Veterinary Surgery, 20, 6, 409-41 2, 1991

Surgical Management of Atlantoaxial Subluxation in 23 Dogs

Twenty-eight surgical procedures were performed in 23 dogs with atlantoaxial subluxation. Dorsal stabilization in seven dogs resulted in two recoveries and five failures of fixation. Ventral decompression and stabilization in 18 dogs resulted in eight recoveries and four failures of fixation. Six dogs died or were euthanatized within 7 days of ventral stabilization. Using either technique, four of seven nonambulatory dogs recovered.

TLANTOAXIAL SUBLUXATION. FIRST described in the A veterinary literature in 1967, has been associated with aplasia or hypoplasia of the dens, dorsal angulation of the dens, nonunion of the dens with the axis, and congenital absence of the transverse ligament.'-' Traumatic luxations may occur in any breed of dog and cat. usually from rup- ture of the atlantoaxial ligaments or fracture of the dens.?- "I Congenital or traumatic instability of the atlantoaxial joint can permit the axis to subluxate dorsally, compress- ing the spinal cord. If the dens is intact when subluxation occurs, it may pierce the spinal cord.' The clinical signs range from cervical pain to tetraplegia. Death may result from respiratory paralysis. ' ' Diagnosis is based on neu- rologic signs of cranial cervical myelopathy and radio- graphic findings.'.'.' '

A dog with minimal neurologic deficit may recover if the neck is splinted and the dog is confined to a cage for 6 weeks." Wiring the dorsal arch ofthe atlas to the spinous process of the axis for surgical stabilization has been de- s~ribed.'.~.' ' Modifications of this technique include using sutures and the nuchal ligament as fixation materials.x.'3 Hemilaminectomy may be required. but reduction ofthe subluxation usually provides adequate decornpressi~n.~." One complication associated with dorsal stabilization is additional spinal cord trauma brought on by the manip- ulations required to reduce the subluxation and pass a wire ventral to the dorsal arch of the atlas. Fatal respiratory arrest may occur because of central nervous system compression during surgery. l 4 Other complications are failure of the fixation material, poor anatomic alignment, and fracture of the dorsal arch of the atlas.'.'.''.'' Meth-

ylmethacrylate and an atlantoaxial retractor have been used for dorsal stabilization."."

A ventral approach for the application of transarticular pins and an autogenous bone graft facilitates odontoidec- tomy and proper anatomic alignment and promotes per- manent fusion of the atlantoaxial Joint.'? Some compli- cations associated with ventral stabilization are improper pin placement and pin m i g r a t i ~ n . ~ . ' ~ Modifications of this technique include the use of threaded pins and transar- ticular lag screw f i ~ a t i o n . ~ . ' ~ The purpose of this study is to evaluate the results of dorsal and ventral surgical tech- niques used for repair of spontaneous atlantoaxial sub- luxation in dogs.

Materials and Methods

The medical records of 25 dogs with atlantoaxial sub- luxation diagnosed at the Auburn University Small An- imal Clinic between 1975 and 1990 were reviewed. Ani- mals with multiple sites of spinal cord injury were ex- cluded. The diagnosis of atlantoaxial subluxation was based on plain spinal radiographs in all dogs and con- firmed at surgery in 2 3 dogs and with necropsy in one dog. One dog with tetraplegia and respiratory distress was euthanatized. The owner ofone dog with ataxia and neck pain did not allow treatment. Each dog's gait was assessed before and after surgery and graded according to the fol- lowing scale: ( 5 ) Normal gait; (4) ataxia or spasticity; ( 3 ) ambulatory paresis: (2) nonambulatory paresis: and ( I ) tetraplegia. Follow-up information was obtained by re- evaluation at Auburn University or a telephone conver-

From the Department of Small Animal Surgery and Medicine, College of Veterinary Medicine, Auburn University, Alabama. Published as publication no. 2234, College of Veterinary Medicine, Auburn University. Reprint requests: William B. Thomas, DVM, Department of Companion Animal and Special Species Medicine, 4700 Hillsborough Street,

Raleigh, NC 27606.

409

41 0 ATLANTOAXIAL SUBLUXATION

Results TABLE 1. Results of Dorsal Fixation of Atlantoaxial Subluxation in Seven Dogs

Number of Dogs ~

Initial Required Neurologic No Multiple

Grade’ Total Recoveredt Improvement Died Procedures

5 0 0 0 0 0 4 2 2 0 0 0 3 2 0 1 0 1* 2 2 0 0 0 251 1 1 0 1 0 0

~~

Total 7 2 2 0 3

Neurologic grades: 5 = normal gait; 4 = ataxia or spasticity; 3

t “Recovered” was defined as a neurologic grade of 5 or 4 and

* Recovered after second dorsal fixation. 5 Died during anesthesia for a second procedure. P First procedure (ventral fixation) failed. Second procedure (dorsal

= ambulatory paresis; 2 = nonambulatory paresis; 1 = tetraplegia.

the absence of neck pain.

fixation) failed. Recovered after third procedure (ventral fixation).

sation with the owner or referring veterinarian. A good outcome was considered to be a neurologic grade of 4 or 5 and the absence of neck pain.

