A Retrospective Comparison of Cervical Intervertebral Disk Disease in Nonchondrodystrophic Large Dogs Versus Small Dogs
Medical records of 144 small-breed dogs (≤15 kg) and 46 medium- to large-breed dogs (>15 kg) with surgically confirmed, Hansen type I, cervical intervertebral disk extrusions were reviewed. The most common clinical presentation was cervical hyperesthesia. The most common sites affected were the second (C2) to third (C3) cervical intervertebral disk space in small-breed dogs and the sixth (C6) to seventh (C7) cervical intervertebral disk space in the larger dogs. Following surgery, 99% of the dogs had resolution of cervical hyperesthesia and were able to ambulate unassisted. Seven (4%) dogs required a second surgery; four of these were large-breed dogs.
Introduction
Intervertebral disk disease (IVDD) is one of the most commonly diagnosed neurological disorders in dogs. Cervical disk disease accounts for approximately 15% of all intervertebral disk extrusions, with dachshunds, beagles, and poodles representing 80% of the cases.12 It has been reported that the most common site for cervical disk extrusion is the second (C2) to third (C3) intervertebral disk space. Disk disease is often divided into two distinct categories, referred to as Hansen type I and Hansen type II.56 Both types of disk disease occur in the cervical region; however, previous reports suggest that type I is more common.7 In animals with IVDD, the nucleus pulposus degenerates and loses its ability to absorb shock. Continued degeneration and subsequent protrusion of the outer fibrous layers of the disk (Hansen type II) or extrusion (Hansen type I) of the nucleus pulposus occurs spontaneously or secondary to mild trauma.7 The amount of herniated disk material, the force of the extrusion or protrusion, and the duration of compression all contribute to the severity of neurological deficits.2568 Older, nonchondrodystrophic breeds are classically described as having slowly progressive, Hansen type II disk disease.8 This retrospective study was prompted by a clinical impression that a substantial number of medium-to large-sized, nonchondrodystrophic dogs are presented with Hansen type I cervical disk extrusions.
The purpose of this study was to compare the historical and clinical features of two sizes of dogs with Hansen type I cervical disk extrusions. For the purpose of this study, dogs were considered to be large if they weighed >15 kg and were considered to be small if they weighed ≤15 kg.
Materials and Methods
The medical records of 190 dogs with a surgically confirmed diagnosis of Hansen type I cervical intervertebral disk extrusion from 1998 through 2002 were reviewed. Information retrieved from the medical records included signalment, body weight, presurgical clinical signs (e.g., ambulatory versus nonambulatory, presence or absence of cervical spinal hyperesthesia), onset and duration of clinical signs, type of diagnostic imaging performed (i.e., myelogram versus magnetic resonance imaging), type of surgery performed (i.e., ventral slot procedure versus dorsal laminectomy), and postoperative neurological status. The specific location of the disk extrusion was identified at surgery or necropsy. The distinguishing criterion between Hansen type I or II lesions was the presence or absence of apparent extruded disk material in the vertebral canal at surgery or necropsy. Dogs with dynamic compressive lesions were excluded from the study.
Acute onset was defined as clinical signs developing in ≤24 hours; chronic onset was defined as clinical signs developing over a period >24 hours. Recovery period was defined as the time from surgery until unassisted ambulation or resolution of cervical spinal hyperesthesia. Follow-up data was obtained by means of physical examination, telephone contact with the owner, a written questionnaire, or all three. The outcome was considered successful if the dog regained or retained the ability to ambulate unassisted postoperatively and was free of cervical spinal hyperesthesia. Recurrence of clinical signs (e.g., cervical spinal hyperesthesia, paresis, paralysis) was also noted.
Results
Signalment
Complete information was available for 190 dogs (46 large and 144 small dogs). A total of 49 breeds were represented. Twenty-four percent (46/190) of the cases were larger, non-chondrodystrophic dogs, with 16 different breeds represented in this category [Table 1]. Labrador retrievers were the most common large-breed dog represented, accounting for 20% of all the large dogs. Mixed-breed dogs represented 9% of all the cases. Seventy-six percent (144/190) of the dogs were small, chondrodystrophic dogs. Dachshunds were the most common small-breed represented, accounting for 36% of all the small dogs.
The mean age was 7.8 years for all dogs (median 7 years; range, 2 to 16 years). The mean age for small dogs was 7.8 years (median 7 years; range, 2 to 16 years). The mean age for the larger dogs was 7.4 years (median 7 years; range, 2 to 13 years). Fifty-five percent of the dogs were males (105/190), and 45% were females (85/190). Of the larger, nonchondrodystrophic dogs, 57% were males (26/46) and 43% were females (20/46). Of the small dogs, 55% (79/144 cases) were males and 45% (65/144 cases) were females.
