Atlantoaxial Subluxation in Two Dogs With Cervical Block Vertebrae
Atlantoaxial (AA) subluxation is an uncommon disorder that can cause various degrees of neurological deficits in dogs. Block vertebra is a congenital deformation involving the fusion of two or more vertebrae. This report describes two dogs with cervical block vertebrae from C2 to C5 and C2 to C4, respectively. We hypothesize that the fused cervical vertebrae created a “fulcrum effect” at the AA joint and predisposed these dogs to traumatic AA subluxation.
Introduction
Atlantoaxial (AA) subluxation is an uncommon disorder that can cause various degrees of neurological deficits ranging from neck pain to tetraplegia and risk of respiratory failure. Atlantoaxial subluxation has been reported in both humans and domestic animals. The first case of AA subluxation in dogs was reported in 1967.1–3 Several congenital and developmental deformities of the AA joint have been documented to cause instability of the vertebral column, which predisposes the animal to AA subluxation, particularly in young, small-breed dogs.2–7 Traumatic AA subluxation can happen in any breed of dog and at any age.
Block vertebrae are rare but well-known congenital vertebral deformations involving the fusion of two or more vertebrae. The vertebral deformity may occur at any level of the vertebral column and usually is considered an incidental radiographic finding; however, block vertebrae may contribute to clinical signs of spinal cord disease in some animals.8
This report describes AA subluxation in two dogs with cervical block vertebrae. We hypothesize that the congenital vertebral fusion in these dogs altered the biomechanics of the vertebral column and created a “fulcrum effect” that predisposed these dogs to traumatic AA subluxation.
Case Reports
Case No. 1
An 18-month-old, 4.4-kg, spayed female, long-haired dachshund was presented for evaluation of spinal pain and ataxia that started 1 week before presentation. The owner reported the dog cried out while playing in the yard. The referring veterinarian initially treated the dog with one dose of methylprednisolone at 30 mg/kg intravenously (IV), and then 15 mg/kg subcutaneously (SC) q 12 hours for three doses. The dog responded to steroid therapy initially but then started showing depression, neck pain, and ataxia again.
On presentation, physical examination revealed right-sided ambulatory hemiparesis. Conscious proprioception was normal in both the left front and left hind limbs, but it was delayed in the right front and rear limbs. No pain was elicited at any level of the spine. Complete blood count and serum biochemical analyses showed a slightly elevated hemoglobin (20.9 g/dL, reference range 12.0 to 18.0 g/dL) and alanine aminotransferase (109 U/L, reference range 10 to 100 U/L). Phosphorus (2.4 mg/dL, reference range 2.5 to 6.8 mg/dL) and amylase (267 U/L, reference range 500 to 1500 U/L) were slightly decreased. Survey spinal radiographs revealed fused cervical block vertebrae from C2 to C5 and an increased distance between the dorsal arch of C1 and the dorsal spinous process of C2. The odontoid process of C2 appeared intact and was normal in size and shape [Figures 1A, 1B]. A myelogram was performed by introducing Omnipaquea (1.9 mL) at the lumbar cistern, which showed thinning of the contrast agent through the cranial cervical spinal canal [Figure 2]. Diagnoses of cervical block vertebrae (C2 to C5) and AA subluxation were made.
Conservative treatment was initiated, which consisted of cage rest with a foam neck braceb for 6 weeks, analgesic medication (tramadolc 25 mg per os [PO] q 12 hours for 7 days), and prednisoloned (2.5 mg PO q 12 hours for 3 days, then tapered over 2 weeks). The owner was instructed to keep the dog’s neck brace clean and dry and closely monitor the dog for progression of neurological signs and the development of an abnormal respiratory pattern. Use of a harness instead of a neck leash or collar when walking the dog was advised for the rest of the dog’s life.
After 6 weeks of conservative therapy, the owner reported the dog had been doing well at home. Physical examination revealed the dog moved normally and did not have any neurological deficits. The neck brace was removed, and the dog did not show any neck pain. Four more weeks of cage confinement was recommended.
