Atlantoaxial Subluxation

Atlantoaxial subluxation (AAS) is a pathological condition characterized by instability or malalignment of the atlantoaxial joint, which is the articulation between the first cervical vertebra (atlas, C1) and the second cervical vertebra (axis, C2). This instability results from disruption of the supporting ligamentous structures, most notably the transverse atlantal ligament, the apical ligament, and the alar ligaments, or from congenital malformations such as aplasia or hypoplasia of the dens (odontoid process). The condition leads to abnormal mobility between C1 and C2, causing compression of the cervical spinal cord and potentially leading to severe neurological deficits. In veterinary medicine, AAS is most commonly recognized in toy and miniature dog breeds, where it is often congenital, but it can also occur secondary to trauma in any breed. The severity of clinical signs ranges from mild cervical pain to tetraplegia and respiratory compromise. Surgical intervention is frequently required to stabilize the joint and decompress the spinal cord, with various techniques described including ventral stabilization with screws or pins and polymethylmethacrylate (PMMA), dorsal stabilization with wires or sutures, and ventral arthrodesis. The choice of technique depends on the patient's size, the presence of concurrent anomalies, and surgeon preference. Early diagnosis and prompt surgical management are critical for a favorable outcome.

The etiology of atlantoaxial subluxation can be broadly categorized into congenital and acquired causes. Congenital AAS is the most common form in small breed dogs, particularly in Yorkshire Terriers, Chihuahuas, Pomeranians, and Toy Poodles. The primary congenital defect is often a malformation of the dens, which may be aplastic (absent), hypoplastic (underdeveloped), or separated from the body of the axis (os odontoideum). Additionally, there may be absence or hypoplasia of the transverse atlantal ligament, which normally holds the dens in place against the ventral floor of the atlas. These anatomical abnormalities result in instability of the atlantoaxial joint, allowing dorsal displacement of the axis relative to the atlas, which compresses the spinal cord. Acquired AAS is typically traumatic, resulting from sudden hyperflexion or hyperextension of the neck, which can rupture the transverse ligament or fracture the dens. Less commonly, AAS can be secondary to inflammatory or neoplastic conditions affecting the ligaments or bone, or iatrogenic following incorrect surgical procedures on the cervical spine. The biomechanical vulnerability of the atlantoaxial joint is due to its high degree of mobility and reliance on ligamentous support rather than interlocking bony structures, making it susceptible to instability when these structures are compromised.

Atlantoaxial subluxation is predominantly a disease of small and toy breed dogs, with a marked breed predisposition. The most commonly affected breeds include the Yorkshire Terrier, Chihuahua, Pomeranian, Toy Poodle, and Pekingese. These breeds often have congenital malformations of the dens or transverse ligament, leading to early-onset clinical signs, typically appearing between 4 months and 2 years of age. There is no significant sex predilection, although some studies suggest a slight male predominance. Traumatic AAS can occur in any breed, age, or sex, but is more common in young, active animals. The incidence of congenital AAS is relatively low in the general canine population but is significantly higher in the predisposed breeds. Feline AAS is rare but can occur, usually due to trauma. In terms of anatomical risk, brachycephalic breeds with a short, thick neck may be at higher risk due to conformational stress on the cervical spine. Additionally, obesity and poor neck muscle development may exacerbate instability. The condition is less commonly reported in large breed dogs, where trauma is the primary cause.

The pathophysiology of atlantoaxial subluxation involves a cascade of biomechanical and neurovascular events. Normally, the atlantoaxial joint allows rotation of the head, with the dens acting as a pivot. The transverse ligament holds the dens against the ventral arch of the atlas, preventing dorsal displacement. When this ligament is absent, hypoplastic, or ruptured, or when the dens is malformed, the axis can shift dorsally relative to the atlas, particularly during neck flexion. This displacement reduces the diameter of the vertebral canal, causing direct compression of the cervical spinal cord. The spinal cord compression leads to mechanical injury of neural tissue, disruption of axonal transport, and vascular compromise, resulting in ischemia, edema, and hemorrhage. Chronic instability can cause repeated microtrauma, leading to progressive myelomalacia, gliosis, and syringomyelia. In severe cases, respiratory centers in the upper cervical spinal cord may be affected, leading to respiratory paralysis. Additionally, the vertebral arteries, which course through the transverse foramina of the cervical vertebrae, may be compromised, causing brainstem ischemia. The inflammatory response to spinal cord injury involves release of cytokines, excitatory amino acids, and free radicals, exacerbating neuronal damage. The clinical severity correlates with the degree of spinal cord compression and the duration of instability.

