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Absence of the dens is rarely described in large breed dogs. In this rottweiler, mild neurological deficits seen at 6 mo of age did not progress for the 9.5 y of the dog’s life despite lack of surgical intervention. This finding underscores the marked differences between small and large breeds.
Absence des dents chez un Rottweiler âgé de 9,5 ans avec des signes cliniques non progressifs. L’absence des dents est rarement décrite chez les chiens des grandes races. Chez ce Rottweiler, de légers déficits neurologiques observés à l’âge de 6 mois n’ont pas progressé pendant 9,5 années de la vie du chien malgré l’absence d’intervention chirurgicale. Cette constatation souligne les différences marquées entre les petites et les grandes races.
(Traduit par Isabelle Vallières)
The absence of a dens as a cause of atlantoaxial subluxation is common in small breeds of dogs. This case report describes the absence of a dens in a rottweiler that was first diagnosed at 6 mo of age. At that time, the neurological signs involving the gait were minimal and despite lack of surgical intervention, these signs did not change over the next 9 y of this dog’s life.
A 6-month-old intact male rottweiler was referred for neurological evaluation and canine extraction at the Cornell University Hospital for Animals in January 1998. At the time of purchase, at 19-weeks-old, the owner noted that the mandibles of the dog were severely shortened and the inferior canines were protruding into the palate bilaterally upon closure of the mouth. In addition, when the dog ran, he moved with marked hypermetria of both thoracic limbs and he often fell.
The dog was bright and alert with normal body condition. The mandibles were approximately 3 cm shorter than expected with the inferior canines positioned caudal to the superior canines. The superior PM1 teeth were mildly displaced caudally and laterally by the inferior canines and an approximately 0.4-cm deep × 0.3-cm round ulcerated, crater was present just caudal to the superior canines, bilaterally and rostral to pre-molar 1 (PM1). Upon closure of the mouth, the inferior canines, which were erupted to approximately one-half of their potential adult length, penetrated into the ulcerated maxillary lesions.
On neurological examination the cranial nerves were normal. When the dog walked in a straight line, the gait was normal. When circled to the left, the right thoracic limb mildly abducted. When trotting, both thoracic limbs exhibited an abnormal cranial extension on protraction resulting in a hypermetric action. The joints were in extension during this action which gave them the appearance of floating or overreaching before weight bearing occurred. Hopping and paw replacement reactions were normal as were spinal reflexes. There was no discomfort elicited on manipulation of the neck. The anatomic diagnosis was a mild C1 to C5 spinal cord segment disorder. The most likely clinical diagnosis was a vertebral malformation-malarticulation with spinal cord compression. Differential diagnoses included cervical trauma, synovial cyst, atlantoaxial instability, congenital vertebral malformation, cervical spondylomyelopathy, leukoencephalomyelopathy of rottweilers, and neuroaxonal dystrophy of rottweilers (1).
All blood parameters, including packed cell volume (PCV), blood urea nitrogen (BUN), creatinine, total protein, alanine aminotransferase (ALT), aspartate aminotransferase (AST), and glucose were normal. Routine castration and extraction of both inferior canine teeth were performed. Lateral and dorsoventral cervical vertebral radiographs were obtained. The dens was absent from the axis (Figure 1) with no associated displacement between the atlas and axis. The vertebral canal was not compressed at this site. In addition, there was asymmetry of the transverse processes of the atlas. Surgical options were considered for therapy; however, the owner elected to not pursue therapy but observe the dog for any progression of the clinical neurological signs. For the next 9 y there was neither change in this dog’s neurological signs nor any development of neck discomfort.
A grade 2 soft tissue sarcoma involving the rostral third of the mandibles was diagnosed when the dog was 9.5 y old. Treatment consisted of surgical resection, but the sarcoma recurred within 1 mo post-excision. In addition, advanced degenerative joint disease of the elbows and hips had developed and the owner elected to have the dog euthanized.
Necropsy was limited to the first 4 cervical vertebrae and associated spinal cord segments. The atlas and axis were severely malformed (Figure 2). The dorsal atlantoaxial membrane and ligament were markedly thickened (~1.3 cm), compared to an age-matched female rottweiler of similar weight and stature (0.4 cm). This tissue was asymmetric with a bulbous, soft region that extended further to the left of midline (1 cm) than to the right of midline (0.7 cm). This connective tissue was white and composed of loosely arranged layers.
The transverse processes of the atlas were asymmetric with the right transverse process being smaller and more round than normal. The left transverse process was caudally displaced, rectangular, and about 1 1/2 times larger than the right transverse process. From a dorsal view the rostral articular fovea and transverse processes were shifted about 10 degrees in a caudal plane. A 1 × 0.2 × 0.3 cm rounded osteophyte was centered to the right of midline on the rostral aspect of the cranial articular fovea of the atlas.
The rostral portion of the spinous process of the axis was shifted to the right. The bulbous proliferation of the dorsal atlantoaxial membrane and ligament protruded from the left side of the spinous process and the deviated spinal process was dorsal to the ligament on the right side of midline. The right transverse process of the axis was about 0.4 cm shorter than the left process. The resultant angulation of the atlantoaxial joint was shifted about 10 degrees caudally. The dens was completely missing as was the apical ligament of the dens. The alar ligaments of the dens were attached laterally to the rostral articular surface of the axis about 0.8 cm apart. The attachments of the alar ligaments to the occipital bone medial to the occipital condyles were normal. The articular cartilage of the cranial articular surface of the axis was tan, mildly pitted, and roughened. There was mild osteophyte production on the ventral cranial articular surface of the axis. Dural ossification was present in the form of 2 hard masses in the dura to the right of the midline ventral to the first 2 spinal cord segments. Both masses were elongate, pink-tan, and hard. The cranial mass was about 0.4 × 0.2 × 0.8 cm. The more caudal mass was 0.3 × 0.2 × 1 cm.
