20 Sharon Kerwin Spinal infection in dogs and cats may involve the vertebral body[1, 2], vertebral physes [3, 4], intervertebral disc, epidural space [5], or paravertebral soft tissues [6]. Spinal infection is being increasingly recognized as imaging techniques become more sophisticated and available [7]. Vertebral physitis is infection centered on the vertebral physis, with no initial involvement of the disc space. It is generally identified in dogs less than 2 years of age and typically observed in the lumbar vertebrae [4]. Spondylitis or vertebral osteomyelitis refers to infection of the vertebral bone and occurs most frequently as a result of direct inoculation of pathogens. Discitis refers to infection of the intervertebral disc only. Discospondylitis (which has also in some literature been termed “intradiscal osteomyelitis”) refers to an infection of the cartilaginous end plates with secondary involvement of the disc [8]. There is no such entity as “discospondylosis” (a term occasionally referred to by veterinarians), and spondylosis deformans is not at all the same thing as discospondylitis, although patients with discospondylitis might consequently develop spondylosis at the affected level(s) (see also Chapter 8). Spinal epidural empyema (abscess) is a suppurative, septic process within the epidural space of the vertebral canal, with accumulation of purulent, septic material [5, 9]. Paraspinal infection refers to infection of the muscles surrounding the spine (longus colli, iliopsoas, epaxial muscles) and may arise from osteomyelitis, discospondylitis, or direct inoculation [6]. The source of spinal infection may be autogenous or iatrogenic. The majority of cases are thought to result from hematogenous spread of infection from a distant site. Infection may become established in the highly vascular, slow flowing metaphyseal and epiphyseal capillary beds with rapid extension into the disc. In humans, it is thought that infection may begin in the anterior longitudinal ligament, which extends along the ventral surface of the spine and serves as the periosteum [10]. As our ability to image cases earlier improves, we may be able to more accurately describe the initial location of infection and subsequent progression [11]. Iatrogenic discospondylitis of the lumbosacral disc has been reported after epidural injection [12, 13]. In both reported cases, intestinal perforation, possibly associated with the use of an excessively long 3.5 inch spinal needle, was theorized as a potential source of infection, based on isolation of enteric organisms (E. coli, Enterococcus spp.). Interestingly, both cases were adult German shepherd dogs. Iatrogenic discospondylitis has also been reported after fenestration carried out during surgery to treat intervertebral disc herniation [14, 15]. In one report, documenting two cases of iatrogenic methicillin-resistant Staphylococcus aureus infection, high-dose steroids were theorized as a possible risk factor [14]. Spinal infection may also occur after migration of foreign bodies [16] or abnormal migration of parasites [17, 18]. In hunting dogs, grass awns are thought to be inhaled, penetrate the lungs into the pleural cavity and then forced between the pleural layers in a caudal direction by respiratory movements. The foreign bodies then migrate along the attachment of the diaphragm and into the sublumbar musculature and infect the L2 to L4 vertebra, where the crura of the diaphragm attach. Commensal organisms in the mucous membrane may colonize the plant material. An oral route of migration through the esophagus or caudal duodenal flexure in the dog has also been proposed [16]. Although most foreign bodies associated with infection around the spine lodge in the musculature, the plant material may also lodge directly within the vertebral body [19]. Clinical signs of spinal infection in dogs can include spinal pain, fever, lameness, anorexia, weight loss, abdominal pain, and neurologic deficits ranging from mild ataxia and paresis to nonambulatory paraplegia with absence of deep nociception. Neurologic deficits are only seen in about half of presenting cases [15]. In cases of exogenous spinal infection (e.g., foreign bodies), fistulous tracts may be present [16]. Although pain on palpation of the affected vertebral column is often present, it may not be detected in stoic dogs. Although presentation can be peracute [20], clinical signs may wax and wane, and the history may be vague and can span over a period of years [19]. Spinal infection is much less commonly reported in cats than in dogs. Signs may include lameness, reluctance to