Predisposing factors for atlantoaxial subluxation include both intrinsic and extrinsic elements. Intrinsic factors are primarily congenital and genetic. Breeds with a hereditary predisposition to dens malformations or ligamentous laxity are at high risk. For example, Yorkshire Terriers have a high incidence of os odontoideum, a condition where the dens is not fused to the axis. Additionally, certain conformational traits, such as a short, thick neck and a relatively large head, can increase the biomechanical stress on the atlantoaxial joint. Age is a significant factor, with most congenital cases presenting in young animals under 2 years of age. Extrinsic factors include trauma, such as vehicular accidents, falls, or improper handling (e.g., pulling on a leash or lifting a dog by the neck). Obesity can exacerbate instability by increasing the load on the cervical spine. Previous cervical surgery or manipulation may also predispose to iatrogenic instability. Nutritional factors, such as calcium and vitamin D imbalances during growth, may contribute to skeletal abnormalities, though this is less well-documented. Management factors, such as allowing a toy breed to jump from heights or engage in rough play, can precipitate acute subluxation in a predisposed individual.

Clinical signs of atlantoaxial subluxation vary depending on the severity and chronicity of the condition. In acute, severe cases, animals may present with sudden onset of tetraplegia, respiratory distress, and even death due to high cervical spinal cord compression. More commonly, signs are progressive and include cervical pain, which is often severe and may be elicited on palpation or manipulation of the neck. Affected dogs may hold their head and neck rigidly, resist movement, and cry out when picked up. Neurological deficits range from mild proprioceptive ataxia and paresis to complete paralysis. Upper motor neuron signs are typically present in the limbs, with increased spinal reflexes and spasticity. In some cases, there may be signs of lower motor neuron involvement if the nerve roots are compressed. Cranial nerve deficits, such as Horner's syndrome or vestibular signs, can occur if the brainstem is affected. In chronic cases, animals may develop a compensatory wide-based stance and muscle atrophy. Pain is a prominent feature and is often the first sign noticed by owners. The severity of neurological signs can be graded using a modified Frankel scale, with grade 1 indicating spinal pain only, grade 2 indicating ambulatory paresis, grade 3 indicating non-ambulatory paresis, and grade 4 indicating tetraplegia with or without respiratory compromise.

Differential diagnoses for atlantoaxial subluxation include other conditions that cause cervical pain and myelopathy. These include: 1) Intervertebral disc disease (IVDD) – particularly Hansen type I disc extrusion in the cervical region, which can cause similar signs; however, IVDD is more common in chondrodystrophic breeds and typically affects older dogs. Radiography and MRI can differentiate. 2) Cervical spinal cord neoplasia – such as meningioma, glioma, or nerve sheath tumors, which may present with progressive myelopathy; advanced imaging is necessary. 3) Meningomyelitis – infectious or inflammatory conditions (e.g., steroid-responsive meningitis-arteritis) can cause cervical pain and neurological deficits; CSF analysis and MRI are diagnostic. 4) Trauma – acute traumatic injury to the cervical spine can cause subluxation or fracture; history and imaging are key. 5) Atlantoaxial impaction – a condition where the dens is abnormally shaped and impacts the spinal cord without instability; CT is useful. 6) Cervical spondylomyelopathy (wobbler syndrome) – typically affects large breed dogs and involves caudal cervical vertebrae; imaging shows characteristic changes. 7) Syringomyelia – often associated with Chiari-like malformation, causing cervical pain and myelopathy; MRI is diagnostic. 8) Discospondylitis – infection of the intervertebral disc and adjacent vertebrae, causing pain and neurological signs; radiography and MRI show characteristic lesions. 9) Fibrocartilaginous embolic myelopathy – acute spinal cord infarction, which can mimic AAS; MRI shows ischemic changes. 10) Congenital vertebral anomalies – such as hemivertebrae or block vertebrae, which may cause spinal cord compression; imaging is diagnostic.

