Equine Peripheral Neuropathy Study Guide

Overview and Clinical Importance

Peripheral neuropathies represent a significant category of neurological disease in horses frequently tested on the NAVLE. These conditions involve damage to peripheral nerves outside the CNS, resulting in motor dysfunction, sensory deficits, or both. The three most commonly tested peripheral neuropathies are facial nerve paralysis (CN VII), femoral nerve paralysis, and radial nerve paralysis.

Peripheral nerve injuries most commonly result from trauma, compression during recumbency (particularly during anesthesia), dystocia in foals, or secondary to systemic diseases such as equine protozoal myeloencephalitis (EPM). Nerve regeneration occurs at approximately 1 mm per day (roughly 1 inch per month).

Classification of Peripheral Nerve Injuries

The Seddon classification categorizes nerve injuries by severity:

Section 1: Facial Nerve Paralysis (CN VII)

Anatomy of the Facial Nerve

The facial nerve originates from the facial nucleus in the rostral medulla oblongata, exits through the petrous temporal bone, then through the stylomastoid foramen. It divides into auricular, auriculopalpebral, buccal, and cervical branches.

• Auricular branches: Motor to ear muscles
• Auriculopalpebral: Motor to eyelid (orbicularis oculi)
• Buccal branches: Motor to lips/nostrils - most vulnerable
• Cervical branch: Motor to platysma

Etiology of Facial Nerve Paralysis

Clinical Signs of Facial Nerve Paralysis

• Muzzle deviation: AWAY from affected side
• Ear droop: Ipsilateral (same side)
• Ptosis: Upper eyelid drooping
• Absent palpebral reflex: Cannot blink
• Lip droop and ptyalism: Drooling
• Feed impaction: Food in cheek (quidding)

Diagnosis and Treatment

Diagnostic Approach:

• Complete neurological examination
• Guttural pouch endoscopy (gold standard for THO)
• Skull radiography or CT
• CSF analysis and EPM serology if indicated
• EMG 5-7 days post-injury

Treatment:

• Corticosteroids: Dexamethasone 0.05-0.1 mg/kg IV q24h
• NSAIDs: Phenylbutazone 2.2-4.4 mg/kg PO q12h
• DMSO 1 g/kg IV as 10% solution
• Eye protection: Artificial tears, fly mask
• Treat underlying cause (EPM: Ponazuril 5-10 mg/kg PO q24h)

Section 2: Femoral Nerve Paralysis

Anatomy of the Femoral Nerve

The femoral nerve arises from L3-L5 (primarily L4-L5) spinal nerves and provides:

• Motor to quadriceps femoris: Extends the stifle
• Motor to iliopsoas: Flexes the hip
• Sensory via saphenous nerve: Medial limb surface

Etiology

• Dystocia (most common in foals): Excessive traction stretches nerve
• Post-anesthetic: Prolonged dorsal recumbency
• Trauma: Ilial, femoral, or pelvic fractures
• Rhabdomyolysis: Associated muscle/nerve damage

Clinical Signs

• Crouched stance: Fetlocks flexed, toes on ground
• Inability to bear weight: Limb buckles
• Stifle collapse: Cannot lock in extension
• Absent patellar reflex: Key diagnostic finding
• Quadriceps atrophy: Within 1-2 weeks
• Can advance limb: Hock flexion intact (sciatic preserved)

Differential Diagnosis

Treatment: Supportive care - stall rest, NSAIDs, discourage standing attempts if acute. Prognosis: Fair to guarded; unilateral better than bilateral.

Section 3: Radial Nerve Paralysis

Anatomy of the Radial Nerve

The radial nerve is the largest brachial plexus nerve, arising from C6-T2 (predominantly T1). It travels through the musculospiral groove of the humerus.

• Motor to triceps brachii: Extends elbow
• Motor to extensor carpi radialis: Extends carpus
• Motor to digital extensors: Extends digits
• Sensory: Craniolateral forearm (variable)

Etiology

• Post-anesthetic: Most common; lateral recumbency compression
• Humeral fracture: Nerve vulnerable in spiral groove
• Direct trauma: Kicks, falls
• Dystocia (foals): Birth compression
• First rib/vertebral fractures: Nerve root laceration

Clinical Signs

High (Proximal) Radial Nerve Paralysis

• "Dropped elbow": Classic pathognomonic sign
• Inability to bear weight: Cannot extend elbow/carpus/digits
• Knuckled-over fetlock: Dorsum of hoof on ground
• Toe dragging: Collapses during weight-bearing
• Muscle atrophy: Triceps, extensors within 2-4 weeks

Low (Distal) Radial Nerve Paralysis

• Can extend elbow: Triceps function preserved
• Can bear weight: If hoof placed flat
• Knuckling at fetlock: Cannot extend carpus/digits

Diagnosis and Treatment

Diagnosis:

• Clinical exam: dropped elbow, knuckled fetlock
• Weight-bearing test: can briefly bear if positioned
• Radiography: rule out humeral fracture
• EMG at 7 days: denervation potentials

Treatment:

• NSAIDs, DMSO
• Limb bandaging/splinting (include carpus)
• Contralateral limb support (prevent laminitis)
• Physical therapy, electroacupuncture
• Strict stall rest

Prognosis: Mild cases (neurapraxia): days to weeks. Severe: months to years or never. Distal injuries better than proximal. No improvement by 2 weeks likely permanent.

Memory Aid - "DROP THE EXTENSION": Dropped elbow, Radial nerve from C6-T2, Only extensor supply, Post-anesthetic most common

Comparative Summary Table