Canine Intervertebral Disc Disease Study Guide

Overview and Clinical Importance

Intervertebral disc disease (IVDD) is the most common cause of spinal cord injury in dogs and represents a significant category of neurological disease on the NAVLE examination. This condition occurs when the intervertebral disc degenerates and herniates, causing compression and/or contusion of the spinal cord. Understanding the pathophysiology, clinical presentation, diagnostic approach, and treatment options is essential for successful case management.

The intervertebral disc consists of two main components: the nucleus pulposus (a gelatinous center rich in proteoglycans and water) and the annulus fibrosus (a concentric ring of fibrous lamellae composed primarily of collagen). Disc degeneration leads to loss of shock-absorbing capacity and can result in herniation and spinal cord compression.

Pathophysiology and Classification

Hansen Classification System

The Hansen classification system, developed in the 1950s, remains the foundation for understanding IVDD pathology. The system is based on the type of disc degeneration and the nature of disc herniation.

Hansen Type I (Disc Extrusion)

Pathogenesis: Chondroid metaplasia of the nucleus pulposus occurs, characterized by replacement of notochordal cells by chondrocytes, loss of glycosaminoglycans, increased collagen content, and decreased water content. This leads to dehydration, mineralization (calcification), and loss of hydroelastic properties. The degenerated nucleus pulposus becomes rock-like and can acutely extrude through a ruptured annulus fibrosus into the vertebral canal.

Clinical features: Acute onset, often following a jump or sudden movement. Causes both contusion (impact injury) and compression of the spinal cord. Most commonly affects young to middle-aged (2-6 years) chondrodystrophic breeds.

Breed predisposition: Dachshund (highest risk), French Bulldog, Beagle, Basset Hound, Pekingese, Cocker Spaniel, Shih Tzu, Welsh Corgi, Lhasa Apso. The FGF4 retrogene on chromosome 12 is associated with chondrodystrophy and early disc degeneration.

Hansen Type II (Disc Protrusion)

Pathogenesis: Fibroid metaplasia occurs with slow maturation of the disc. The nucleus becomes fibrocyte-like and there is progressive infiltration of the annulus fibrosus by fibroid nuclear material. The annulus thickens dorsally and protrudes into the vertebral canal without rupture.

Clinical features: Chronic, progressive onset over weeks to months. Primarily compressive rather than contusive injury. Clinical signs may wax and wane. Mineralization is rare.

Breed predisposition: Older (5+ years) non-chondrodystrophic large breed dogs, especially German Shepherd Dog, Labrador Retriever, Doberman Pinscher, Rottweiler.

Hansen Type III / ANNPE (Acute Non-Compressive Nucleus Pulposus Extrusion)

Pathogenesis: Also known as high-velocity, low-volume disc extrusion or traumatic disc extrusion. A small amount of healthy, hydrated nucleus pulposus extrudes through an annular tear under high pressure, typically during exercise or trauma. Causes severe spinal cord contusion with minimal compression.

Clinical features: Peracute onset (often during exercise), non-progressive, asymmetric deficits common. Non-painful in many cases after initial event. Prognosis generally favorable if deep pain intact.

Comparison of Hansen Types

Neuroanatomical Localization

Anatomical Regions

Thoracolumbar IVDD (T3-L3): Accounts for 66-87% of all IVDD cases. Most commonly affected disc spaces are T12-T13 and T13-L1 in chondrodystrophic breeds, and T13-L1, L1-L2, L2-L3 in large breed dogs. This region is predisposed due to the transition from a rigid thoracic spine (with rib support) to a more mobile lumbar spine.

Cervical IVDD (C1-C5 or C6-T2): Accounts for approximately 15-16% of cases. The larger vertebral canal-to-spinal cord ratio in the cervical region allows larger disc extrusions to occur without severe neurological deficits. C2-C3 and C3-C4 are most commonly affected.

Lumbosacral IVDD (L4-S3): Affects the cauda equina (peripheral nerve roots) rather than the spinal cord itself. Common in large breed working dogs. Presents with lumbosacral pain, pelvic limb lameness, and lower motor neuron signs.

Clinical Signs by Neuroanatomical Localization

Board Tip - Memory Aid for UMN vs LMN: "UMN = Upper (increased reflexes, spastic paralysis, no muscle atrophy) vs LMN = Lower (decreased reflexes, flaccid paralysis, rapid muscle atrophy)." The lesion affects UMN segments above and LMN segments at the level of injury.

Neurological Grading System

The 5-point neurological grading scale is critical for determining treatment approach and prognosis. This scale reflects the progressive loss of spinal cord function from peripheral (proprioception) to central (deep pain perception) pathways.

Deep Pain Perception Assessment

Deep pain perception (DPP) is the most critical prognostic indicator in IVDD. It is tested by applying heavy pressure with hemostats to the bone of a toe. A positive response requires a conscious behavioral response such as turning the head toward the stimulus, vocalizing, or trying to bite.

