What are the step-by-step investigations for localizing the lesion in patients with foot drop for neurology residents and fellows?

🧠 Step-by-Step Localization of Foot Drop Lesions

For neurology residents and fellows evaluating foot drop, systematically localize the lesion from cortex to peripheral nerve using targeted clinical examination followed by electrodiagnostic studies and imaging tailored to the suspected level. 1, 2

🎯 Step 1: Clinical Localization Through Examination

🔍 Upper Motor Neuron vs Lower Motor Neuron Signs

Start by determining if this is a central or peripheral lesion:

• Look for hyperreflexia, spasticity, and Babinski sign → suggests cortical, subcortical, or spinal cord pathology 1, 3
• Look for hyporeflexia/areflexia, muscle atrophy, and fasciculations → suggests anterior horn cell, nerve root, plexus, or peripheral nerve lesion 1, 3

🦵 Pattern of Weakness Distribution

Map out exactly which muscles are weak beyond the dorsiflexors:

• Isolated ankle/toe dorsiflexion weakness with intact foot eversion → common peroneal nerve at fibular head (most common cause) 1, 3
• Dorsiflexion AND eversion weakness, but preserved plantar flexion → sciatic nerve (peroneal division) or common peroneal nerve proximal to fibular head 1, 2
• Dorsiflexion weakness PLUS hip abduction/knee flexion weakness → lumbosacral plexus or L5 root 1, 2
• Dorsiflexion weakness PLUS other L5 myotomal involvement (hip extension, knee flexion, foot inversion) → L5 radiculopathy 1, 2
• Bilateral foot drop with upper motor neuron signs → parasagittal cortical lesions (consider brain metastases or stroke) 4, 5

🔬 Sensory Examination Patterns

Sensory loss patterns are critical for localization:

• Sensory loss over dorsum of foot between first and second toe only → superficial peroneal nerve or common peroneal nerve 1, 2
• Sensory loss extending up lateral leg → common peroneal nerve or L5 root 1, 2
• Sensory loss in L5 dermatomal distribution (lateral leg, dorsum of foot, medial foot) → L5 radiculopathy 1, 2
• Sensory loss in sciatic distribution (posterior thigh, entire lower leg/foot except medial leg) → sciatic neuropathy 1, 2

🦴 Reflex Testing

Check ankle reflexes bilaterally:

• Preserved ankle reflex with foot drop → common peroneal nerve or L5 root (ankle reflex is S1) 6, 1
• Diminished/absent ankle reflex with foot drop → sciatic nerve, lumbosacral plexus, or combined L5/S1 radiculopathy 6, 1

🩺 Special Examination Maneuvers

Look for specific clinical clues:

• Tinel's sign at fibular head → common peroneal nerve compression 1, 3
• Straight leg raise positive → L5 radiculopathy 1, 2
• Pain in buttock/posterior thigh → sciatic neuropathy or lumbosacral plexopathy 1, 2
• History of habitual leg crossing → common peroneal neuropathy (most frequent reversible cause) 1

⚡ Step 2: Electrodiagnostic Studies (First-Line Confirmatory Test)

Nerve conduction studies (NCS) and electromyography (EMG) are the first-line tests for lesion localization, establishing severity, and predicting recovery. 1, 2

📊 Nerve Conduction Studies

Perform these specific studies:

• Common peroneal motor NCS → look for conduction block or slowing across fibular head (confirms peroneal neuropathy at fibular head) 1, 2
• Tibial motor NCS → if abnormal with peroneal abnormality, suggests sciatic nerve or lumbosacral plexus lesion 1, 2
• Superficial peroneal sensory NCS → if absent, confirms peroneal nerve involvement 1, 2
• Sural sensory NCS → if abnormal, suggests sciatic nerve or polyneuropathy rather than isolated peroneal nerve 1, 2

🔌 Electromyography Patterns

EMG needle examination localizes the lesion by identifying denervation patterns:

• Denervation in tibialis anterior, extensor hallucis longus, extensor digitorum brevis ONLY → common peroneal nerve distal to fibular head 1, 2
• Denervation in above muscles PLUS peroneus longus/brevis → common peroneal nerve at or proximal to fibular head 1, 2
• Denervation in peroneal-innervated muscles PLUS tibial-innervated muscles (gastrocnemius, tibialis posterior) but SPARING short head of biceps femoris → sciatic nerve below sciatic notch 1, 2
• Denervation in all sciatic-innervated muscles INCLUDING short head of biceps femoris → sciatic nerve at sciatic notch or lumbosacral plexus 1, 2
• Denervation in L5 myotomes (tibialis anterior, tibialis posterior, gluteus medius) with SPARING of S1 muscles and normal paraspinals → lumbosacral plexus 1, 2
• Denervation in L5 myotomes INCLUDING paraspinal muscles → L5 radiculopathy 1, 2

🖼️ Step 3: Imaging Based on Localization

MR neurography has emerged as the key imaging modality for foot drop, but tailor imaging to the suspected lesion level. 2, 3

🧲 For Suspected L5 Radiculopathy

Order lumbar spine MRI with focus on L4-L5 and L5-S1 levels:

• Look for disc herniation, foraminal stenosis, or facet hypertrophy compressing L5 nerve root 2, 3
• MRI is the imaging modality of choice for radiculopathy 2

🧲 For Suspected Lumbosacral Plexopathy

Order MRI of lumbar plexus and pelvis:

• Look for masses, hematomas, or infiltrative processes in psoas muscle or pelvis 2, 3
• Consider contrast enhancement to identify tumors or inflammatory plexopathy 2

🧲 For Suspected Sciatic Neuropathy

Order MRI of pelvis and thigh (sciatic nerve course):

• Look for masses, hematomas, or nerve compression along sciatic nerve from sciatic notch to popliteal fossa 2, 3
• MR neurography can characterize nerve signal abnormality and identify exact site of injury 2

🧲 For Suspected Common Peroneal Neuropathy

Order MRI or ultrasound of knee/fibular head:

• Ultrasound is increasingly used for peroneal nerve evaluation at fibular head 2, 3
• MR neurography can identify nerve compression, masses (ganglion cysts), or intraneural pathology 2, 3
• CT may identify fibular head fractures or bony abnormalities 2

🧠 For Bilateral Foot Drop or Upper Motor Neuron Signs

Order brain MRI immediately:

• Look for parasagittal cortical lesions (stroke, metastases, or other mass lesions) 4, 5
• Bilateral foot drop from central causes is rare but critical not to miss 4, 5
• Consider spinal cord MRI if myelopathy suspected 2, 3

⚠️ Critical Pitfalls to Avoid

🚨 Common Diagnostic Errors

• Don't assume all foot drop is peroneal neuropathy → 30-40% of cases are NOT at the fibular head 1, 2
• Don't miss bilateral foot drop from parasagittal brain lesions → always examine both legs and check for upper motor neuron signs 4, 5
• Don't rely on imaging alone without electrodiagnostic studies → EMG/NCS establishes severity and prognosis 1, 2
• Don't forget to ask about leg crossing habits → most common reversible cause of peroneal neuropathy 1

🎯 When to Suspect Central Causes

Consider brain imaging when:

• Bilateral foot drop is present 4, 5
• Upper motor neuron signs are present (hyperreflexia, spasticity, Babinski) 3, 5
• Electrodiagnostic studies are normal despite clear clinical foot drop 4, 2
• Patient has known cancer or recent subarachnoid hemorrhage 4, 5

🔄 Algorithmic Approach Summary

1️⃣ Clinical exam → Determine UMN vs LMN, map weakness/sensory pattern, check reflexes 1, 3

2️⃣ Electrodiagnostic studies → NCS/EMG to confirm level and severity 1, 2

3️⃣ Targeted imaging → MRI of spine (radiculopathy), pelvis/thigh (plexus/sciatic), knee (peroneal), or brain (central) 2, 3

4️⃣ Identify reversible causes → Leg crossing, masses, hematomas, compressive lesions 1, 3

5️⃣ Determine surgical candidacy → Nerve decompression for compressive peroneal neuropathy or spine surgery for radiculopathy 3