Following 10 weeks of conservative treatment, the owner reported the dog was normal at home since removal of the neck brace 4 weeks prior. The dog was fully ambulatory and not exhibiting any neck pain. Under sedation, lateral radiographs of the cervical spine in a neutral and cautiously flexed position were taken [Figures 3A, 3B]. The increase in distance between the dorsal arch of C1 and the dorsal spinous process of C2 on the radiograph of the spine in the flexed position showed the persistent instability at the AA joint. Surgical stabilization was discussed, but the owner declined. Ten months following initial presentation, the owner was contacted by telephone and reported the dog was acting normally. No evidence of neck pain or neurological dysfunction had been observed since the last examination.
Case No. 2
A 1-year-old, 1.68-kg, intact female toy poodle was presented for acute onset of neck pain after being stepped on the day before presentation. The dog was ambulatory but moved slowly and exhibited pain when turning her head. The dog was tense and guarded in her neck. Proprioceptive positioning of all four limbs and segmental reflexes were normal. While the dog was sedated, cervical spinal radiographs were taken. Radiographs revealed fusion of C2 to C4 and an increase in the distance between the dorsal arch of C1 and the dorsal spinous process of C2 on the lateral view of the flexed cervical spine [Figure 4]. The odontoid process of C2 appeared to be normal. Diagnoses of cervical block vertebrae C2 to C4 and AA subluxation were made based on radiographic findings.
Surgical and medical treatment options were discussed with the owner, and conservative treatment was elected. A foam neck braceb was applied, and strict confinement for 6 weeks was recommended. Oral butorphanol syrup was made by mixing the injectable butorphanole with syrup, and a total dose of 0.09 mg/kg butorphanol PO q 12 hours for 10 days was prescribed as needed for pain management. Avoidance of a neck leash or collar for the rest of the dog’s life was recommended. Owners were instructed to monitor the dog for neurological progression. The dog acted normally and showed no pain on neck palpation 4 weeks after neck brace placement. The brace was removed 6 weeks after initial presentation. The owners were warned of the risks of future AA instability and were instructed to avoid potential neck injuries. Eight months after the initial presentation, the owners were contacted by telephone and reported the dog had been doing well with no recurrence of clinical signs.
Discussion
The AA joint is a pivot joint that permits the head and atlas to rotate around the longitudinal axis. The odontoid process of the axis (dens, C2) is the center of the rotation.9 The dens projects rostrally into the bony canal of the atlas and articulates with the concave fovea dentis of the atlas.10 The transverse atlantal ligament functions to hold the dens against the ventral arch of the atlas.10 The AA joint is also stabilized by the dorsal AA ligament between the dorsal arches of the atlas and the axis, as well as by the apical ligaments connecting the dens to the ventral part of the foramen magnum and occipital condyles.10
Atlantoaxial subluxation may result from trauma, congenital and developmental anomalies, or a combination of the three.1–3,9,11 The instability of the AA joint allows excessive flexion of the joint and causes spinal cord compression from the dorsally displaced cranial portion of the axis.1–3,9,11 Traumatic AA subluxation results from forceful overflexion of the head, which may tear the ligaments or cause a fracture of the dens or dorsal arch of the axis.1–3,9,11 Considerable impact may be required to cause such injuries in a normal AA joint.9,11
Possible congenital or development anomalies include dysplasia, hypoplasia, dorsal angulation, and separation of the dens, as well as absence of the transverse ligament.2–7 Congenital and developmental anomalies have been reported to contribute to AA subluxation or predispose dogs and cats to AA subluxation following minor trauma.1–7,11 Small breeds of dog including Yorkshire terriers, Chihuahuas, toy poodles, Pomeranians, and Pekingese are most often affected by the congenital and developmental anomalies that predispose the animal to AA subluxation.12
Block vertebrae are the congenital deformations involving the fusion of two or more vertebrae resulting from faulty segmentation in the developing embryo.13 Block vertebrae may occur at any level of the vertebral column.13 Although congenitally fused vertebrae are usually considered an incidental radiographic finding, spinal stenosis, angulation, or instability of the vertebral column associated with this condition may cause pain and neurological deficits.8
Two dogs presented to our hospital possessed congenital cervical block vertebrae of the cranial cervical spine (C2 to C5 and C2 to C4, respectively). In normal dogs, the cervical segments C2 to C3, C3 to C4, and C4 to C5 have the greatest motion from maximum flexion to maximum dorsal extension.14 The fused segments would limit the range of motion of the affected portion of the cervical spine. Consequently, the segment of immobilized cervical vertebrae shifts the load to both ends (fulcrum effect). Therefore, the normal, unfused, intervertebral joints adjacent to the block vertebrae will absorb most of the stress during cervical motion.
