NAVLE Nervous

Equine Cauda Equina Syndrome Study Guide

📅 March 28, 2026 ⏱ 25 min read

Overview and Clinical Importance

Cauda equina syndrome in horses, more accurately termed polyneuritis equi (PNE), is an uncommon but clinically significant neurological disease characterized by progressive granulomatous inflammation of the sacrococcygeal nerve roots and occasionally cranial nerves. First described by Dexler in 1897, this condition represents an immune-mediated polyneuropathy with pathologic similarities to Guillain-Barre syndrome in humans.

The term "cauda equina" derives from Latin meaning "horse's tail," referring to the bundle of spinal nerve roots extending from the caudal spinal cord. While this anatomical structure exists in all mammals, horses are highly susceptible to this specific inflammatory condition affecting these nerve roots.

High-YieldOn the NAVLE, polyneuritis equi (PNE) is the preferred terminology over "cauda equina neuritis" because cranial nerves are frequently affected beyond the cauda equina.

Structure	Description	Clinical Significance
Conus Medullaris	Tapered distal end of spinal cord at S2-S3	Contains sacral segments; lesions cause bilateral symmetric signs
Cauda Equina	Bundle of L6-S5 and coccygeal nerve roots	Primary site affected in PNE; innervates tail, perineum, bladder, rectum
Filum Terminale	Fibrous band extending from conus to coccyx	Anchors spinal cord; can be involved in inflammation
Lumbar Cistern	CSF-filled subarachnoid space	Site for lumbosacral CSF collection

Anatomical Foundations

The Cauda Equina in Horses

In horses, the spinal cord terminates at the sacrococcygeal junction (S2-S3 level), forming the conus medullaris. Distal to this point, the lumbosacral and coccygeal spinal nerve roots continue within the vertebral canal as the cauda equina, suspended in cerebrospinal fluid within the lumbar cistern.

Key Anatomical Structures

Cauda Equina Nerve Root Functions

Nerve Roots	Motor Function	Sensory Function
Sacral (S1-S5)	Anal sphincter, bladder detrusor, urethral sphincter, tail muscles	Perineum, caudal thigh, tail base
Coccygeal	Coccygeal muscles, tail movement	Tail sensation
Pudendal (S2-S4)	External anal/urethral sphincters, penis retraction	Perineum, external genitalia
Pelvic (S2-S4)	Parasympathetic: bladder contraction, rectal motility	Visceral sensation from pelvic organs

Etiology and Pathogenesis

Proposed Immune-Mediated Mechanism

The exact etiology remains unknown, but strong evidence supports an immune-mediated pathogenesis. The disease shares similarities with Guillain-Barre syndrome in humans. The prevailing theory suggests autoimmunity against myelin P2 protein causes nerve fiber destruction.

Key pathogenic features:

Anti-P2 myelin protein antibodies detected in affected horses
T-lymphocyte and B-lymphocyte infiltration of affected nerves
CD8+ cytotoxic T-cells predominate over CD4+ helper T-cells
Primary demyelination progressing to granulomatous polyneuritis

Proposed Infectious Triggers

Pathologic Findings

Gross Pathology

Thickened, firm extradural and intradural nerve roots of cauda equina
Nerves may appear diffusely white or discolored by hemorrhage
Marked muscle atrophy of sacrocaudalis dorsalis and ventralis
Sabulous cystitis secondary to bladder atony

Histopathology

Granulomatous neuritis with lymphocytes, macrophages, multinucleated giant cells
Demyelination and axonal degeneration
Epineurial, perineurial, and endoneurial fibrosis

Agent	Evidence	Status
EHV-1	Isolated from some cases	Not confirmed causative
Equine Adenovirus	Isolated from cauda equina	Bystander mechanism proposed
Streptococcus equi	History of strangles in some cases	Possible post-infectious trigger
Sarcocystis neurona	Recent research suggests association	Under investigation

Epidemiology

Factor	Details
Species	Horses and ponies of all breeds
Age	Adults; rarely yearlings, NEVER foals
Sex	No definitive predisposition; some reports suggest females
Breed	No breed predisposition
Geography	Reported in North America and Europe
Incidence	Uncommon; true incidence unknown

Clinical Signs

Disease Progression

PNE typically presents as a slowly progressive, chronic disease. Signs progress through characteristic phases:

Early Phase (Hyperesthesia): Tail rubbing, hypersensitivity to touch

Progressive Phase (Hypoesthesia): Decreased sensation progressing to analgesia

Late Phase (Paralysis): Complete paralysis of tail, anus, rectum, bladder

Cauda Equina Signs (Most Common)

Cranial Nerve Signs (Distinguishes PNE)

Cranial nerve involvement distinguishes "polyneuritis equi" from "cauda equina neuritis." Cranial signs are typically asymmetric, while caudal signs are symmetric.