Clinical Presentation

Thoracolumbar IVDD

• Spinal hyperesthesia (pain on palpation of affected region)
• Reluctance to walk, jump, or climb stairs
• Kyphosis (arched back posture)
• Hindlimb ataxia or paresis progressing to paralysis
• Urinary and fecal incontinence in severe cases
• Schiff-Sherrington posture in severe acute cases (rigid forelimb extension with hindlimb paralysis)

Cervical IVDD

• Cervical hyperpathia (severe neck pain)
• Low head carriage, muscle spasms, guarded neck movement
• Root signature (unilateral forelimb lameness from nerve root compression)
• Tetraparesis or tetraplegia in severe cases (less common than TL region)

Diagnostic Approach

Survey Radiography

Role: Screening tool; useful for identifying disc calcification, narrowed disc spaces, and ruling out other pathology (fractures, neoplasia, diskospondylitis). However, radiographs only confirm IVDD in approximately 60% of cases.

Radiographic findings: Narrowed intervertebral disc space (70% of extrusions); mineralized/calcified disc material within disc space or vertebral canal (42% of extrusions); increased opacity of intervertebral foramen; spondylosis deformans (more common in Type II).

Advanced Imaging

MRI (Gold Standard): Best modality for evaluating soft tissue structures including spinal cord, intervertebral discs, and nerve roots. Allows assessment of spinal cord edema, hemorrhage, and compression severity. Can differentiate between Type I, Type II, ANNPE, and other myelopathies. T2-weighted hyperintensity in the spinal cord may indicate edema or myelomalacia.

CT (with or without myelography): Excellent for visualizing mineralized disc material and bone detail. Faster acquisition than MRI (shorter anesthesia). CT myelography combines CT with intrathecal contrast to outline spinal cord compression. Particularly useful for surgical planning.

Myelography: Injection of contrast into subarachnoid space to outline spinal cord compression. Largely superseded by CT and MRI but remains a reasonable option when cross-sectional imaging unavailable. Shows extradural compression pattern with IVDD.

Treatment Options

Conservative (Medical) Management

Indications: Grade 1-2 IVDD on first presentation; financial constraints; anesthetic risk patients; patient improving on medical therapy.

Components:

• Strict cage rest: 4-6 weeks in a crate or small confined area. Only leash walks for elimination. This is the MOST important component of conservative management.
• Analgesia: Multimodal approach preferred. Options include NSAIDs (carprofen, meloxicam), gabapentin/pregabalin for neuropathic pain, muscle relaxants (methocarbamol, diazepam), and opioids for severe pain.
• Anti-inflammatory therapy: NSAIDs OR low-dose corticosteroids (never combine). If using steroids, anti-inflammatory doses only (0.5-1 mg/kg/day prednisone).
• Bladder management: Manual expression or indwelling catheter if urinary retention present.
• Physical rehabilitation: After initial rest period, gradual return to activity with physiotherapy support.

Common Medications for IVDD

Surgical Management

Indications: Grade 3-5 IVDD; failure of conservative management; rapidly deteriorating neurological status; recurrent episodes; severe uncontrolled pain.

Goals: Decompression of spinal cord; removal of extruded disc material; restoration of blood flow to spinal cord.

Surgical Procedures

Prognosis by Grade and Treatment

Complications

Progressive Myelomalacia (PMM)

Definition: Progressive ascending and/or descending hemorrhagic necrosis of the spinal cord following acute IVDE. This is a devastating, typically fatal complication.

Incidence: Occurs in 2-14.5% of dogs with Grade 5 (DPP-negative) IVDD. Risk is higher with: younger age (less than 5.8 years), lumbar intumescence involvement (L4-S3), peracute onset (non-ambulatory within 24 hours), and French Bulldog breed (up to 33% in one study).

Clinical signs: Cranial progression of cutaneous trunci reflex cutoff; transition from UMN to LMN signs in hindlimbs (flaccid paralysis); loss of anal tone; progressive tetraparesis; eventual respiratory paralysis and death.

Prognosis: Uniformly fatal. No effective treatment. Euthanasia recommended when diagnosis confirmed.

Recurrence

Recurrence of clinical signs at the same or different disc space occurs in 2-27% of surgically treated dogs (studies vary) and approximately 50% of medically managed dogs. Dogs with multiple calcified discs at initial presentation are at higher risk. Prophylactic fenestration may reduce recurrence risk.

Memory Aids for NAVLE

DACHSHUND = "D.A.C.H.S.H.U.N.D." Disc disease (Type I most common) Acute onset typical Chondrodystrophic breed Hemilaminectomy for T-L surgery Strict cage rest for conservative Rx Hansen classification UMN signs with T3-L3 lesions Neurological grading 1-5 Deep pain = most important prognostic factor

Neurological Grade Memory: "Pain → Paresis → Paralysis → Pain-free" Grade 1: Pain only (best prognosis) Grade 2: Paretic but walking (ambulatory paresis) Grade 3: Paretic, cannot walk (non-ambulatory paresis) Grade 4: Paralyzed but feels pain (DPP+) Grade 5: Paralyzed AND no deep pain (DPP-, worst prognosis)