Analogously, biomechanical alterations in the vertebral column have been observed in humans and dogs with surgically fused cervical vertebrae.15 Aphenomenon termed the “domino effect” of compressive lesions, developing at an intervertebral space adjacent to the surgically stabilized and fused cervical vertebrae, has been reported in dogs with caudal cervical spondylomyelopathy.15–18 Fusion of an intervertebral space may increase stress on the adjacent intervertebral spaces, accelerate degenerative changes of the adjacent intervertebral disks, and subsequently cause spinal cord compression.15–18 Degenerative changes of the adjacent vertebral segments have been noted in long-term investigations after cervical fusion in humans.19 Eliminating motion through fusion shifts the load to the adjacent level, causing significant increases in intra-disk pressure, strain, and stress during normal range of motion.19–22 The increase in intra-disk pressure is believed to negatively affect the metabolism of the intervertebral disk and contribute to the progressive degeneration of the disks adjacent to the surgically fused cervical vertebrae.19–22
In a normal dog, the ventral flexion and dorsal extension of the head and neck can be described as a suspension mechanism. The AA joint is the fulcrum for all downward forces of the head, and it counteracts the stress from the ventral movement of the head to maintain stability.9 In this report, the dogs with block vertebrae originating at C2 experienced even more stress at the AA joint. This situation would predispose the animal to damage of the dorsal AA, alar, apical, and transverse ligaments or fracture of the dens. Consequently, a relatively minor trauma may have caused AA subluxation in these dogs.
A comparative congenital anomaly has been found in human medicine. Congenital fusion of two or more cervical vertebrae in the human is defined as Klippel-Feil syndrome, which usually is associated with a high incidence of concurrent congenital anomalies in the genitourinary, skeletal, or cardiovascular systems.23 Klippel-Feil syndrome patients rely on the unfused cervical vertebrae and intervertebral spaces to compensate for cervical flexion, extension, or lateral bending.23–25 The congenitally fused vertebral segment forms a lever arm and places stress on the nonfused segments.23–25 The instability and hypermobility of the upper cervical segment adjacent to the fused vertebrae have been suggested as risk factors for neurological compromise, whereas the lower segment is predisposed to degenerative disease.24 Risk of injury contributing to AA instability is of great concern in Klippel-Feil syndrome patients with fused vertebrae at C2 to C3.24,25
Potential pathological embryological mechanisms responsible for the anomaly associated with fusion of cervical vertebrae have been proposed in humans but have not been reported in dogs.23 No concurrent congenital anomalies were diagnosed in these two cases. More cases are needed to determine whether there is a similar association of multisystemic congenital anomalies in dogs.
Canine AA instability was first described in 1967.1–3 Surgical stabilization of the AA joint had been advocated as the standard of care for years.26 Several surgical techniques consisting of either dorsal or ventral stabilization have been described, and they carry various success rates and complications.11 The goals of treatment are to reduce the subluxation and to decompress the spinal cord by stabilizing the AA joint.11 Nonsurgical management strategies such as external cervical splinting, restriction of exercise, and corticosteroid therapeutics are also associated with complete recovery.27
Retrospective studies of the surgical management of AA subluxation have reported various success rates, ranging from 28% to 75% for dorsal procedures and 44% to 90% for ventral procedures.26,28–30 McCarthy et al compared the success rates of different surgical procedures and reported a significantly higher success rate for the ventral lag screw technique. However, when all dorsal stabilization techniques were combined and compared with all ventral stabilization techniques, no significant differences were seen in the overall outcomes with the ventral and dorsal stabilization methods.11 The Table provides a summary of reported success rates in retrospective studies for dorsal stabilization, ventral stabilization, and conservative management.12,26,28–30 Overall success rates for dorsal and ventral surgical procedures and conservative treatment were 59%, 69%, and 57%, respectively. Based on a review of the veterinary literature, dorsal surgical procedures and conservative treatment carried similar results, and ventral surgical procedures carried a higher success rate.12,26,28–30