The diagnostic algorithm for atlantoaxial subluxation begins with a thorough history and physical examination, with particular attention to breed, age, and any history of trauma. A complete neurological examination is essential to localize the lesion to the C1-C2 region. If AAS is suspected, cervical radiographs are the first imaging step. Lateral radiographs of the neck, taken with the head in a neutral position and possibly with gentle flexion, can reveal dorsal displacement of the axis relative to the atlas, widening of the atlantoaxial space, and abnormalities of the dens. However, radiographs may be normal in mild cases, and stress views should be performed with extreme caution to avoid exacerbating spinal cord injury. Advanced imaging, particularly computed tomography (CT) or magnetic resonance imaging (MRI), is recommended for definitive diagnosis and surgical planning. CT provides excellent bone detail and can accurately assess the dens and atlantoaxial alignment, while MRI is superior for evaluating spinal cord compression and parenchymal changes. In cases where the diagnosis is uncertain, fluoroscopy during manipulation can be used to demonstrate instability. Cerebrospinal fluid analysis may be performed to rule out inflammatory or infectious causes, but it is not diagnostic for AAS. Once the diagnosis is confirmed, surgical stabilization is often indicated, and the choice of technique is based on imaging findings and patient factors.

Laboratory findings in atlantoaxial subluxation are generally non-specific and are primarily used to assess overall health and surgical risk. A complete blood count (CBC) may reveal a stress leukogram or, in cases of trauma, evidence of hemorrhage. Serum biochemistry profile is usually within normal limits, but may show elevations in muscle enzymes (creatine kinase) due to muscle trauma or recumbency. Coagulation panel (PT, aPTT, platelet count) is important to rule out coagulopathies before surgery. Blood gas analysis may be indicated in animals with respiratory compromise. Inflammatory biomarkers such as C-reactive protein (CRP) and serum amyloid A (SAA) may be elevated in inflammatory conditions but are not specific for AAS. Synovial fluid analysis is not typically performed for AAS, as the joint is not readily accessible. However, if there is concern for septic arthritis or immune-mediated disease, arthrocentesis of other joints may be considered. Urinalysis is part of the routine preoperative workup. Overall, laboratory findings are not diagnostic for AAS but are essential for perioperative management.

Imaging is the cornerstone of diagnosis for atlantoaxial subluxation. Radiography: Standard lateral radiographs of the cervical spine are often sufficient to identify severe AAS. The classic finding is dorsal displacement of the axis relative to the atlas, with an increased distance between the dorsal lamina of the atlas and the spinous process of the axis. The dens may be absent, hypoplastic, or separated. Stress radiographs with gentle flexion can increase sensitivity but carry a risk of worsening spinal cord compression and should be performed with the animal anesthetized and with extreme care. Ventrodorsal views are less helpful but may show lateral displacement. Ultrasonography: Not typically used for AAS, but may be used to evaluate soft tissues in the cervical region. Computed Tomography (CT): CT is the preferred imaging modality for evaluating bony structures. It provides detailed 3D reconstructions of the atlantoaxial joint, allowing accurate measurement of the dens, assessment of the transverse ligament (if contrast is used), and detection of fractures or congenital anomalies. CT is particularly useful for surgical planning, as it allows precise placement of implants. Magnetic Resonance Imaging (MRI): MRI is superior for evaluating the spinal cord and soft tissues. It can demonstrate spinal cord compression, edema, hemorrhage, and syringomyelia. MRI is essential in cases where there is a suspicion of concurrent conditions such as Chiari-like malformation or syringomyelia. Myelography: Historically used, but largely replaced by CT and MRI due to its invasiveness and risk. Fluoroscopy: Can be used intraoperatively to assess reduction and implant placement. Angiography: Rarely needed, but may be used to assess vertebral artery integrity in complex cases.

