Animals may present with a variety of diseases that involve the spine or spinal cord. Often, the clinical signs and diagnostic evaluation allows a specific diagnosis to be made. This ultimately leads to an appropriate treatment plan that brings about resolution of the clinical signs. A certain percentage of animals, however, have spinal diseases that are difficult to diagnose, misdiagnosed, difficult to treat, or poorly responsive to available treatments. These cases can be frustrating for both the veterinarian and owner alike. Some of these diseases are discussed here with an emphasis on clarifying clinical diagnosis and planning for proper treatment.
Intervertebral Disk Disease
Intervertebral disk disease (IVD) remains a common cause of spinal disease. Considering the frequency that IVD is encountered in clinical practice, it is surprising that many aspects of diagnosis and treatment for this disease are controversial. This is due, in part, to inaccurate diagnoses, incomplete diagnoses, and poor objective follow-up evaluations on treatment outcomes. The clinical spectrum of IVD is varied and is only just beginning to be clarified due to increased used of transverse (MR or CT) diagnostic imaging.
Two basic types of intervertebral disk disease are described; however, Hansen’s classification scheme does not reflect the scope of anatomical and pathophysiologic changes encountered. Intervertebral disks may extrude nucleus pulposis or fragments of anulus fibrosis around, and occasionally, into the spinal cord. Intervertebral disk material may come to lie ventrally, laterally, and dorsally to the spinal cord. In some instances, intervertebral disk material may extrude laterally from the disk, and encounter and impinge upon exiting peripheral nerves. This latter syndrome results in pain as a primary complaint and is more common that previously recognized.
Creating a diagnostic dilemma, some dogs with disk disease (especially Hansen’s Type II) also have underlying primary spinal cord disease (e.g., degenerative myelopathy) that may contribute to the clinical signs (multiple disease processes). These underlying diseases may have a significant influence on outcomes following surgery.
Anomalies involving the spinal cord or vertebral column may result from abnormal development of the neural tube. Components of spinal dysraphism include spina bifida (defective fusion of the vertebral arch), meningocele (protrusion of meninges through the defect), myelocele (protrusion of spinal cord through the defect), and meningomyelocele (protrusion of both meninges and spinal cord). Occasionally, intramedullary defects such as hydro- or syringomyelia may accompany these other more obvious defects (see further).
Most congenital diseases of the spine are recognized earlier in life. Occasionally, however, congenital vertebral or spinal defects do not result in spinal abnormalities until adulthood or later. Some of these intraspinal cystic abnormalities may be missed with routine myelography. Intraspinal abnormalities are more often found using CT/myelography or MR imaging.
These diseases can be frustrating to treat as underlying anatomical abnormalities may not be reversible. Additionally, other underlying diseases can be present concurrent with these congenital defects making accurate diagnosis of the extent of spinal abnormalities imperative.
Syringomyelia and hydromyelia
Of all of the intraspinal cord abnormalities, syringomyelia and hydromyelia are most prevalent. Syringomyelia and hydromyelia are cystic abnormalities of the spinal cord. Syringomyelia refers to abnormal cavities filled with liquid within in the substance of the spinal cord. A “syrinx” refers to one of these cavities. Hydromyelia refers to a pathologic condition characterized by accumulation of fluid within an enlarged central canal of the spinal cord. In both of these instances, the fluid that accumulates is similar, if not identical, to cerebrospinal fluid (CSF).
The pathologic mechanisms that result in syringomyelia and hydromyelia are multiple. Possible pathogenic mechanisms of cyst formation include changes in CSF pressure relationships within the spinal cord (as occurs with hydrocephalus or foramen magnum abnormalities), loss of spinal parenchyma, stenosis of the central canal, and obstruction to CSF flow via inflammation or tumor. Each of these mechanisms is possible in dogs and cats but have yet to be definitively proved.
