Canine Peripheral Neuropathy Study Guide

Overview and Clinical Importance

Peripheral neuropathy refers to disorders affecting the peripheral nervous system (PNS), which includes all nerves outside the brain and spinal cord. These conditions cause dysfunction of motor, sensory, and/or autonomic nerve fibers, resulting in weakness, ataxia, decreased reflexes, and muscle atrophy. Peripheral neuropathies represent a significant category of neurological disease in canine medicine and are frequently tested on the NAVLE due to their clinical importance and diagnostic complexity.

The peripheral nervous system connects the central nervous system to muscles, organs, and sensory receptors throughout the body. Dysfunction can occur at the level of the nerve cell body (neuronopathy), the axon (axonopathy), the myelin sheath (demyelinating neuropathy), or at the neuromuscular junction. Understanding the anatomical and pathophysiological basis of peripheral neuropathies is essential for accurate diagnosis and management.

Anatomy and Pathophysiology

Peripheral Nervous System Structure

The peripheral nervous system consists of cranial nerves (12 pairs), spinal nerves (36 pairs in dogs: 8 cervical, 13 thoracic, 7 lumbar, 3 sacral, and 5-7 caudal), and the autonomic nervous system (sympathetic and parasympathetic components).

Nerve Structure Components

• Neuron cell body: Located in dorsal root ganglia (sensory) or ventral horn of spinal cord (motor)
• Axon: Long projection that conducts electrical impulses; can extend from spinal cord to distal extremities
• Myelin sheath: Fatty insulation produced by Schwann cells that enables rapid saltatory conduction
• Nodes of Ranvier: Gaps between myelin segments where action potentials are regenerated

Pathological Processes in Peripheral Neuropathy

Classification of Canine Peripheral Neuropathies

Classification by Nerve Type Affected

Acquired Peripheral Neuropathies

Acute Polyradiculoneuritis (Coonhound Paralysis)

Acute polyradiculoneuritis (APN) is the most common acute peripheral neuropathy in dogs. This immune-mediated condition is considered the canine equivalent of Guillain-Barré syndrome (GBS) in humans. The classic form, "Coonhound paralysis," occurs 7-14 days after raccoon exposure, but APN can occur without raccoon contact and has been associated with Campylobacter jejuni infection (often from raw chicken consumption), vaccination, and other antigenic stimuli.

Pathophysiology

An immune-mediated attack primarily targets the ventral (motor) nerve roots and peripheral nerves. Anti-ganglioside antibodies (particularly anti-GM2) have been detected in affected dogs. The inflammatory infiltrate causes demyelination and axonal damage, with clinical signs reflecting the severity and distribution of nerve involvement.

Clinical Signs

• Onset: Acute, rapidly progressive over 24-72 hours
• Progression: Ascending paralysis starting in pelvic limbs, progressing to tetraparesis/tetraplegia
• LMN signs: Flaccid paralysis, hyporeflexia to areflexia, hypotonia, rapid muscle atrophy (within 3-5 days)
• Sensory function: Preserved (sensation intact despite motor paralysis)
• Cranial nerve involvement: Facial weakness, dysphonia (voice change), dysphagia possible
• Tail wagging: Often preserved (helpful diagnostic feature)
• Respiratory involvement: Severe cases may develop respiratory failure requiring ventilation

Diagnostic Findings

Treatment and Prognosis

Hypothyroid Neuropathy

Hypothyroid neuropathy is the most common neurological manifestation of canine hypothyroidism. It predominantly affects older, large-breed dogs and can present with a variety of neurological syndromes affecting both peripheral and central nervous systems.

Clinical Presentations

• Generalized polyneuropathy: LMN weakness, hyporeflexia, muscle atrophy, proprioceptive deficits; often pelvic limbs initially, progressing to all four limbs
• Peripheral vestibular syndrome: Head tilt, nystagmus, ataxia; 9 of 29 dogs in one study
• Facial nerve paralysis: Unilateral or bilateral; inability to blink, lip droop, decreased palpebral reflex
• Laryngeal paralysis: Stridor, exercise intolerance, voice change
• Megaesophagus: Regurgitation, aspiration pneumonia risk

Diagnostic Workup and Treatment

Tick Paralysis

Tick paralysis is an acute ascending motor paralysis caused by neurotoxins in tick saliva. In North America, Dermacentor variabilis (American dog tick) and Dermacentor andersoni (Rocky Mountain wood tick) are the primary causative species. In Australia, Ixodes holocyclus causes a more severe form with higher mortality.

Pathophysiology and Clinical Signs

The tick neurotoxin inhibits the release of acetylcholine at the neuromuscular junction (presynaptic block), preventing muscle contraction. Clinical signs develop 5-9 days after tick attachment and include ascending LMN paralysis beginning in the pelvic limbs, progressing to tetraparesis, respiratory muscle involvement, and potentially death if untreated.

Diabetic Neuropathy

Diabetic neuropathy is more common in cats than dogs but can occur in poorly controlled diabetic dogs. Prolonged hyperglycemia causes metabolic dysfunction leading to axonal degeneration and demyelination. Clinical signs include tibial nerve dysfunction resulting in a plantigrade stance ("dropped hocks"), pelvic limb weakness, and proprioceptive deficits. Treatment focuses on strict glycemic control with insulin; neurological improvement typically occurs over weeks to months with adequate blood glucose regulation.

Inherited Peripheral Neuropathies

Inherited polyneuropathies are breed-specific conditions typically presenting in young dogs. Many are analogous to Charcot-Marie-Tooth disease in humans. Recognition of breed predispositions is essential for diagnosis and genetic counseling.

Breed-Specific Inherited Neuropathies

Diagnostic Approach to Peripheral Neuropathy

Step 1: Neurological Localization

The first step is confirming that clinical signs are consistent with peripheral nervous system disease (LMN signs) rather than upper motor neuron or neuromuscular junction disease.

Step 2: Diagnostic Testing

Minimum Database

• CBC, serum chemistry, urinalysis: Screen for systemic disease (diabetes, renal disease)
• Thyroid panel (TT4, fT4, TSH): Essential in any older dog with peripheral neuropathy
• Thoracic radiographs: Evaluate for megaesophagus, aspiration pneumonia, thymic mass (myasthenia gravis)
• Acetylcholine receptor antibody titer: If myasthenia gravis suspected

Electrodiagnostic Testing

Electromyography (EMG) and nerve conduction velocity (NCV) studies are the gold standard for evaluating peripheral nerve function. These tests require general anesthesia and specialized equipment.

Advanced Diagnostics

• CSF analysis: Albuminocytologic dissociation (elevated protein, normal cells) in polyradiculoneuritis
• Nerve/muscle biopsy: Definitive diagnosis for inherited and inflammatory neuropathies
• Genetic testing: Available for some breed-specific neuropathies (Alaskan Malamute - NDRG1 mutation)
• Infectious disease testing: Neospora caninum, Toxoplasma gondii titers if infectious cause suspected

Differential Diagnosis of Acute LMN Tetraparesis

Treatment Principles

Supportive Care for Recumbent Patients

• Padded bedding: Prevent pressure sores; turn patient every 2-4 hours
• Bladder care: Manual expression or catheterization; monitor for UTI
• Nutrition: Elevated feeding if megaesophagus; feeding tube if severe dysphagia
• Respiratory monitoring: Pulse oximetry, blood gas; prepare for mechanical ventilation if needed
• Physical therapy: Passive range of motion, hydrotherapy, massage to prevent contractures and atrophy

Medications for Neuropathic Pain and Support