Atlantoaxial subluxation

From Dog
Cervical radiograph of a 6-month-old miniature poodle, showing the dorsal subluxation of the axis due to the absence of the dens. Courtesy Prof. Alexander de Lahunta
Necropsy specimen of a 3-month-old toy poodle that was tetraplegic due to atlantoaxial subluxation. The axis is on the left, with no visible dens. The atlas is on the right, showing its caudal surface and the abnormal transverse plane of the ligament of the dens (1) that was forced into this position by the displaced cranial articular surface of the axis. Courtesy Prof. Alexander de Lahunta

Atlantoaxial subluxation is a rare genetic musculoskeletal disorder that results in neurological disease of dogs[1]. This condition should be distinguished from Wobbler syndrome which involves cervical vertebral compression.

Atlantoaxial subluxation is instability of the atlantoaxial articulation that produces excessive flexion of the joint causing the cranial aspect of the axis to rotate dorsally into the vertebral canal with subsequent spinal cord compression often resulting in severe, acute neurological deficits.

The disorder occurs most frequently in dogs and it may result from fracture, separation, absence, or malformation of the dens (chiari malformations), from hypoplastic deformity of the dens along with shortening of the atlas, from fracture of the body of the axis or fracture of the atlas, from rupture or stretching of the atlantoaxial ligaments with an intact normal dens, or from absence of the transverse ligament of the atlas[2].

Cause

The pathogenesis of the developmental malformations remains uncertain. Anatomical studies indicate that the dens develops from two separate ossification centers[3]. In this report, the authors suggested that dens dysplasia was unlikely to be a result of failure of development of one of the ossification centers but that vascular-related ischemia might lead to postnatal resorption of at least the middle part of the dens and result in dens dysplasia with subsequent atlantoaxial subluxation.

While hereditary factors may be involved in some lines of miniature and toy breeds of dogs in which this congenital anomaly is most common (e.g., Yorkshire Terrier, Chihuahua, Pomeranian, Japanese Chin, Toy Poodle, Pekingese, etc.), fracture and insufficient ligamentous support of the dens may occur in any breed.

The condition has been reported as a congenital disorder in several large breeds, including Rottweiler and Doberman Pinscher[4][5]. Congenital atlantoaxial subluxation occurs most commonly in dogs less than one year of age; however, older animals exposed to various stresses also may be affected.

Atlantoaxial subluxation may also occur in association with occipitoatlantoaxial malformation (OAAM)[6][7], a congenital deformity of the upper cervical spine that is characterized by absence of occipital condyles with fusion of the atlas to the occiput and hypoplasia of the atlas, axis and dens[8]. The atlanto-occipital and atlantoaxial joints are regarded as a single complex on both anatomical and biomechanical grounds[9]. Fusion of the atlanto-occipital joint may exacerbate instability of the atlantoaxial joint and complicate surgical attempts to correct it[10]. Atlantoaxial subluxation associated with dorsal dens angulation[11] and abnormal occipitoatlantal articulation[12] have been seen in adult Cavalier King Charles Spaniel dogs with Chiari malformations.

Clinical signs

Clinical signs vary according to the degree of luxation. They may range from cervical rigidity and pain to spastic paraparesis, and sometimes tetraplegia (see cervical syndromes). The signs may develop slowly over several months or they may occur acutely. In some instances, hemorrhage and edema from severe trauma to the upper cervical cord may extend to the caudal brainstem resulting in cranial nerve deficits[13].

When atlantoaxial subluxation is suspected, survey radiographs should be made without anesthesia before manipulating the animal excessively. Lateral view radiographs will reveal widening of the space between the arch of the atlas and spinous process of the axis, angulation of the axis relative to the atlas, a fractured dens, or the rounded end of the axis indicating the absence of the dens. Oblique lateral or ventrodorsal views may be useful in determining the presence or absence of the dens. Open mouth frontal and flexed lateral views are not necessary in most cases and are likely to cause severe compression of the spinal cord[14]. Use of specialized neuroimaging techniques, including computed tomography (CT) and computed tomographic myelography, are also recommended[15]. The prognosis is guarded. In a recent study on risk factors affecting the outcome of surgery for atlantoaxial subluxation in 46 dogs, an age of onset less than 24 months, duration of clinical signs less than 10 months, and preoperative neurological status were found to be significant positive prognostic factors[16]. In an earlier surgical study involving 23 dogs with atlantoaxial subluxation, only 4 of 7 non-ambulatory dogs recovered [17]. A potential complication from myelography and/or surgical manipulation in animals with atlantoaxial subluxation is cardiopulmonary arrest[18]. Medical treatment involves similar protocols as outlined under acute spinal trauma. Neck and head splinting in extension for animals with mild luxations and cervical pain or minimal neurological deficits followed by cage rest for at least 6 weeks has been successful[19].

Treatment

Internal stabilization of the luxation generally is regarded as the therapy of choice[20], especially in animals with moderate to severe neurological deficits or in animals treated conservatively having recurring episodes of pain.

Results of several studies[21] suggest that vertebral stabilization using a ventral approach[22] may be safer than dorsal stabilization of the atlas and axis. The application of ventral pins and polymethylmethacrylate has been used successfully in the surgical treatment of congenital and traumatic atlantoaxial instability[23].

In one case report involving an 8 month old Rottweiler with hypoplasia of the dens, use of cannulated screws was considered superior to K-wires and conventional screws for arthrodesing the atlantoaxial joint[24]. Stabilization with bone plates via a ventral approach has been successful in dogs[25]. The subluxated atlantoaxial joint of a tetraplegic Yorkshire Terrier was reduced and secured in position by means of a novel cross pinning technique applied via a dorsal approach[26].