Syrinxes that do not communicate with the central canal (extracanalicular syrinxes) at any level are often acquired due to spinal injury or damage from disease (hemorrhage and inflammation). These syrinxes tend to occur in the central grey matter or dorsal and lateral white matter, possibly associated with changes in vascular distribution (“watershed zones”). Another important cause of syringo-/hydromyelia in humans is abnormal CSF dynamics at the level of the fourth ventricle/foramen magnum area. Abnormal pressure/fluid dynamics may then result in spinal cord cavitation or dilation of the central canal. Proposed explanations for this include arterial pulsation of CSF with pressure waves being transmitted to the cervical cord and increased CSF pressures resulting in transmedullary passage of CSF from the central canal and epidural space into the spinal cord parenchyma. Obstruction of CSF flow from the cranium to the spinal cord may result in pressure differentials that impact the spinal cord. Increased or differential CSF pressures within the spinal cord may result in vascular compromise to the cord, impairing venous drainage, and predispose to parenchymal damage.
Hydromyelia is often associated with hydrocephalus in humans. This has been reported rarely in dogs, but is likely under recognized. In an experimental hydrocephalic model in dogs, it was shown that with increased intraventricular pressure there are increases in pressures within the syrinx cavity.
More recent studies of humans with posterior fossa abnormalities (referred to as Chiari malformations), have shown that disequilibration and movement of CSF from the intracranial to the spinal subarachnoid space may be the underlying factor in perpetuating syringomyelia. If the foramen magnum is obstructed due to caudal displacement of the cerebellum, CSF cannot move in either direction. Cerebrospinal fluid, which cannot leave the intracranial space during systole, causes increased intracranial pressure. The pulsatile increase in pressure is transmitted down the spinal cord and appears to be an important factor in perpetuating the syrinx cavity. Cerebrospinal fluid may enter the syrinx by multiple microscopic connections of the syrinx with the subarachnoid space. Reversal of these excessive pressure pulsations occurs after decompressive surgery (craniectomy and durotomy) of the foramen magnum.
These malformations are frequently associated with hydrocephalus and syringo-/hydromyelias are commonly encountered concurrently. Obstruction of CSF flow at the foramen magnum seems to be the primary mechanism of both of these pathologic changes. Similar abnormalities may occur in dogs and other animals. Other abnormalities resulting in hydrocephalus and a dilated fourth ventricle may also be associated with syringomyelia. The Dandy-Walker syndrome in humans is one such example. With this disease, there is a malformation of the cerebellum resulting in a cyst-like abnormality in the cerebellum. The lateral and third ventricles are commonly dilated concurrently. This is a congenital problem assumed to be associated with abnormal embryogenesis. Examples of a similar syndrome have been described in dogs and other animals. The spinal cord abnormalities have not been described, however, the spinal cords of affected animals may not have been examined pathologically. Interestingly, a number of dogs and humans with syringomyelia have associated scoliosis.
The diagnosis of syringomelia can be difficult, as the abnormality is often not apparent following routine myelography. With lumbar injections, it is sometimes possible to fill the central canal with contrast medium, making a hydromyelia apparent. This is inconsistent—if the contrast medium does not fill the syringo-/hydromyelic cavity, myelography may be normal or show only an expanded spinal cord.
Other imaging studies, such as CT or magnetic resonance MR imaging are often more helpful in establishing a diagnosis. Magnetic resonance imaging may be better than CT for defining intraparenchymal spinal cord abnormalities.
In humans, the treatment of cranial cervical syringomyelia with or without caudal fossa abnormalities remains controversial. Surgical approaches that have been employed include incision (decompression) of the syrinx via myelotomy, posterior fossa decompression via a suboccipital craniectomy and associated cervical vertebral laminectomy, and syringosubarachnoid shunting. In one report comparing the latter two procedures, no difference was found between patient groups with both procedures being equally effective in causing syrinx collapse. Clouding the issue further, some syrinxes have spontaneously regressed, resulting in some authors questioning the role of surgery as treatment for this problem. Additionally, direct syrinx drainage without shunting, may also be helpful is some instances.
The type of pathology present influences treatment responses. Human patients with hydromyelia in association with hydrocephalus may be likely to benefit from ventriculoperitoneal shunting, whereas those with hydromyelia that does not communicate with the fourth ventricle, may not. Extracanalicular syrinxes that do not communicate with the fourth ventricle may require direct shunting. Additionally, as this type of syrinx often results in irreversible damage to spinal tracts, overall treatment responses may not be as favorable as with hydromyelia.