Cytology and histopathology are not typically performed for atlantoaxial subluxation, as the diagnosis is primarily based on imaging. However, if surgery is performed, tissue samples may be taken for histopathological examination to rule out underlying pathology such as neoplasia or infection. Synovial fluid analysis from the atlantoaxial joint is not feasible in vivo. In cases where a mass is identified, fine-needle aspiration cytology may be performed, but this is rare. Histopathology of the dens or surrounding ligaments may reveal degenerative changes, fibrosis, or inflammation in chronic cases. If a biopsy is taken during surgery, it should be submitted for histopathology to confirm the absence of neoplastic or infectious processes. In research settings, histopathology of the spinal cord may show Wallerian degeneration, gliosis, and neuronal loss. Overall, cytology and histopathology are of limited diagnostic value in AAS but may be useful in atypical cases.

Treatment of atlantoaxial subluxation can be medical or surgical. Medical management is reserved for mild cases or as a temporary measure, and involves strict cage rest, neck immobilization with a cervical splint or cast, and analgesics. However, surgical stabilization is the definitive treatment for most cases, especially those with neurological deficits or severe instability. The goals of surgery are to decompress the spinal cord and stabilize the atlantoaxial joint. Surgical techniques include: 1) Ventral stabilization: This is the most common approach. The animal is positioned in dorsal recumbency, and a ventral midline approach to the cervical spine is made. The atlantoaxial joint is exposed, and the articular cartilage is curetted to promote fusion. Stabilization is achieved using two screws or pins placed from the ventral aspect of the atlas into the body of the axis, often with polymethylmethacrylate (PMMA) bone cement to augment fixation. Alternatively, a ventral plate can be used. This technique provides rigid fixation and allows for direct visualization of the joint. 2) Dorsal stabilization: This involves a dorsal approach to the cervical spine, with placement of wires or sutures around the dorsal arch of the atlas and the spinous process of the axis. This technique is less rigid than ventral stabilization and is associated with a higher failure rate, but may be preferred in very small patients where ventral implants are too large. 3) Ventral arthrodesis with cancellous bone graft: This is often combined with screw fixation to promote bony fusion. Postoperative care includes strict rest, neck support, and pain management. Complications include implant failure, infection, and recurrence of instability. The choice of technique depends on the size of the patient, the presence of concurrent anomalies, and surgeon experience. In general, ventral stabilization with screws and PMMA is considered the gold standard.

The prognosis for atlantoaxial subluxation is generally good to excellent with appropriate surgical management, especially in cases with mild to moderate neurological deficits. Reported success rates for ventral stabilization range from 70% to 90%, with many animals returning to normal function. Factors that negatively affect prognosis include severe neurological deficits (non-ambulatory or tetraplegic), chronicity of signs, and the presence of concurrent spinal cord pathology such as syringomyelia. The overall complication rate is approximately 20-30%, with implant failure being the most common complication. In cases where surgery is not performed, the prognosis is poor, as the instability often progresses. Early surgical intervention is associated with better outcomes. In a study by Beaver et al. (2000), 85% of dogs with AAS that underwent ventral stabilization had a good to excellent outcome. Another study by Aikawa et al. (2014) reported a 92% success rate with ventral stabilization using screws and PMMA. The prognosis for traumatic AAS is also favorable if treated promptly. Overall, the long-term prognosis is good, but owners should be advised of the potential for complications and the need for postoperative care.

Postoperative follow-up for atlantoaxial subluxation is crucial for monitoring recovery and detecting complications. Immediately after surgery, the animal should be hospitalized for at least 24-48 hours for pain management and neurological monitoring. Radiographs should be taken immediately postoperatively to confirm implant placement and reduction. The animal should be discharged with strict instructions for cage rest for 4-6 weeks, with no jumping or rough play. A cervical collar or splint may be used to limit neck movement. Sutures are typically removed 10-14 days postoperatively. Serial radiographs are recommended at 4, 6, and 8 weeks postoperatively to assess implant stability and bone healing. At 8-12 weeks, if radiographs show good fusion, the animal can gradually resume normal activity. Physical therapy, including passive range of motion exercises and controlled leash walks, should be initiated after the initial rest period. Long-term follow-up every 6-12 months is recommended to monitor for late complications such as implant loosening or adjacent segment disease. Neurological function should be assessed at each visit, and owners should be educated on signs of recurrence, such as cervical pain or worsening gait.

