日本医科大、駒込学園、根津、千駄木、谷中、上野、田端、西日暮里、犬、猫

Disease description:
Anatomy
Intervertebral disks permit mobility of the vertebral column by acting as deformable tissue between individual vertebral bodies. In simple terms, an intervertebral disk is a shock absorber. Normally, the disk is subject to five possible loading conditions, namely axial compression, tension, bending, shear, and torsion. It must be flexible enough to permit the extremes of movement, yet rigid enough to withstand the normal physiological forces acting on the vertebral column. The disk is composed of 2 parts, the inner nucleus pulposis and the outer fibrous ring (called the annulus fibrosis). The nucleus pulposis normally consists of a gelatinous type of material, whereas the annulus fibrosis is a fibrous type of material that holds the nucleus in place.

Etiology
The annulus fibrosis is thicker ventrally than dorsally, which may be one reason why most disk extrusions/protrusions affect the spinal cord. The pathway of least resistance in a disk rupture is through the thinnest part of the annulus fibrosis. Degenerative changes within the disk structure (i.e. disk disease) can result in disk herniation and spinal cord injury.

Pathophysiology
Classically there are two types of disk herniation, extrusions (Hansen type I) and protrusions (Hansen type II). Which type that occurs is often based on the type of degeneration that occurs in the nucleus pulposis of the disk. Differences exist between the two types in the onset of clinical signs and the breeds affected.

Hansen Type I disk degeneration is most frequently associated with sudden disk extrusions in chondrodystrophoid dogs. Degeneration of the disk occurs in these breeds within the first 2 years of life but disk herniations may never occur or may occur at any time thereafter. The disk dehydrates and the nucleus pulposis is replaced by hyaline cartilage, which can sometimes be observed on survey radiographs as a mineralized density within the intervertebral disk space. The dehydrated disk loses it shock absorbing capacity and normal movements can result in herniation of the nucleus through the annulus fibrosis. This type of herniation is often sudden in onset. The amount of prolapsed material can be large and occupy more than 50% of the neural canal.

Hansen Type II disk degeneration is associated with more gradual onset of clinical signs caused by a fibroid metamorphosis of the nucleus pulposis and degeneration of the annulus fibrosus. Disk herniation may result in an incomplete rupture of the annulus, with partial protrusion of nuclear material into the spinal canal. Alternatively, the nuclear material may be contained within a bulging annulus and the bulging annulus compresses the spinal cord. These protrusions are considerably smaller and have a more limited and regular shape than Hansen Type I extrusions. They may occur in any breed but older, large breed dogs and non-chondrodystrophoid breeds are more typically affected. Type II protrusions are frequently accompanied by arthritic changes of the vertebral joints.

Nondegenerated Nucleus Pulposis Extrusion, a third type of disk herniation, has been recognized in recent years.¹ This type of herniation is associated with strenuous exercise or trauma and results in spinal cord contusion, with little or no residual cord compression.

Clinical Signs
Hansen Type I disk herniations are rather explosive. They are often associated with significant hemorrhage, spinal cord trauma, and sudden pain, weakness, or paralysis.

Hansen Type II disk protrusions are usually slower in onset and are often progressive over weeks to months. Conscious proprioceptive deficits, ataxia, and weakness are the predominate signs. Some type II protrusions also are lateralized to one side, producing weakness on one side or in one limb.^2-4

Diagnosis
Definitive diagnosis of intervertebral disk herniation is usually made on the basis of advanced diagnostic imaging (i.e. magnetic resonance imaging [MRI], computed tomography [CT], CT/myelography, myelography).⁵ MRI has been shown to be superior to other modalities in identifying the location of disk herniation and degree of spinal cord compression.⁶ With experience, careful positioning and technique, plain radiography may be used to identify thoraco-lumbar disk herniation but is not currently considered sufficiently diagnostic or definitive for surgical intervention.

Disease description in this species:
Intervertebral disk disease (IVDD) and herniation are more common in the chondrodystrophic breeds, especially the dachshund.⁷ A recent study suggests that a major locus on chromosome 12 harbors genetic variations that affect the development of intervertebral disc calcification in the dachshund.⁸ Disc degeneration occurs before 2 years of age in chondrodystrophic breeds but age of onset of actual disc herniation is usually 3-6 years. In non-chondrodystrophic breeds, the peak incidence of disk herniations is 6-8 years of age.

Cervical Herniations: In a recent study of cervical disk lesions, the most common site for Type I herniation was between cervical vertebrae 2 and 3 (C2-3), while C6-7 was the most common site for Type II herniation.⁹ With cervical herniations, neck pain is usually present. In one retrospective study, 90/103 dogs manifested neck pain.¹⁰ Because of the larger space present in the cervical vertebral canal, a large amount of material may herniate without causing tetraplegia. However, if the extrusion is very large or explosive, tetraplegia may occur.¹¹ Occasionally the disk may protrude towards one side creating compression of a nerve root and subsequent lameness (i.e. root signature sign of holding up or dragging a forelimb).

Thoracolumbar Herniations: Over 3/4 of thoracolumbar disk lesions occur between thoracic vertebrae T11-12 and lumbar vertebrae L1-2. When herniations occur in the thoracolumbar region, sudden pelvic limb paralysis or severe weakness may occur. The spinal canal is much smaller in this area, so there is no extra space for any extraneous material. However, if the extrusion is small, the affected patient may only exhibit acute onset of pain, ataxia, and/or conscious proprioceptive deficits in the pelvic limbs. Lateralized disc herniation may produce acute lameness.^2-4 As more spinal cord damage occurs, pelvic limb weakness increases, paralysis can occur, and deep pain perception may be lost. Surgical intervention is considered for cases that have persistent pain, discomfort, or weakness that does not respond to medical treatment; for cases that are paralyzed with intact deep pain perception; and for cases that are progressing despite appropriate medical treatment.

