to prevent progression of neurological deficits.

Neurological Causes

1. Lumbar Radiculopathy (L2-L4)

• L2-L4 radiculopathy is a primary cause of diminished patellar reflexes 1
• Compression of nerve roots from herniated discs, spinal stenosis, or other space-occupying lesions
• Often accompanied by:
  • Low back pain radiating to the anterior thigh
  • Sensory changes in L2-L4 dermatomes
  • Possible quadriceps weakness
• In a study of patients with diminished patellar reflexes, 95.5% of those with L2-L4 radiculopathy had recordable but abnormal adductor reflexes 2

2. Peripheral Neuropathies

• Diabetic peripheral neuropathy commonly affects patellar reflexes
  • Prolonged latencies of reflexes are characteristic 2
  • Often bilateral but can be asymmetric
• Other neuropathies:
  • Alcoholic neuropathy
  • Vitamin B12 deficiency
  • Guillain-Barré syndrome (typically presents with ascending weakness and areflexia) 3
  • Chronic inflammatory demyelinating polyneuropathy

3. Cauda Equina Syndrome

• Compression of nerve roots from the lower cord segments
• Requires immediate imaging and intervention
• Associated with:
  • Urinary retention or incontinence
  • Bilateral motor weakness
  • Saddle anesthesia 3
  • Often multiple diminished reflexes

Non-Neurological Causes

1. Total Knee Arthroplasty

• Total knee arthroplasty significantly diminishes the patellar tendon reflex on the operative side
• Patients with unilateral TKA have a patellar reflex that is approximately 55.1% of their contralateral uninvolved side 4
• This is statistically significant compared to patients with knee arthritis awaiting TKA (96.03%) and those without knee arthritis (102.2%)

2. Knee Joint Pathology

• Severe knee arthritis can affect reflex testing but typically doesn't significantly diminish the reflex 4
• Knee effusion or pain may inhibit normal reflex response

3. Other Musculoskeletal Causes

• Quadriceps tendon injury
• Patellar tendon injury or inflammation
• Severe muscle atrophy

Diagnostic Approach

Initial Assessment

1. Neurological examination:

   • Test other reflexes (ankle, upper extremity)
   • Assess muscle strength in quadriceps and other muscle groups
   • Check for sensory deficits in dermatomal distribution
   • Evaluate for upper motor neuron signs (Babinski, hyperreflexia elsewhere)

2. Special tests:

   • Straight-leg-raise test for radiculopathy
   • Check for crossed adductor reflex (may be present with L2-L4 radiculopathy) 2
   • Assess for toe flexor response upon patellar percussion (sign of L3-L4 pathology) 5

Imaging

• MRI lumbar spine without IV contrast is the preferred initial imaging for suspected radiculopathy with persistent symptoms beyond 6 weeks of conservative management 1, 3
• Immediate MRI is indicated for:
  • Progressive neurological deficits
  • Suspected cauda equina syndrome
  • Suspected infection or malignancy 1
• CT may be considered when MRI is contraindicated 1

Clinical Pearls and Pitfalls

Important Considerations

• A unilateral diminished patellar reflex is more concerning than bilateral diminished reflexes
• Always check for history of knee surgery or trauma, as TKA can reduce the reflex by approximately 45% 4
• Asymmetry between right and left reflexes is more significant than absolute reflex strength
• Consider age-related changes in reflexes, as they may diminish with advancing age

Pitfalls to Avoid

• Misattributing a diminished reflex to lumbar radiculopathy in a patient with prior knee replacement 4
• Failing to recognize an absent quadriceps reflex with toe flexor response, which can indicate L3-L4 pathology 5
• Overlooking cauda equina syndrome, which requires emergency intervention 3
• Relying solely on imaging findings without clinical correlation, as high rates of asymptomatic degenerative changes exist on imaging 1

By systematically evaluating patients with diminished right patellar reflex and considering both neurological and non-neurological causes, clinicians can accurately diagnose the underlying condition and implement appropriate management strategies to prevent further neurological deterioration.