Use of the nuchal ligament as a means of securing the spinous process of the axis to the dorsal arch of the atlas has also been reported to be successful in small- and large-breed dogs[27]. In dogs with OAAM, a combination of substantial internal and external fixation, odontectomy, and arthrodesis of the atlantoaxial articulation is recommended[28]. Poor results have been reported using the ishigami atlantoaxial tension band[29].

References

  1. Vite, Ch (2004) Developmental disorders. In: Braund's Clinical Neurology in Small Animals: Localization, Diagnosis and Treatment. IVIS, Ithaca, New York, USA
  2. Ladds P, Guffy M, Blauch B, et al (1970) Congenital odontoid process separation in two dogs. J Small Anim Pract 12:463-471
  3. Watson AG, Stewart JS. (1990) Postnatal ossification centers of the atlas and axis in Miniature Schnauzers. Am J Vet Res 51:264-268
  4. Hoerlein BF. (1971) Canine Neurology: Diagnosis and Treatment. 2nd ed. Philadelphia: WB Saunders Co, pp:269
  5. Wheeler SJ. Atlantoaxial subluxation with absence of the dens in a Rottweiler. J Small Anim Pract 33:90-93
  6. Watson AG. (1979) Congenital occipitoatlantoaxial malformation (OAM) in a dog. Anat Histol Embryol 8:187
  7. Watson AG, Hall MA, de Lahunta A. (1985) Congenital occipitoatlantoaxial malformation in a cat. Compend Contin Educ Pract Vet p:7
  8. Read R. (1987) Instability of the occipito-atlanto-axial joint complex. Aus Vet Pract 17:179-182
  9. Watson AG, Lahunta Ad, Evans HE. (1986) Prenatal development of the composite occipito-atlanto-axial synovial joint cavity in the dog. Anat Rec 216:423-433
  10. Read R, Brett S, Cahill J. (1987) Surgical treatment of occipito-atlanto-axial malformation in the dog. Aus Vet Pract 17:184-189
  11. Bynevelt M, Rusbridge C, Britton J. (2000) Dorsal dens angulation and a Chiari type malformation in a Cavalier King Charles Spaniel. Vet Radiol Ultrasound 41:521-524
  12. Churcher RK, Child G. (2000) Chiari 1/syringomyelia complex in a King Charles Spaniel. Aust Vet J 78:92-95
  13. Bailey CS, Morgan JP. (1992) Congenital spinal malformations. Vet Clin North Am Small Anim Pract 22:985-1015
  14. Oliver JE, Jr. , Lewis PE. (1973) Lesions of the atlas and axis in dogs. J Am Anim Hosp Assoc 9:304-313
  15. van Ham LM, Bree HJv, Tshamala M, et al (1995) Use of computed tomography and computed tomographic myelography for assessment of spinal tumoral calcinosis in a dog. Vet Radiol Ultrasound 36:115-118
  16. Beaver DP, Ellison GW, Lewis DD, et al (2000) Risk factors affecting the outcome of surgery for atlantoaxial subluxation in dogs: 46 cases (1978-1998). J Am Vet Med Assoc 216:1104-1109
  17. Thomas WB, Sorjonen DC, Simpson ST. (1991) Surgical management of atlantoaxial subluxation in 23 dogs. Vet Surg 20:409-412
  18. Huibregtse BA, Smith CW, Fagin BD. (1992) Atlantoaxial luxation in a Doberman Pinscher. Canine Practice 17:7-10
  19. Gilmore DR. (1984) Nonsurgical management of four cases of atlantoaxial subluxation in the dog. J Am Anim Hosp Assoc 20:93-96
  20. Gage ED, Hoerlein BF. (1973) Surgical repair of cervical subluxation and spondylolisthesis in the dog. J Am Anim Hosp Assoc 9:385-390
  21. Denny HR, Gibbs C, Waterman A. (1988) Atlanto-axial subluxation in the dog: a review of thirty cases and an evaluation of treatment by lag screw fixation. J Small Anim Pract 29:37-47
  22. Sorjonen DC, Shires PK. (1981) Atlantoaxial instability: a ventral surgical technique for decompression, fixation, and fusion. Vet Surg 10:22-29
  23. Schulz KS, Waldron DR, Fahie M. (1997) Application of ventral pins and polymethylmethacrylate for the management of atlantoaxial instability: results in nine dogs. Vet Surg 26:317-325
  24. Rochat MC, Shores A. (1999) Fixation of an atlantoaxial subluxation by use of cannulated screws. Vet Comp Orthop Traumatol 12:43-46
  25. Stead AC, Anderson AA, Coughlan A. (1993) Bone plating to stabilise atlantoaxial subluxation in four dogs. J Small Anim Pract 34:462-465
  26. Jeffery ND. (1996) Dorsal cross pinning of the atlantoaxial joint: new surgical technique for atlantoaxial subluxation. J Small Anim Pract 37:26-29
  27. LeCouteur RA, McKeown D, Johnson J, et al (1980) Stabilization of atlantoaxial subluxation in the dog, using the nuchal ligament. J Am Vet Med Assoc 177:1011-1017
  28. Jaggy A, Hutto VL, Roberts RE, et al (1991) Occipitoatlantoaxial malformation with atlantoaxial subluxation in a cat. J Small Anim Pract 32:366-372
  29. van Ee RT, Pechman R, van Ee RM. (1989) Failure of the atlantoaxial tension band in two dogs. J Am Anim Hosp Assoc 25:707-712