Clinical Signs
Eighty-six (45%) dogs had acute onset of clinical signs. A slower onset of clinical signs occurred in 104 (55%) dogs. Twenty (43%) large dogs had an acute onset of signs, and 26 (57%) had a slower onset of clinical signs. Sixty-six (46%) small dogs had an acute onset of clinical signs, and 78 (54%) had a chronic onset. The most common clinical sign was cervical spinal hyperesthesia (in 87% of the dogs). Cervical spinal hyperesthesia was found in 90% of the small dogs and 80% of the large dogs. Sixty-three (33%) dogs were presented with cervical spinal hyperesthesia as the only clinical sign. Of this group, 52 (36%) were small dogs and 11 (24%) were large dogs. Sixteen (8%) dogs were presented with thoracic limb lameness and evidence of root signature, of which six (13%) were large dogs and 10 (7%) were small dogs.
Neurological deficits with intact deep pain sensation were present in 121 (64%) dogs. Eighty (42%) dogs were tetraparetic but ambulatory, including 14 (30%) large dogs and 66 (46%) small dogs. Forty-one (22%) dogs were tetra-paretic and nonambulatory, of which 18 (39%) were larger dogs and 23 (16%) were small dogs. Six (3%) dogs were tetraplegic, including three (7%) large dogs and three (2%) small dogs.
Myelography (179/190) or magnetic resonance imaging (MRI) (11/190) were used to localize the lesion in all dogs. Myelograms were performed in 91% (42/46) of the large dogs and 95% (137/144) of the small dogs. Magnetic resonance imaging was performed in 9% (4/46) of the large dogs and 5% (7/144) of the small dogs. Overall, the C2 to C3 inter-vertebral disk space was affected most often. The second most commonly affected intervertebral disk space was C3 to the fourth cervical (C4). The most common intervertebral disk space affected in the larger, nonchondrodystrophic dogs was the sixth cervical (C6) to the seventh cervical (C7) [Table 2]. The most common site for the small dogs was C2 to C3.
Surgery and Outcome
In each case, a ventral slot procedure (163/190) or dorsal laminectomy (26/190) was performed to confirm the site of disk extrusion. The surgical approach was chosen for 189 dogs based on the location of the spinal cord compression as demonstrated on either the MRI or myelogram. In one large dog, the site of disk extrusion was confirmed at necropsy. Seventy-two percent (33/46) of the large dogs and 90% (130/144) of the small dogs had ventral slot surgeries performed. Twenty-six percent (12/46) of the large dogs and 10% (14/144) of the small dogs had dorsal laminectomies performed.
Neurological status after surgery as compared to that prior to surgery was evaluated in 189 dogs. Overall, 99% of the dogs had successful outcomes, which was defined as maintaining or regaining the ability to ambulate unassisted. Of the 143 dogs that were ambulatory prior to surgery, 100% had successful outcomes. Of the 41 tetraparetic, non-ambulatory dogs with deep-pain perception prior to surgery, 100% had successful outcomes. Two of four surviving dogs that were nonambulatory and had no deep pain perception regained ambulatory status. One dog that was negative for deep pain perception experienced respiratory difficulties (i.e., hypoventilation) intraoperatively and was euthanized.
Following surgery, the time to unassisted ambulation was evaluated and was compared to the severity of presurgical clinical signs and the duration of clinical signs. All 143 dogs that were ambulatory at presentation remained ambulatory after surgery. For the 41 dogs that were nonambulatory at presentation and had deep pain sensation, the mean time to ambulation was 6 days. In this latter group, the mean times to ambulation were 4.5 days for 23 small dogs and 7.0 days for 18 large dogs. Two dogs (one small and one large) with loss of deep pain sensation prior to surgery had not regained ambulatory status 2 months postoperatively. The remaining two small-breed dogs with loss of deep pain sensation prior to surgery were able to ambulate within 4 days postoperatively.
Eighty-six dogs that had an acute onset of clinical signs were walking in a mean of 5 days (median 0 days; range, 0 to 75 days). Seventeen of these dogs were presented as non-ambulatory tetraparetics. The mean time to ambulation after surgery for these nonambulatory dogs was 12 days (versus <1 day for dogs ambulatory at presentation). For the 104 dogs that had a chronic onset of clinical signs, the mean time to walking was 4 days (median 0 days; range, 0 to 60 days). There was no difference in the time to unassisted ambulation between dogs having a ventral slot procedure versus dorsal laminectomy surgery.
Recurrences
Eighteen (10%) of the dogs (8% of the small dogs and 13% of the large dogs) had a recurrence of cervical spinal hyperesthesia. The mean time to recurrence after surgery was 91 days for all dogs. The mean times to recurrence were 49 days for large dogs and 112 days for small dogs.
Seven dogs (4%) required a second surgery to alleviate recurrent clinical signs, and four were large dogs. For all 190 dogs, more large dogs (8.7%) required a second surgery than did small dogs (2.1%). Two small dogs required a second surgery on the same disk space as the first surgery. Clinical signs recurred within 7 days of the initial surgery in these two dogs. A different disk space was operated in the remaining five dogs, and the mean time to recurrence for these dogs was 58 days. All seven dogs had successful outcomes after the second surgery.