Clinical Signs
Grading Scheme for Clinical Signs
Severity of clinical signs seen with thoracolumbar IVDD can be graded according to the following scheme:

• Grade 1: Pain only

• Grade 2: Ataxia, conscious proprioceptive deficits, paresis

• Grade 3: Paraplegia

• Grade 4: Paraplegia, urinary retention and overflow

• Grade 5: Paraplegia, urinary retention and overflow, loss of deep pain perception

Myelomalacia
A small portion of dogs with acute disk hernation develop myelomalacia of the spinal cord, which is a progressive, often fatal condition that arises secondary to hemorrhagic or ischemic necrosis of the spinal cord parenchyma. At surgery, the consistency of the spinal cord is soft and more like toothpaste rather than the normal firmness. Clinically, affected dogs have upper motor paraplegia (i.e. exaggerated pelvic limb spinal reflexes) and no deep pain perception in the pelvic limbs. Within hours to days neurologic findings may change. If the malacia descends from the original thoracolumbar site, the pelvic limb reflexes become depressed or absent, and tail tone and anal reflexes may be loss. If the malacia ascends, then the cutaneous trunci reflex moves cranially. For example, if it was originally absent at L1, it may now be absent at T11 or above. Some dogs develop a fever, which may be due to pyrogens released as the spinal cord becomes malacic.

Sometimes the malacia continues to ascend until the front limbs and respiration are affected. In other cases the malacia stops for unknown reasons. Deep pain perception and the ability to walk/ambulate are never regained in myelomalacic cases. Predisposing factors for the development of this condition have not been identified. No treatment is successful in halting the progression of signs.

Schiff-Sherrington Signs
The term, Schiff-Sherrington Syndrome, has been applied to the clinical description of thoracic limb hypertonia/rigidity and paraplegia seen in dogs with acute, severe thoracolumbar spinal cord lesions. Even though the disk herniation is caudal to T2, the front limbs and sometimes the neck muscles are hyperextended. Motor ability of the front limbs is not affected; however, so these dogs should be able to perform hopping and wheel-barrowing motions when supported.

Hyperextension of the thoracic limbs can be explained by the fact that neurons located in the lumbar spinal cord (i.e. "border cells" located at L1-L7) give rise to axons that ascend (via the fasciculus propruis tract) to the cervical spinal cord that inhibit neurons to the extensor muscles of the front legs. When inhibition of the extensor muscles is lost, the front limbs assume a rigid, extended position. Although Schiff-Sherrington signs imply a severe spinal cord injury, prognosis is not necessarily grave unless the dog also has a concurrent lack of deep pain perception in the pelvic limbs.

Etiology:
Genetic, hereditary
Intervertebral disc disease
Trauma

Breed predilection:
American cocker spaniel
Beagle
Brachycephalic breeds
Chondrodystrophic breeds
Dachshund
Dandie dinmont terrier
Doberman pinscher
German shepherd dog
Large breed dogs
Pekingese
Pembroke Welsh corgi
Shih tzu

Sex predilection:
None

Age predilection:
Middle-aged
Young adult

Treatment/Management/Prevention:
SPECIFIC THERAPY
Surgical Treatment
Recurrent episodes of disk disease, unresolved pain, or evidence of neurologic deficits are indications for surgical intervention.

1) Treatment for cervical intervertebral disk herniation is usually by ventral slot decompression. Dorsal laminectomy and hemilaminectomy may also be considered.¹² Caudal cervical disk protrusions may require surgical distraction and stabilization in addition to ventral slot decompression.¹³

2) Treatment of thoracolumbar intervertebral disk herniation is usually by hemilaminectomy or dorsal laminectomy.¹⁴ Both procedures allow removal of material from within the spinal canal. In a study comparing the two methods, dogs that had undergone hemilaminectomy had significantly improved neurologic recovery at discharge and less deterioration of neurologic signs postoperatively.¹⁵
a) A small percentage of dogs may require a second surgery if not all the disk material is removed.¹⁶ Most dogs that are re-operated; however, actually have another disk herniation at another site and time.^16,17
b) Fenestration has been shown to reduce the incidence of subsequent intervertebral disk herniation following surgery.¹⁷

3) Treatment of lumbosacral intervertebral disk herniation is usually by dorsal laminectomy, which allows decompression.

Medical Treatment
Medical therapy and supportive care are recommended for dogs with pain and only slight neurological deficits, especially when the herniation is a first occurrence.

1) A consensus regarding the use of steroids for the treatment of spinal cord trauma, specifically for intervertebral disk herniation has been difficult to achieve in veterinary medicine, largely due to the lack of prospective, double-blinded clinical studies. In human medicine the routine use of steroids for spinal cord injuries was largely abandoned and considered "a harmful standard of care" following a 2002 report citing insufficient evidence to support the benefit of methylprednisolone sodium succinate (MPSS).^18,19 Veterinarians are urged to review the current literature regarding the potential benefits and side effects of corticosteroids in the treatment of spinal cord injury prior to prescribing these medications.^20,21,22

2) Nonsteroidal antiinflammatory drugs, opioids, gabapentin and acupuncture may be considered for unremitting pain.²³ Caution must be exercised in the use of these analgesics because if pain and inflammation subside, the patient may become more active and thereby cause more disk material to herniate.