Twenty-eight surgical procedures were performed on 23 dogs by an experienced staff surgeon or a resident. Multiple procedures were performed on some dogs be- cause of failure of fixation. Two dorsal procedures were performed on one dog, two ventral procedures on one dog, a dorsal and a ventral procedure on one dog, and two ventral procedures and one dorsal procedure on one dog. Eight dorsal procedures were performed on seven dogs (Table 1). The dorsal arch of the atlas was wired to the spinous process of the axis with 22- or 24-gauge wire.’ A left hemilaminectomy of the axis and atlas was per- formed concurrently on four dogs.” Twenty ventral sta- bilization procedures were performed on 18 dogs (Table 2). Transarticular pins were used to stabilize the atlan- toaxial joint of 17 dogs.I7 Fixation was achieved with 1.5 mm cortical screws,* a 1.5 mm straight plate with 1.5 mm cortical screws,* and a 1.5 mm T-plate with 1.5 mm cortical screws* in one dog each. The articular cartilage was always removed from the articular facets between the axis and atlas. In two dogs, a cancellous bone graft har- vested from the humerus was placed in the debrided at- lantoaxial joint spaces. ‘’ Odontoidectomy was performed on 12 dogs with a The neck was splinted post- operatively in all but one dog. Dexamethasone or pred- nisolone was administered postoperatively to all dogs. Analgesics were not required.

* Synthes, Paoli, Pennsylvania

The 23 dogs represented eight small breeds, including Yorkshire terrier (n = 8), miniature or toy poodle (n = 7), Pomeranian (n = 3), Pekingese (n = 2), Australian terrier (n = l), Bichon Frise (n = l), and Schipperke (n = 1). Twelve dogs (52%) were 12 months of age or younger when presented for evaluation. Gait dysfunction ranging from ataxia to tetraplegia was seen in 22 dogs (94%). No dog had a loss of deep pain perception. Signs of neck pain were evident in 13 dogs (57%). The clinical signs in seven dogs occurred acutely after minor trauma, such as falling from a chair.

On lateral radiographs of all dogs, there was dorsal dis- placement of the axis relative to the atlas and an increase in space between the dorsal arch of the atlas and the spi- nous process of the axis. The dens appeared to be of nor- mal size in six dogs. It was smaller than normal in 1 1 dogs, and was absent in six dogs. The dens appeared to be disunited with the body of the axis in four dogs and angulated dorsally in two dogs.

Dorsal fixation was successful in two dogs with mild ataxia and spasticity, with no neck pain (grade 4) at year 7 (Table 1). Dorsal fixation failed in five dogs. The wire broke in two dogs, and it fractured the dorsal arch of the atlas in three dogs. No further surgical procedure was done in two of these dogs, and they remained ambulatory par- etic (grade 3). Two dogs underwent another surgical pro-

TABLE 2. Results of Ventral Fixation of Atlantoaxial Subluxation in 18 Dogs

Number of Dogs

Initial Required Neurologic No Multiple

Grade* Total Recoveredt Improvement Died Procedures ~~ ~

5 2 1 0 1* 0 4 3 1 0 2*§ 0 3 7 3 0 34111 I**

2 3 2 0 0 I t t 1 3 1 0 i n I**

Total 18 8 0 7 3 ~

* Neurologic grades: 5 = normal gait; 4 = ataxia or spasticity; 3

t “Recovered” was defined as a neurologic grade of 5 or 4 and

* Neurologic improvement but died within 7 days. 5 Euthanatized due to failure of fixation. I[ Dyspneic after surgery. 11 Dyspneic after surgery; euthanatized. ** Recovered after stabilization with plate. tt First (ventral fixation) and second (dorsal fixation) procedures

$$ No improvement after dorsal fixation.

= ambulatory paresis; 2 = nonarnbulatory paresis; 1 = tetraplegia.

the absence of cervical pain.

failed; recovered after fixation with transarticular screws.

THOMAS, SORJONEN, AND SIMPSON 41 1

cedure, one dorsal and one ventral, and recovered. One dog died during anesthesia for a second surgical procedure.

A good outcome was achieved in one of four dogs after dorsal fixation by an experienced staff surgeon. One of three dogs that underwent dorsal stabilization by a less experienced surgeon recovered well.

Eight dogs were doing well (grade 4 or 5 ) 3 months to 10 years after ventral stabilization with pins (Table 2 ) . One pin broke in one dog and pin migration occurred in three dogs, but the atlantoaxial joint remained stable. Ventral fixation with a T-plate was successful, although radiographs made 5 days after surgery revealed slight mi- gration of the caudal screw. At month 12, the dog was doing well with slight spasticity and no neck pain (grade 4), and there was no further screw migration.

