How do I distinguish all possible nerve lesions that cause foot drop?

Distinguishing Nerve Lesions That Cause Foot Drop

To accurately diagnose the specific nerve lesion causing foot drop, perform a systematic neurological examination focusing on muscle strength patterns, sensory changes, and reflex abnormalities, then confirm with electrodiagnostic studies.

Anatomical Localization of Foot Drop

Common Peroneal Nerve Lesion (Most Common Cause)

• Location: At the fibular neck/head
• Key findings:
  • Weakness of ankle dorsiflexion and eversion
  • Sensory loss over lateral lower leg and dorsum of foot
  • Preserved inversion strength and posterior tibial function
  • Normal ankle reflex
• Common causes:
  • Compression at fibular head (leg crossing, prolonged squatting)
  • Trauma/fractures
  • External compression (casts, tight bandages)
  • Habitual leg crossing (most frequent cause) 1

L5 Radiculopathy

• Location: L5 nerve root
• Key findings:
  • Weakness of ankle dorsiflexion, inversion AND eversion
  • Weakness of hip abduction (gluteus medius)
  • Back pain radiating to lateral leg
  • Sensory loss in L5 dermatome (lateral leg, dorsum of foot, first web space)
  • Preserved ankle reflex

Sciatic Nerve Lesion (Partial)

• Location: Proximal sciatic nerve with predominant common peroneal division involvement
• Key findings:
  • Foot drop with variable hamstring weakness
  • Sensory loss in peroneal AND tibial distributions
  • Possible weakness of plantar flexion
  • Diminished or absent ankle reflex

Lumbar Plexopathy

• Location: Lumbar plexus
• Key findings:
  • Foot drop with hip flexion/adduction weakness
  • Quadriceps weakness (L4 involvement)
  • Sensory loss in femoral and peroneal distributions
  • Diminished knee and ankle reflexes

Anterior Horn Cell Disease (Motor Neuron Disease)

• Location: Spinal cord
• Key findings:
  • Foot drop with widespread fasciculations
  • No sensory loss
  • Hyperreflexia (upper motor neuron) or hyporeflexia (lower motor neuron)
  • Progressive weakness in multiple limbs

Peripheral Neuropathy (especially diabetic)

• Location: Distal symmetric polyneuropathy
• Key findings:
  • Bilateral, often asymmetric foot drop
  • Distal sensory loss in "stocking" distribution
  • Loss of protective sensation (10g monofilament test) 2
  • Often associated with pain, especially at night

Diagnostic Approach

Step 1: Focused Neurological Examination

1. Muscle strength testing of key muscles:

   • Tibialis anterior (dorsiflexion) - L4/L5, deep peroneal nerve
   • Peroneus longus/brevis (eversion) - L5/S1, superficial peroneal nerve
   • Tibialis posterior (inversion) - L4/L5, tibial nerve
   • Gluteus medius (hip abduction) - L5, superior gluteal nerve
   • Quadriceps (knee extension) - L2-L4, femoral nerve 3

2. Sensory examination:

   • Test pinprick, temperature, vibration (128-Hz tuning fork) 2
   • Map sensory loss pattern to identify specific nerve/root involvement

3. Reflex testing:

   • Ankle reflex (S1)
   • Knee reflex (L3-L4)
   • Plantar response (Babinski sign for upper motor neuron lesions)

Step 2: Electrodiagnostic Studies

• Nerve conduction studies to:
  • Localize site of lesion
  • Determine severity (demyelinating vs axonal)
  • Assess prognosis for recovery 1
• Electromyography (EMG) to:
  • Identify denervation patterns
  • Distinguish between acute and chronic lesions
  • Detect subclinical involvement of other muscles

Step 3: Imaging Studies

• MRI of lumbosacral spine: For suspected radiculopathy
• MRI of knee/fibular head: For suspected peroneal neuropathy
• Ultrasound: To visualize nerve compression or mass lesions
• CT/MRI of pelvis: For suspected plexopathy or proximal sciatic lesion

Special Considerations

Diabetic Neuropathy Assessment

• Perform comprehensive foot examination annually in all diabetic patients 2
• Test for loss of protective sensation (LOPS) using:
  • 10g monofilament test PLUS at least one other neurological assessment
  • Pinprick, temperature perception, ankle reflexes, or vibration with 128-Hz tuning fork 2
• Assess for peripheral arterial disease (PAD) which often coexists 2

Acute vs. Chronic Presentation

• Acute onset: Consider trauma, compression, vascular causes
• Subacute/chronic: Consider neuropathy, radiculopathy, tumor
• Progressive: Consider motor neuron disease, hereditary neuropathy

Bilateral vs. Unilateral Presentation

• Unilateral: More likely local compression, trauma, radiculopathy
• Bilateral: More likely polyneuropathy, motor neuron disease

Common Pitfalls to Avoid

1. Failing to distinguish peroneal neuropathy from L5 radiculopathy

   • Test tibialis posterior (inversion) strength: preserved in peroneal neuropathy, weak in L5 radiculopathy

2. Missing partial sciatic nerve lesions

   • Check hamstring strength and ankle reflex

3. Overlooking diabetic neuropathy

   • Always assess for loss of protective sensation in diabetic patients 2

4. Neglecting ankle sprains as a cause

   • Inversion ankle sprains can cause peroneal nerve injury through traction or compression 4

5. Missing central causes of foot drop

   • Check for upper motor neuron signs (spasticity, hyperreflexia, Babinski sign)

By systematically evaluating muscle strength patterns, sensory changes, and reflex abnormalities, clinicians can accurately localize the site of nerve injury causing foot drop and direct appropriate treatment.