NAVLE TipWhen you see symmetric cauda equina signs + asymmetric cranial nerve signs, think PNE immediately. This combination is the clinical hallmark.

System	Clinical Signs	Mechanism
Tail	Flaccid paralysis, absent tail tone	Coccygeal nerve damage
Perineum	Analgesia; loss of perineal reflex	Pudendal nerve sensory loss
Bladder	Urinary incontinence, dribbling, distension	Pelvic nerve dysfunction
Rectum/Anus	Fecal retention, hypotonic sphincter	Pudendal and pelvic nerve damage
Male Reproductive	Penile paralysis, prolapse	Pudendal nerve motor loss
Hindquarters	Gluteal atrophy, mild pelvic limb weakness	Lumbosacral plexus extension

Diagnosis

Polyneuritis equi is a diagnosis of exclusion. No definitive antemortem test exists; confirmation requires histopathology at necropsy.

Diagnostic Tests

High-YieldLumbosacral CSF collection is the most useful antemortem test. Xanthochromic CSF with elevated protein strongly supports PNE. Atlantooccipital CSF may be normal.

Cranial Nerve	Clinical Signs	Frequency
CN VII (Facial)	Facial paralysis, ear/lip droop, decreased tears	Most common
CN VIII (Vestibulocochlear)	Head tilt, nystagmus, vestibular ataxia	Common
CN V (Trigeminal)	Masseter atrophy, decreased facial sensation	Less common
CN IX, X	Dysphagia, difficulty swallowing	Rare

Differential Diagnosis

Exam Focus: Sacral fractures are MORE COMMON than PNE - always rule out first! PNE affects ADULTS (never foals) and is slowly progressive, while fractures are acute.

Test	Expected Findings	Utility
Lumbosacral CSF	Xanthochromia; protein 100-300 mg/dL; mixed pleocytosis	Most useful antemortem test
Sacral Radiography	Normal (rules out fracture)	Essential to exclude trauma
Rectal Palpation	Normal sacrum; distended bladder	Rules out fracture; assesses complications
Ultrasonography	Enlarged, hypoechoic nerve roots	Supportive; visualizes thickened nerves
Muscle Biopsy	Neurogenic atrophy; inflammatory infiltrates	Supports antemortem diagnosis

Treatment and Management

There is no effective curative treatment. Management is palliative.

Differential	Distinguishing Features	Key Tests
Sacral Fracture	Acute onset; history of trauma; MORE COMMON	Rectal palpation; radiography
EPM	Asymmetric ataxia; multifocal signs	S. neurona serology/CSF
EHV-1	Acute; fever; multiple horses affected	PCR nasal swab; serology
Sorghum Toxicosis	History of sorghum pasture; cystitis	History; remove from pasture
Rabies	Progressive; behavioral changes; ALWAYS consider	Postmortem brain FA

Prognosis

The prognosis is POOR to GRAVE. Disease is slowly progressive and irreversible. Most horses are euthanized.

Disease progression is relentless despite treatment
Corticosteroids provide only temporary, short-lived improvement
Secondary complications significantly impact quality of life
Euthanasia typically recommended within weeks to months

High-YieldOn NAVLE questions, "poor prognosis" or "euthanasia" is almost always correct regarding PNE outcome.

Treatment	Protocol	Notes
Corticosteroids	Dexamethasone 0.1-0.2 mg/kg IV/IM; taper	Short-lived benefit; monitor for laminitis
Bladder Management	Manual expression 2-3x daily; treat UTI	Prevent overdistension; culture urine
Fecal Management	Manual evacuation; dietary management	Monitor for colic; hydration
Skin Care	Clean/dry perineum; barrier creams	Prevents urine scald dermatitis

Memory Aids

"CAUDA" Mnemonic

C - Coccygeal/tail paralysis

A - Anal sphincter hypotonia

U - Urinary incontinence

D - Dysesthesia/anesthesia (perineal)

A - Atrophy (gluteal and tail muscles)

Key Diagnostic Clues

"Adult horse + Progressive tail/perineal paralysis + NO trauma = Think PNE"

"Symmetric caudal + Asymmetric cranial = Classic PNE"

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