ambulate, spinal pain, paresis, or paralysis. A history of trauma, in particular bite wounds adjacent to the spinal column, may be present [21]. Male dogs were twice as likely as females to be affected in one large study, and urinary tract infections were the most commonly diagnosed concurrent disease. Great danes, boxers, rottweilers, English bulldogs, German shepherd dogs, and Doberman pinschers appeared to be at higher risk than mixed breed dogs [15]. In the United States, discospondylitis appears to be more prevalent in the southern region of the country. Initial radiographic changes of vertebral physitis are a lucent widening of the caudal vertebral physis accompanied by hazy loss of definition of the metaphyseal and epiphyseal margins of the physis. This later gives way to increasing sclerosis in the surrounding cancellous bone, collapse of the physis, and remodeling of the ventrocaudal aspect of the affected vertebra, with preservation of the caudal end plate and absence of disc space narrowing [4]. Discospondylitis is first characterized by symmetric loss of definition of the vertebral end plates of the affected disc space with progressive lucency and loss of bone from the subchondral bone plate or may be first observed as a collapse of the intervertebral disc space alone [8]. Radiographic changes have been reported to lag behind clinical signs by several weeks in dogs [14] and 2–8 weeks in humans [10]. Progressive lysis and sclerosis of both end plates and vertebral bodies, leading to extensive remodeling and partial collapse of the disc space, occur over time, with ventral spondylosis and eventual ankylosis of the space [8]. Vertebral osteomyelitis, with sparing of the disc space and end plates, has been described in the dog as osteolytic and osteoproliferative changes including intense periosteal reaction of the vertebral bodies [2]. Myelography may be useful in combination with radiographs for diagnosis of space-occupying extradural lesions, determining the degree of spinal cord compression, and assessing vertebral instability in association with spinal infections but is being supplanted by advanced imaging in many practices [5, 18, 22]. Computed tomography (CT) is a very effective tool for diagnosing osteomyelitis of bone as it has the ability to detect bone changes earlier than conventional radiography, and the cross sectional anatomy is helpful for surgical planning. Combination CT with myelography has been used to diagnose epidural empyema in the dog. The additional use of intravenous contrast medium aids detection of enhancing lesions both in the epidural space and within the surrounding soft tissues. Multiple areas of punctate osteolysis may be present in the end plates surrounding involved disc space in discospondylitis [21, 23, 24]. Although CT documentation in small animals has been reported in several types of spinal infections in dogs and cats, a case series describing sensitivity and specificity both for diagnosis and for monitoring treatment has not been reported. Magnetic resonance imaging (MRI) is considered the most sensitive and specific imaging modality for inflammatory and infectious disease of the human spine and was reported for the diagnosis of discospondylitis in the dog as early as 1998 [25], as well as for the diagnosis of canine paraspinal infections [6, 16]. Carrera et al. [7] reported MRI findings in 13 dogs with confirmed (culture positive, presumed hematogenous origin) discospondylitis. There was always involvement of two adjacent vertebral end plates and the associated disc. The involved end plates and adjacent marrow were hypointense on T1-weighted imaging, and all dogs also had contrast enhancement of end plates and paravertebral soft tissues after administration of paramagnetic contrast agents. The end plates and involved marrow were hyperintense on short tau inversion recovery (STIR) images (generally used to eliminate fat from the image), and the affected intervertebral discs were hyperintense in T2-weighted and STIR images. The intervertebral discs exhibited contrast enhancement in 15 of 17 (88%) of affected sites. End plate erosion could be observed in 15 sites, but was not observable either on MRI or plain radiography in two sites. Epidural extension could be seen on contrast at 15 sites and all affected dogs had neurologic signs [7]. Harris et al
Discospondylitis and Related Spinal Infections in the Dog and Cat
Introduction
Definitions
Pathophysiology
History and clinical signs
Risk factors
Imaging
Radiographs
Myelography
Computed tomography
Magnetic resonance imaging
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