Six transverse sections of the cervical spinal cord from C1 to C4 were examined histopathologically. There were occasional dilated myelin sheaths containing macrophages within the dorsal lateral tracts within the transverse sections of spinal cord cranial to the atlantoaxial joint. Rare similar dilated myelin sheaths were located within the deep portion of the ventral funiculus at the level of C2–C3 articulation. Only 2 to 3 dilated myelin sheaths were noted per transverse section of spinal cord examined.
In summary, the necropsy findings included: 1) malformation of the transverse processes of the atlas; 2) degenerative joint disease with osteophyte production on the atlas cranioventrally; 3) absence of the dens of the axis; 4) spinous process malformation (deviated to the right rostrally) of the axis; 5) transverse process malformation (right process is shorter) of the axis, 6) absence of the apical ligament of the dens; 7) moderate to severe degenerative joint disease of the atlantoaxial joint with osteophyte production on the axis cranioventrally; 8) severe dorsal atlantoaxial membrane and ligament fibrosis; 9) multi-focal dural ossification ventral to the C1 and C2 spinal cord segments; and 10) mild, chronic ascending and descending Wallerian degeneration of the spinal cord.
The atlas and axis have a unique development from the 4th occipital and first 3 cervical somites (2,3). Ossification centers develop in the portions of these somites that migrate ventral to the first 2 spinal cord segments. Normally, the dens and cranial articular surface of the axis are formed by the ossification of centrum 1. Agenesis of the dens represents a partial failure of this ossification. The transverse processes of the atlas and axis develop from the ossification of their neural arches. The spine of the axis develops from both of its neural arches. The growth factors and genes involved in this development in the dog are unknown.
Malformations of the craniocervical region have been described in many species including occipitoatlantoaxial malformation of the horse, hypoplasia of the odontoid process in the cat, and congenital hypoplasia or aplasia of the dens of humans (4–7). Agenesis or resorption of the dens is most commonly seen in small and toy breed dogs including miniature and toy poodles, Yorkshire terriers, miniature dachshunds, Chihuahuas, and pomeranians (3,8–10). This lesion is commonly associated with atlantoaxial subluxation in affected patients. Computed tomography (CT) scans have shown that in 46% of cases with atlantoaxial subluxation there is absence or hypoplasia of the dens. In an additional 30% of cases, the dens is malformed. Only 24% of cases with atlantoaxial subluxation have a normal dens. In many of the latter cases, abnormalities of the transverse ligament of the atlas may lead to displacement of the normal dens into the vertebral canal, thus compressing the spinal cord (9). Alternatively, atlantoaxial subluxation can occur secondary to trauma. Most cases in large breed dogs are associated with cervical trauma. However, rare cases of agenesis of the dens in large breeds have been documented in the literature. These have included a rottweiler, saint bernard, doberman pinscher, basset hound, standard poodle, weimaraner, and a German shepherd (11–15). Most of these involve adult dogs presenting for acute signs of paresis and ataxia secondary to spinal cord compression (3,10,11,13). In these dogs, it is impossible to eliminate fracture of the dens with subsequent resorption. A traumatic injury when this dog was a few weeks old could have resulted in the resorption of the fractured dens as well as the asymmetry of the atlas and axis, the degenerative joint disease of the atlantooccipital and atlantoaxial joints, and the marked fibrosis of the dorsal atlantoaxial membrane and ligament. Dural ossification, as noted in this case, is a form of metaplasia that is common in older large breed dogs unassociated with clinical signs.
Atlantoaxial subluxation is often associated with neck discomfort and mild to severe tetraparesis and ataxia in all 4 limbs. Occasionally the affected dog becomes recumbent and unable to stand. In some dogs, the thoracic limbs may appear more affected than the pelvic limbs. Asymmetric signs may occur in some dogs (9). Surgical stabilization of the atlantoaxial joint is the recommended therapy, but non-surgical treatments including external stabilization with a neck brace may be helpful. Surgical procedures vary from suturing the spine of the axis to the arch of the atlas, to various applications of pins between the ventral portions of the atlas and axis, to fusion of these 2 bones with bone grafts or methyl methacrylate (10,16).
Acute atlantoaxial subluxation with clinical signs attributed to spinal cord injury can be associated with fracture, congenital agenesis, or resorption of the dens in any breed of dog. Large breed dogs, however, may be more resistant to development of clinical signs due to their larger neck muscle mass and stability that this muscle mass can provide to prevent a bony displacement of the atlantoaxial joint. Comparisons to other breed-, age-, and sex-matched dogs to evaluate the joint structures were attempted by comparing this case to that of another rottweiler dog of similar stature and size. Although the sexes were not matched, the dorsal ligament of this particular dog was much thicker than that of the female dog, despite similar muscle mass. In this particular case, the marked hypertrophy of the atlantoaxial joint capsule, membrane, and ligament likely contributed to the stability of the joint. It was not possible to reliably correlate the spinal cord lesions with the clinical signs due to the paucity of both. In the nearly 9 y, the initial lesions of axonal and myelin injury may well have disappeared. We could speculate that the residual dorsolateral funicular lesions affecting the dorsal spinocerebellar tracts that serve the thoracic limbs may have been responsible for the mild thoracaic limb signs. The case presented here highlights that large breed dogs may live long lives with minimal or no clinical manifestations associated with absence of the dens without surgical or medical treatments. CVJ
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