Six dogs died or were euthanatized within 7 days of ventral stabilization. Two dogs were tetraplegic and dys- pneic after surgery until death 6 hours and 2 days later, respectively. Tracheostomy was performed in one dog that was dyspneic after ventral stabilization with two plates. Neurologic deficits improved, but the dog was euthana- tized on day 7 because of progressive respiratory distress. At necropsy, there was tracheal collapse and pneumonia. The mucosa and lamina propria of the trachea were ul- cerated and necrotic. The atlantoaxial joint was well aligned and stable, but focal myelomalacia was observed on microscopic examination. Narrowing of the tracheal lumen caudal to the larynx was visible i n postoperative radiographs of one dog. This dog was ambulatory but coughed and gagged after surgery and died suddenly with respiratory distress on day 7. A necropsy performed by the owner, a physician, revealed a focal area of tracheal necrosis and perforation just caudal to the larynx. Two dogs were ambulatory and doing well after surgery but died suddenly on days 4 and 7 , respectively. Necropsies were not done.

Poor pin placement resulted in pin migration with sub- sequent subluxation in four dogs. One dog was euthan- atized, one dog recovered after stabilization with transar- ticular screws, one dog recovered after fixation with a T- plate, and one dog remained paretic after dorsal fixation.

A good outcome was achieved by ventral stabilization in six of 14 dogs operated on by an experienced staff sur- geon and in two of four dogs operated on by a less ex- perienced surgeon.

Discussion

The surgical objectives for correction of atlantoaxial subluxation are to decompress the spinal cord and reduce and stabilize the atlantoaxial joint without causing mor- bidity and mortality.' ' 3 ' ' Dorsal techniques provide for decompression, reduction, and stabilization of the atlan- toaxial joint. 1.7.8.12-16 Because fusion of the atlantoaxial

joint is not generally achieved with dorsal techniques, failure of the fixation material may be associated with subsequent ' . I h which occurred in five of eight dogs in the present study.

The ventral approach also permits decompression of the spinal cord by allowing alignment and stabilization of the atlantoaxial joint." The ventral approach has the advantage of providing a means for joint f ~ s i o n . ' ~ In four dogs, the pins eventually migrated or broke but the atlan- toaxial joint remained well aligned and stable, with ra- diographic evidence ofjoint fusion. Two dogs in this study had a dorsally angulated dens. Decompression by odon- toidectomy is indicated in such cases" and requires a ventral approach.

Several complications associated with ventral stabili- zation occurred. In two dogs, signs of severe central ner- vous system injury appeared after surgery. One dog had a preoperative grade of I and one dog had a preoperative grade of 3. Both dogs had irregular respiration suggestive of injury to the medulla oblongata or rostra1 segments of the spinal cord. followed by cardiac arrest. The cause of death was probably additional injury to the central ner- vous system by surgical manipulation. Similar compli- cations have been reported after dorsal stabilization. I 4

Tracheal necrosis developed in two dogs. Pneumonia complicated the respiratory disease in one dog. Trauma due to intubation and the tracheal retraction necessary for exposure ofthe atlantoaxial joint may have contributed to injury of the trachea. although preexisting subclinical disease of the trachea was possible. The most common complications associated with an anterior approach to the cervical spine in humans are laryngeal edema and dys- phagia because of retraction of the larynx and trachea. Occasionally. tracheostomy is required because of upper airway obstruction caused by extensive retraction. '') Ne- crosis of the tracheal cartilage resulting in stenosis has been reported in a dog, possibly due to overinflation of an endotracheal tube." The cause of sudden death in two dogs was unknown, and necropsies were not done. A single case of acute death of unknown cause 1 week after ventral stabilization has been reported. I4

Failure of ventral fixation resulting in subluxation was attributed to improper angulation of the transarticular pins. Improper pin placement with inadequate bony pur- chase leads to mig ra t i~n . '~ Malformation ofthe atlas and axis in two dogs made it difficult to assess proper pin angulation intraoperatively. No relationship between the results and experience of the surgeon could be detected in this study.

Ventral fixation with a plate or transarticular screws is an alternative to fixation with transarticular pins. Fixation with a plate in a dog with traumatic atlantoaxial sublux- ation and a fractured axis has been reported." In one

41 2 ATLANTOAXIAL SUBLUXATION

report, transarticular lag screw fixation is considered the treatment of choice for atlantoaxial ~ubluxation. '~

Although the prognosis is considered good for dogs with minimal neurologic deficits and guarded for dogs with tetraplegia,'2 comparisons of the initial neurologic dys- function with the outcome after surgery have not been made. Four of seven initially nonambulatory dogs in this study had a good outcome (grade 4 or 5) after surgery. These results suggest that significant recovery is possible if the spinal cord compression can be relieved and the atlantoaxial joint stabilized. However, the morbidity and mortality associated with the procedures used suggest a need for improved surgical techniques for this problem.

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