Wobbler’s syndrome encompasses a number of cervical vertebral abnormalities. These include vertebral malarticulation/malformation, disk extrusion, articular facet hypertrophy, and ligament hypertrophy. Two types of “Wobbler’s syndrome” exist. One is exemplified by the older Doberman pinschers where the disease is characterized by ventral compressive lesions of the caudal cervical area from ligamentous hypertrophy and disk (usually anulus) protrusion. Younger, large breed dogs such as the Great Dane, have a similarly named disease, but with a differing pathophysiology. In this instance, disease of the dorsal articular facets predisposes to hypertrophy of the associated joint capsule and ligaments, resulting also in spinal cord compression. Similar-type compression is rare in older dogs.
Medical treatments, including exercise restriction, analgesics, and corticosteriods, are often inadequate to control clinical signs. A number of surgical techniques for the disease have been described, suggesting that no one technique is adequate. This is due to a relatively poor understanding of the pathophysiological mechanisms that result in these diseases. Many dogs have short-lived improvements following surgery, only to have a recurrence of clinical signs within six months to one year. In general, many older dogs with ventral lesions have both a static as well as a dynamic compressive component to their disease. It seems reasonable then, that a decompressive as well as a stabilization surgical treatment may be warranted.
In the younger, large dogs with dorsal articular facet disease, a dorsal decompressive procedure is more appropriate. Fusion of the associated articular facets seems most beneficial in our hospital for long-term improvements after surgery. This group of dogs tends to have better long-term outcomes compared to the older dogs with ventral compressive disease. In the older group of dogs, dorsal instability and joint failure most likely contribute to the pathophysiology of the disease. Unfortunately, by the time the disease is recognized, ventral compression is severe enough that dorsal extension of the neck results in worsening spinal compression. This dynamic worsening often precludes dorsal fixation in extension. If, however, the disease could be recognized in its early stages, dorsal fusion may be beneficial in decreasing the progression of the disease.
Lumbosacral disease can present as a diagnostic dilemma, as clinical signs of this disease may mimic a number of pelvic limb musculoskeletal conditions. Clinical signs include pain upon palpation dorsal of the LS area, fecal and/or urinary incontinence, and lower motor neuron (LMN) signs in the pelvic limbs (sciatic areas). Pelvic limb tremor may reflect weakness or pain. A painful response may be seen with dorsal extension of the LS joint, however, can also be seen with hip dysplasia. It is important to palpate the LS area when the animal is laterally recumbent, as pressure placed here in a standing animal with, for example, coxofemoral joint disease, may also result in pain. This painful response to pressure distributed though the coxofemoral area may be misinterpreted at LS joint pain. A digital rectal examination is also warranted to exclude intrapelvic masses.
Diagnosis is based upon documentation of nerve compression via myelography, epidurography, diskography, CT, or MR studies. The dural sac will sometimes terminate caudal to the LS joint in larger dogs, and compression in this area may be seen with myelography. Myelography is also helpful to exclude spinal lesions of the lumbosacral intumescence. Because there is often only nerve root compression with this disease, however, myelographic studies of this area will not adequately document this compression.
Advanced imaging modalities (MR, CT) have the advantage of providing a transverse view of the LS joint, allowing an assessment of the foraminal areas where exiting nerves are often compressed. Cerebrospinal fluid analysis is usually not helpful, as the spinal cord proper has terminated cranial to the level of the compression. Electromyography (EMG) and nerve conduction (NCV) studies of the pelvic limbs and tail may reveal spontaneous activity consistent with denervation or slowed conduction velocities, respectively. While these changes are not specific for nerve compression, abnormal results of these studies tend to incriminate LMN disease as a cause of the clinical signs.
Treatment involves surgical decompression and/or stabilization of this area. Commonly, a dorsal laminectomy and decompression is used to free entrapped nerves in the LS area. More debatable is the need for surgical stabilization of this area. In our experience, however, instability at this joint contributes to degeneration of this area and may result in recurrence of clinical signs. Because of this, we have elected to stabilize the LS joint following decompression hoping to improve clinical signs more rapidly and decrease the risk of recurrence.