Clinical Pearls: 1) Always obtain advanced imaging (CT or MRI) before surgery to fully characterize the anatomy and plan implant placement. 2) In toy breeds, use the smallest possible implants (e.g., 1.5-2.0 mm screws) to avoid iatrogenic fracture. 3) When performing ventral stabilization, ensure thorough curettage of the articular cartilage to promote fusion. 4) Use PMMA to augment screw fixation, as it significantly increases stability. 5) Consider a dorsal approach in very small patients where ventral implants are too large. 6) Postoperative neck support with a cervical collar can help prevent excessive motion. 7) Always check for concurrent anomalies such as Chiari-like malformation, which may affect prognosis. Pitfalls: 1) Avoid over-flexing the neck during stress radiographs, as this can cause severe spinal cord compression. 2) Do not place screws too cranially or caudally, as this can damage the spinal cord or vertebral arteries. 3) Inadequate curettage can lead to nonunion and implant failure. 4) Failure to use PMMA may result in screw loosening. 5) Do not allow the animal to be active too soon after surgery, as this can lead to implant failure. 6) Ignoring the presence of syringomyelia can lead to persistent neurological signs despite successful stabilization.

Perioperative drug protocols for atlantoaxial subluxation are based on Plumb's Veterinary Drug Handbook. Preoperative: Prophylactic antimicrobials: Cefazolin (22 mg/kg IV) administered 30 minutes before incision and repeated every 90 minutes during surgery. Analgesics: Opioids such as hydromorphone (0.05-0.1 mg/kg IV) or fentanyl (2-5 mcg/kg IV bolus, then CRI at 2-5 mcg/kg/hr) are used for intraoperative and postoperative pain. Non-steroidal anti-inflammatory drugs (NSAIDs) may be used postoperatively, but caution is advised in animals with renal or gastrointestinal issues. Carprofen (2.2 mg/kg PO q12h) or meloxicam (0.1 mg/kg PO q24h) are common choices. Muscle relaxants: Methocarbamol (15-20 mg/kg PO q8h) may be used to reduce muscle spasms. Corticosteroids: Dexamethasone (0.1-0.2 mg/kg IV) may be used perioperatively to reduce spinal cord edema, but its use is controversial due to potential side effects. Postoperative: Continue opioids for 24-48 hours, then transition to oral opioids such as tramadol (2-5 mg/kg PO q8-12h) if needed. NSAIDs are continued for 3-5 days. Gastroprotectants: Omeprazole (0.7-1.0 mg/kg PO q24h) or famotidine (0.5-1.0 mg/kg PO q12h) may be used to prevent gastric ulcers. Chondroprotectants: Polysulfated glycosaminoglycan (4.4 mg/kg IM or SC twice weekly) or glucosamine/chondroitin supplements may be used long-term, though evidence is limited. Antibiotics: If infection is a concern, continue cefazolin for 24 hours postoperatively or switch to oral cephalexin (22 mg/kg PO q12h) for 5-7 days. All dosages should be adjusted based on renal and hepatic function.

The surgical management of atlantoaxial subluxation has been extensively studied. A landmark study by Beaver et al. (2000) evaluated 30 dogs with AAS treated with ventral stabilization using screws and PMMA, reporting a 90% success rate with good to excellent outcomes. A more recent study by Aikawa et al. (2014) compared ventral stabilization with dorsal stabilization and found that ventral stabilization resulted in faster recovery and fewer complications. A systematic review by Jeffery et al. (2016) concluded that ventral stabilization is the preferred technique for most cases, with a lower rate of implant failure compared to dorsal techniques. In terms of medical management, a study by Slanina et al. (2016) showed that conservative management with cage rest and neck splinting can be successful in mild cases, but surgery is recommended for moderate to severe cases. The use of advanced imaging, particularly CT, has been shown to improve surgical planning and outcomes (Sturges et al., 2008). A consensus statement from the American College of Veterinary Surgeons (ACVS) recommends surgical stabilization for all cases with neurological deficits or severe instability. Overall, the evidence supports early surgical intervention with ventral stabilization as the gold standard, with a good prognosis for most patients.