Discussion
Two basic forms of IVDD, Hansen type I and type II, have been described in the dog.569 It has been previously reported that type I disk extrusion primarily occurs in chondrodystrophic dogs between 2 and 7 years of age (peak incidence, 4 to 5 years), and type II disease affects older (8 to 10 years of age), nonchondrodystrophic dogs.9 In the study reported here, type I cervical IVDD occurred in 46 dogs of medium to large, nonchondrodystrophic breeds. The mean age of affected dogs was 7.8 years, and this is consistent with previous reports.2311 No dog was <2 years of age. Based on these findings, it is highly unlikely for young dogs to have cervical IVDD.
The dachshund and beagle have been described as the breeds most commonly affected with cervical intervertebral disk extrusion.1–31011 In this study, 49 different breeds of dogs were represented. The dachshund and beagle were among the most common small dogs, and the Labrador retriever, German shepherd dog, rottweiler, and Dalmatian were among the most common large dogs affected.
The most common clinical sign of the dogs in this study was cervical spinal hyperesthesia. Unlike thoracolumbar disk extrusions, cervical disk extrusions are less likely to result in compressive myelopathy sufficient to cause paresis or paralysis.34 The larger diameter of the vertebral canal in the cervical area is the most likely explanation for this difference in clinical presentation.34 The location of extruded disk fragments within the vertebral canal is the most important factor in determining whether affected dogs have pain or tetraparesis.2 If disk material extrudes in a dorsolateral direction (i.e., between the dorsal longitudinal ligament and vertebral venous sinus), nerve root compression and pain occur. This is the most common direction of cervical disk extrusion in dogs.2 If disk material extrudes toward the mid-line (i.e., between fibers of the dorsal longitudinal ligament), it is more likely to cause spinal cord compression and subsequent tetraparesis/tetraplegia. Animals with mid-line extrusions may also exhibit cervical pain as a result of meningeal irritation.2 Occasionally, dogs have cervical spinal hyperesthesia and thoracic limb lameness (i.e., mono-paresis) as a result of dorsolateral disk extrusion in the lower cervical spine (C4–C7) that entraps a nerve root supplying the brachial plexus. Pressure from disk material on the nerve root can cause nerve root ischemia and severe pain. Pain is often intermittent and manifests as thoracic limb lameness (root signature).2
The most common sites of disk extrusion for the small-breed dogs were the C2–C3 and C3–C4 intervertebral disk spaces, and these locations were consistent with previous studies.1–411 The most common site of disk extrusion for the larger, nonchondrodystrophic dogs was the C6–C7 inter-vertebral disk space. Previous reports have suggested that cervical disk disease in large-breed dogs usually occurs from a Hansen type II protrusion at the fifth cervical (C5)–C6 or C6–C7 disk spaces.4 Previous reports have also stated that the Doberman pinscher was the only nonchondrodystrophoid breed commonly affected, and that cervical IVDD was most likely associated with cervical spondylomyelopathy.4 In the study reported here, only three of the large dogs were Doberman pinschers. This may be a result of current breed popularity, as Doberman pinschers are less popular than Labrador retrievers, who were the most represented large-breed dogs in this study.
If deep pain perception was present prior to surgery, surgical outcomes in these dogs were good. Outcomes were no different for the ambulatory versus nonambulatory dogs with intact deep pain perception. The surgical procedure performed also had no effect on the surgical outcome. Previous studies have reported that dogs with caudal cervical intervertebral disk extrusions respond less favorably and are more severely affected than dogs with cranial cervical disk extrusions.1411 These clinical differences have been explained by anatomical differences between the cranial and caudal cervical spine.11 The spinal cord is more confined in the caudal cervical region because of the relative increase in cord size at the cervical intumescence (i.e., enlargement at C6 to the second thoracic vertebra.11
In this study, a greater percentage of large dogs were nonambulatory preoperatively than the small dogs. Because the large dogs more commonly had intervertebral disk extrusions in the caudal cervical region (i.e., C6–C7), it would have been reasonable to expect that their clinical signs would be more severe and their recovery rates lower. However, regardless of the intervertebral disk space involved, there were no differences noted between the ambulatory and nonambulatory dogs with intact deep pain. These results differed from previous reports in which large-breed dogs only had a 66% success rate following surgery.12 These reports, however, included dogs treated for Hansen type II disk disease and caudal cervical spondylomyelopathy. No prior reports have exclusively evaluated Hansen type I cervical IVDD in large-breed dogs. As expected, the dogs in this study that had loss of deep pain sensation prior to surgery had less favorable outcomes.
Ten percent of the dogs in this study had recurrence of cervical spinal hyperesthesia, which is in contrast to previous studies that have reported recurrence rates as high as 33%.1 Seven dogs in this study required a second surgery to alleviate recurrence of clinical signs. More large dogs required a second surgery (8.7%) than did small dogs (2.1%). A second disk herniation at a site distinct from the initial lesion was the most common reason for a second operation. Similar results have been reported for thoracolumbar disk extrusions.13
Conclusion
This study showed that Hansen type I cervical intervertebral disk extrusion is not uncommon in medium- to large-breed, nonchondrodystrophic dogs. Clinical signs are similar to small dogs with the same type of IVDD. Comparable outcomes following surgery can be expected for both small and large dogs that are either ambulatory or nonambulatory and have intact deep pain perception prior to surgery.
