Peripheral Nervous System Pathology Overview

Nerve Injury

Classification of Peripheral Nerve Disease

By Nerve Type	By Anatomy	By Pathology
Mixed	NMJ	Neuronopathy : The Whole nerve cell
Motor	Distal / Proximal	Axonopathy : The axon
Sensory	Root / Spinal Cord	Demyelination: Schwann cell

Types of Injury

Neuropraxia:	Temporary interruption of conduction - no histological change.
Axonotmesis:	Axon interupted but basal lamina intact (e.g. crush injury) Regeneration occurs along original path - prospect for functional recovery good.
Neurotmesis:	Axon and basal lamina interupted (e.g. transection injury) Success of regeneration depends on finding the correct distal stump

Clinical Signs with Peripheral Nerve Disease

Motor Neuropathy Signs (LMN signs)	Sensory Neuropathy	Autonomic Neuropathy
Flaccid paresis/paralysis of innervated structures	Decreased or abherrant 'pain' response or sensation	Anisocoria (dilated / constricted pupils)
Neurogenic muscle atrophy (very rapid)	Proprioceptive defects	Decreased tear production
Reduced / absent reflexes and muscle tone	Abnormal sensation (paraesthesia)/sensitivity (dysthesia)	Decreased salivation
	Reduced / absent reflexes without muscle atrophy	Bradycardia

Peripheral Nerve Disease in Small Animals

Disease	Pathology	Clinical Signs	Diagnosis	Prognosis + Treatment
Distal Denervating Disease	Distal axonopathy, possibly of toxic origin	Tetraparesis Dysphonia / Aphonia Neck Weakness Tetraparesis developing to tetraplegia in severe cases floppy dog Sensory function preserved: Pain but no withdrawal	Biopsy / EMG unhelpful	Very good prognosis Recovery in 3-6 weeks by axon regeneration Good nursing to prevent pressure sores in the recumbant animal
Idiopathic Polyradiculoneuritis	Immune mediated demyelination often involving roots, directed against unknown epitopes in peripheral myelin. May be post infection May get axonal injury if severe	As for distal denervating (although more rapid onset) May have sensory involvement.	By clinical signs CSF changes if roots involved Slower nerve conduction H reflexes and F-waves may be lost	Very good prognosis Recovery in 3-6 weeks by remyelination Immunosuppresive levels of corticosteroids (although may delay remyelination)
Cauda Equina Traction	Tail pull injury Esp. cats after RTA Lesion via longitudinal traction Sacrocaudal dislocation/fracture	Limp tail Incontinence Hindlimb Paresis	Diagnosis on Clinical signs and history +/- Radiographs to show dislocation/fracture	Prognosis difficult to predict Poor prognosis if tail limp & no anal tone Supportive treatment Persist for >3 months if possible
Peripheral Nerve Tumours	Nerve sheath tumours Generally tumours of Schwann cells Common in dog Brachial plexus	Chronic single forelimb lameness Weakness Muscle Atrophy Pain in Axilla LMN signs (may get UMN if grows into spinal cord	Clinical signs Imaging	Prognosis dependant on location Poor prognosis if in spinal cord Amputation of limb?
Feline Dysautonomia	Common in 1980's - Sporadic now Degeneration (toxic?) of autonomic ganglia	Mainly Parasympathetic Vomiting 3rd Eyelid protrusion Dilated pupils + poor PLR Reduced Lacrimation Megaoesophagus Bradycardia	Clinical signs	Poor Prognosis Supportive therapy only Similar sporadic disease seen in dogs

Peripheral Nerve Disease in Large Animals

Disease	Pathology	Clinical Signs	Diagnosis	Prognosis + Treatment
Idiopathic Laryngeal Hemiplagia (ILH)	Progressive loss of fibres (esp. large diameter) within left recurrent laryngeal nerve compared to right Dying back axonopathy with myelin sheath involvement: Degenerating fibres Regenerating clusters De/Re-myelination Flattening of nerve between aorta & trachea	Inspiratory 'roaring' noise - flapping of vocal fold	'Slap test' - adduction of contralateral arytenoid during expiration Endoscopic examination - assymetric arytenoids, poor abduction of left vocal fold. Palpable atrophy of laryngeal musculature	Laryngeal Ventriculectomy (Hobday procedure) 'Tie Back' Laryngeal muscle prosthesis Prognosis usually good
Cauda Equina Neuritis	Equine version of Idiopathic polyradiculoneuritis Extradural nerve roots of cauda equina thickened and discoloured Inflammatory infiltrate (lymphocytes, plasma cells, macrophages) Extensive axonal damage and demyelination Cranial nerve involvement often occurs Aetiology unknown: Antecedent infection? Antibodies to PNS myelin?	Paralysis & anaesthesia of tail Urinary incontinence Loss of anal reflex Failure to defaecate Pain/hypersensitivity in gluteal/tail-head area	Clinical signs	Recovery unlikely - most animals are destroyed.
Stringhalt	Causes poorly understood - similar signs caused by sweat pea plant ingestion (lathyrism) Distal axonopathy (esp. large diameter fibres)	Abrupt onset continuous / intermittent hyperflexion of one or both hind limbs May also have ataxia, urinary incontinence, perineal flaccidity	Differential diagnosis : Upward fixation of patella	May get spontaneous recovery Move pasture Tenectomy of lateral digital extensor may help
Equine Motor Neuron Disease	Generalised LMN disorder Degeneration of Ventral horn motor neuron Chromatolysis Neurofilament accumulation Gliosis Wallerian degeneration Denervation changes in muscle	Weight Loss Muscle atrophy Generalised Weakness Short strided gait + narrow based stance Trembling Sweating and fasiculations Increased recumbency	Elevated CK CSF protein Denervation of EMG	May progress to constant recumbency (destroy), stabilise or improve
Suprascapular Nerve Injury	"Sweeney" Commonly damaged by horse colliding into objects Fibrous entrapment as nerve reflected around wing of scapula Atrophy of supra- and infra- spinatous muscles	Lateral luxation of shoulder when weight bearing Muscle wasting around shoulder	Clinical signs History	No more than 30cm regrowth in 12 months expected due to irreversible muscle fibrosis.

Other Polyneuropathies

• Tick Paralysis
• Vincristine Toxicity
• Endocrine polyneuropathy
  • Dogs - hypothyroidism, insulinoma, diabetes mellitus
  • Cats - diabetes mellitus
• Genetic polyneuropathies
• Protozoan polyradiculoneuritis
• Idiopathic facial paralysis (e.g. Bell's Palsy)
• Botulism