Jonathan M. Levine,     College Station, Texas

Pathophysiology of Intervertebral Disk Disease

The intervertebral disk has three anatomic zones: the anulus fibrosus, nucleus pulposus, and cartilaginous end plate. The anulus fibrosus arises from mesenchymal cells and consists of overlapping lamellae, which are predominantly composed of type I collagen. These lamellae attach to the cartilaginous end plate and are capable of parallel motion during biomechanical loading. In some species, the outermost portions of the anulus are supplied by minute blood vessels and innervated by small, penetrating nerve fibers. The nucleus pulposus is centrally located, bounded by the anulus fibrosus, and contains an abundance of extracellular matrix. It arises from remnant notochordal and chondrocyte-like cells. These cell populations are responsible for the synthesis and maintenance of proteoglycans that bind extracellular water. The cartilaginous end plate serves as a connection between the bony end plate and anulus fibrosus. The hyaline cartilage that composes this structure has pores, which are essential in providing nutrition and removing waste materials from the largely avascular nucleus and anulus.

Epidemiology and Clinical Signs

Dogs with IVDH typically are young to middle-aged males of chondrodystrophoid breeds. Dachshunds have been reported to represent 48% to 72% of affected animals and may have a lifetime incidence of IVDH that approaches 20% (Levine et al, 2011). In one study, 83.6% of dogs with IVDH had compression located in the thoracolumbar vertebral column, whereas 16.4% had lesions in the cervical vertebral column (Gage, 1975).

The clinical signs associated with IVDH are variable. In the cervical vertebral column, IVDH may result in hyperesthesia, root signature, tetraparesis, and general proprioceptive ataxia in all limbs. Severely affected dogs may be tetraplegic and require mechanical ventilation. Many dogs with cervical IVDH have hyperesthesia as their only clinical sign, perhaps because the ratio of vertebral canal diameter to spinal cord diameter is larger in the cervical region than in other vertebral column locations.

In dogs with thoracolumbar IVDH either the T3 to L3 or L4 to S3 spinal cord can be involved. Animals may have hyperesthesia, paraparesis or paraplegia, pelvic limb general proprioceptive ataxia, urinary voiding disability, fecal incontinence, and loss of pelvic limb nociception. Animals with acute IVDH localized to T3 to L3 may have Schiff-Sherrington syndrome. In some instances of acute IVDH, spinal shock can occur. Spinal shock results from interruption of corticospinal tracts and is manifest as a transient loss of pelvic limb spinal reflexes in the setting of an upper motor neuron lesion (Smith and Jeffery, 2005). About 5% to 10% of dogs with IVDH that lack pelvic limb deep nociception develop ascending-descending myelomalacia. The clinical signs of myelomalacia may include severe hyperesthesia, loss of pelvic limb reflexes in the setting of a compression located in the T3 to L3 vertebral column, anal dilation, flaccid bladder paralysis, and cranial migration of the cutaneous trunci reflex. In some instances myelomalacia may result in thoracic limb paresis and ventilatory compromise due to involvement of the cervical spinal cord.

Spinal Cord Injury Scores

Recently several groups have advocated the routine use of physical examination–based SCI scores in dogs with IVDH and other myelopathies. Examples of currently validated systems are the modified Frankel score, Texas Spinal Cord Injury Score, and 14-point gait score (Table 233-1) (Levine et al, 2011). These systems allow for objective, reliable measurement of clinical facets of SCI, such as nociception and gait. Scoring systems enhance medical record keeping, facilitate clinician communication, and provide objective functional milestones during recovery.