What conditions can cause leg weakness, hyperreflexia, and femoral nerve enhancement?

Differential Diagnosis: Leg Weakness with Femoral Nerve Enhancement and Hyperreflexia

The combination of leg weakness, femoral nerve enhancement on imaging, and hyperreflexia most strongly suggests an inflammatory lumbosacral plexopathy, though atypical Guillain-Barré syndrome (particularly acute motor-sensory axonal neuropathy variant) must be considered when hyperreflexia is present.

Primary Diagnostic Consideration: Inflammatory Lumbosacral Plexopathy

The triad of symptoms you describe points toward a focal inflammatory process affecting the lumbosacral plexus with motor fiber predominance 1. Key distinguishing features include:

• Asymmetric fascicular enhancement of lumbosacral plexus components on MR neurography
• Proximal lower limb weakness with relative distal sparing
• Preserved or brisk reflexes (including possible ankle clonus) despite peripheral nerve involvement
• Severe pain provoked by minimal stretch maneuvers (straight leg raise, hip extension)
• Relapsing course with episodic worsening despite immunotherapy

The presence of hyperreflexia in this context is paradoxical but explained by selective motor fiber involvement at the plexus level while preserving upper motor neuron pathways 1. Electrodiagnostic studies typically show reduced compound muscle action potentials in femoral distribution, abundant fibrillation potentials in proximal muscles, but preserved sensory responses and no conduction block 1.

Secondary Consideration: Atypical Guillain-Barré Syndrome

GBS can present with preserved or even hyperactive reflexes in specific variants, particularly acute motor axonal neuropathy (AMAN) or acute motor-sensory axonal neuropathy (AMSAN) subtypes 2, 3. This occurs in:

• Pure motor variants without sensory involvement
• AMAN subtype on electrophysiological examination
• Early disease course before areflexia develops

However, classic GBS features that would support this diagnosis include 2:

• Ascending pattern starting distally in legs, progressing to arms and cranial muscles
• Bilateral and relatively symmetric involvement (though asymmetry can occur)
• Acute onset reaching maximum disability within 2 weeks (not beyond 4 weeks)
• CSF showing albuminocytologic dissociation (elevated protein, normal cell count)
• Preceding infection within 6 weeks (particularly Campylobacter jejuni, cytomegalovirus, Zika virus)

The presence of femoral nerve enhancement on imaging is NOT typical for GBS, which primarily affects nerve roots and would show nerve root enhancement rather than isolated femoral nerve enhancement 2.

Critical Diagnostic Algorithm

Step 1: Imaging Characteristics

• MR neurography of lumbosacral plexus (not routine spine MRI) with orthogonal views, T1-weighted, T2-weighted, fat-saturated sequences 4
• Look for: fascicular enhancement pattern, asymmetry, associated neurogenic muscle edema
• Femoral nerve-specific enhancement → inflammatory plexopathy more likely
• Nerve root enhancement → consider GBS or radiculopathy

Step 2: Pattern of Weakness

• Proximal > distal weakness with femoral distribution → plexopathy
• Ascending symmetric weakness → GBS
• Check for upper motor neuron signs: sustained clonus, Babinski sign (if present, consider central causes like spinal cord pathology 5)

Step 3: Reflex Pattern

• Brisk reflexes with peripheral nerve enhancement → inflammatory plexopathy with selective motor involvement 1
• Hyperreflexia with AMAN/AMSAN features on EMG → atypical GBS 3
• Progressive hyperreflexia developing over time → reconsider central etiology 5

Step 4: Electrodiagnostic Studies

• Reduced motor amplitudes with preserved sensory responses → motor-predominant plexopathy 1
• Demyelinating features (conduction block, temporal dispersion) → AIDP variant of GBS
• Axonal features with reduced amplitudes, no conduction block → AMAN/AMSAN or axonal plexopathy

Step 5: Clinical Course

• Relapsing-remitting with treatment resistance → inflammatory plexopathy 1
• Monophasic with plateau phase → GBS 2
• Evolving upper motor neuron signs → urgent repeat imaging for spinal cord pathology 5

Common Pitfalls to Avoid

Do not assume all peripheral neuropathies cause hyporeflexia. Hyperreflexia can occur in motor-predominant plexopathies and AMAN variants of GBS 2, 1, 3. The key is recognizing that selective motor fiber involvement at the plexus level can spare reflex arcs while upper motor neuron pathways remain intact.

Do not order routine spine MRI when plexopathy is suspected. Standard spine imaging lacks the specialized sequences needed to visualize plexus pathology 4. Specifically request "MR neurography of lumbosacral plexus" with dedicated plexus protocols.

Do not dismiss central causes if hyperreflexia worsens or new upper motor neuron signs emerge. Spinal cord infarction can initially mimic peripheral neuropathy but will show progressive spasticity and require urgent vascular workup 5.

Other Etiologies to Consider

Compressive or perioperative femoral neuropathy 6, 7, 8:

• Most common causes of isolated femoral neuropathy (40% compressive, 35% perioperative stretch)
• Typically presents with weakness (96%), sensory loss (73%), pain (53%)
• Reflexes are typically reduced or absent, not hyperactive
• Enhancement pattern would be focal to femoral nerve, not diffuse plexus involvement
• Recovery prolonged (mean 14.8 months) and often incomplete (only 48% near-complete recovery)

The presence of hyperreflexia makes isolated femoral neuropathy from compression or stretch injury unlikely 6, 7.