Although severe neurological deficits in dogs with AA subluxation indicate severe spinal cord damage, significant improvements in neurological signs have been observed after both surgical and conservative treatments.12,26 Age at onset of clinical abnormalities, duration of clinical abnormalities prior to surgery, and preoperative neurological status are risk factors for success of surgical management of AA subluxation in dogs.26 The age at onset of clinical abnormalities (24 months) was significantly associated with greater odds that the first surgery and final outcome would be successful. Age at onset of abnormalities was also associated with a lower grade of postoperative neurological status. The lower neurological grade represented the status of no neurological impairment or ambulatory with ataxia or paresis.26 Duration of clinical abnormalities (10 months) was associated with greater odds of a successful final outcome and a lower final neurological grade.26 Apreoperative neurological status of no neurological impairment or ambulatory ataxia and paresis was significantly associated with a low final neurological grade.26 A good prognosis was anticipated if the animal survived the perioperative period.26 Choosing a dorsal or ventral approach depended on the clinician’s preference. Long-term outcomes for dogs that had a second surgery were as good as the outcomes for dogs that had only one surgery.26
Reports are few regarding the nonsurgical, conservative treatment of AA subluxation.12,27,30 The concern with conservative treatment was loss of any improvement after removal of the neck brace and return to normal activity.31 Sixteen (57%) of 28 dogs in retrospective reports had good final outcomes.12,28,30 This success rate was similar to that of surgical stabilization [see Table].12,26,28–30 Havig et al reported that dogs affected with AA subluxation for <30 days had significantly better long-term outcomes with conservative treatment regardless of the neurological grade on admission, radiological appearance of the dens, and age at onset.12 The duration of symptoms related to AA subluxation before admission has been consistently associated with the outcome in either surgical or nonsurgical treatment.12,26
Conclusion
This report illustrates two unusual cases of AA subluxation in dogs with congenital block vertebrae involving the C2. Congenital cervical fusion shifted the load to both ends of the fused segments, increasing the lever force on the AA joint. We hypothesize that the altered biomechanics predisposed these dogs to subluxation of the AA joint after minor trauma. Both dogs completely recovered with conservative therapy, but the AA joint remained unstable in one dog (case no. 1). The dens of this dog is small and not angulated into the spinal canal, which may cause less compression to the spinal cord and explain the clinical recovery. Avoiding any cervical trauma is essential to prevent relapse in these dogs. The long-term follow-up interviews (i.e., 10 and 9 months after initial diagnosis) revealed that both dogs in this report returned to normal life and activity levels. To our knowledge, this is the first report that describes congenital block vertebrae involving C2 and concurrent AA subluxation in small animals.
Omnipaque; GE Healthcare, Inc., Princeton, NJ 08540
Foam collar; Jorgensen Laboratories, Inc., Loveland, CO 80538
traMADOL; Mutual Pharmaceutical Co., Inc., Philadelphia, PA 19124
Prednis Tab; Lloyd, Inc., Shenandoah, IA 51601
Torbugesic; Fort Dodge Animal Health, Fort Dodge, IA 50501
Citation: Journal of the American Animal Hospital Association 45, 6; 10.5326/0450305
Citation: Journal of the American Animal Hospital Association 45, 6; 10.5326/0450305
Citation: Journal of the American Animal Hospital Association 45, 6; 10.5326/0450305
Citation: Journal of the American Animal Hospital Association 45, 6; 10.5326/0450305
Lateral (A) and ventrodorsal (B) cervical radiographs of case no. 1, showing fusion of C2 to C5 (congenital block vertebrae) and an increased distance between the dorsal arch of C1 and the dorsal spinous process of C2.
Lateral cervical myelogram radiograph of case no. 1, showing thinning of the dorsal contrast column between C1 and C2.
The lateral views of radiographs of neutral (A) and flexed (B) cervical position, showing that the distance between the dorsal arch of C1 and the dorsal spinous process of C2 in flexed position was greater than the distance between these points in neutral position.
Lateral radiograph of slightly flexed cervical position of case no. 2, showing the fused cervical vertebrae from C2 to C4 and the prominent space between the dorsal arch of C1 and the dorsal spinous process of C2.
