Atlantoaxial Rotatory Displacement
Atlantoaxial rotatory displacement (AARD), also known as atlantoaxial rotary subluxation (AARS), is a spinal condition characterized by a fixed rotation of the first cervical vertebra (C1ā, or atlas) on the second cervical vertebra (C2ā, or axis). This condition is a significant and common cause of torticollis in children. AARD exists on a spectrum of disease, from a mild subluxation to a more severe, fixed facet dislocation.
Pathophysiology
The underlying mechanism of AARD is believed to be related to ligamentous laxity. The condition can be triggered by several factors, with infections being the most common cause, accounting for approximately 35% of cases. Trauma is the second most frequent cause, responsible for about 24% of cases, followed by recent head or neck surgery, which contributes to around 20% of instances. Autoimmune processes can also be a cause. In some cases, the cause is idiopathic.
A specific manifestation of AARD linked to infection is Grisel's Disease, which occurs following a respiratory infection or retropharyngeal abscess. The development of Grisel's Disease is thought to be connected to lymphatic edema in the cervical spine region. Patients with AARD due to infection may present with a history of laryngitis or otitis media.
The integrity of the transverse ligament plays a crucial role in the severity of AARD. When this ligament is intact, spinal canal stenosis only occurs with severe rotation and facet dislocation. However, if the transverse ligament is ruptured, anterolisthesis greater than five millimeters can occur, leading to spinal canal stenosis with less rotation (around 45 degrees). The vertebral arteries are also at risk in such scenarios.
Several conditions are associated with an increased risk of developing AARD, including Down syndrome, rheumatoid arthritis, tumors, and certain congenital anomalies.
Anatomy
Axis (C2) Osteology: The axis is composed of an odontoid process (dens) and a body. It develops from five ossification centers. A key anatomical feature is the subdental or basilar synchondrosis, a cartilaginous junction between the dens and the vertebral body that typically fuses by the age of six. A secondary ossification centre appears around age three and fuses to the dens by age twelve.
Ligamentous Stability: The stability of the occipital-C1-C2 complex is maintained by the odontoid process and its supporting ligaments. The primary ligaments involved are:
- Transverse ligament: This ligament limits the anterior translation of the atlas.
- Apical and Alar ligaments: These ligaments are responsible for limiting the rotation of the upper cervical spine.
Classification
The Fielding classification is the most widely used system for categorizing AARD and is divided into four types:
- Type I: This is the most common and benign type, characterized by unilateral facet subluxation with an intact transverse ligament. The odontoid acts as a pivot point. There is no anterior displacement. (most common type)
- Type II: This type involves unilateral facet subluxation with 3 to 5 millimeters of anterior displacement and an injured transverse ligament. One facet acts as a pivot point, with the contralateral lateral mass displaced anteriorly.
- Type III: Characterized by bilateral anterior facet displacement of greater than 5 millimeters, this type is rare and carries a high risk of neurological involvement or instantaneous death.
- Type IV: This rare type involves the posterior displacement of the atlas (C1) and is associated with an odontoid fracture or a hypoplastic dens. It also has a high risk of neurologic complications or death.
Clinical Presentation
Patients with AARD typically present with an acute "cock-robin" neck position followed by a suboccipital headache.
The characteristic "cock-robin" head position, refers to with ipsilateral rotation and contralateral tilt of the head in relation to the lateral mass of C1. The chin is rotated to the side opposite the facet subluxation. For example, a right-sided facet subluxation will cause the chin to rotate to the left.
A key diagnostic sign is spasm of the contralateral sternocleidomastoid muscle. This spasm occurs on the same side as the chin's rotation and is a protective mechanism to prevent further subluxation. For instance, with a right-sided facet subluxation, the chin rotates left, and the left sternocleidomastoid muscle will be in spasm. This is in contrast to congenital muscular torticollis, where the sternocleidomastoid spasm is primary and occurs on the opposite side of the chin's rotation. Reduced cervical rotation is another common finding on physical exam.
A thorough neurologic examination is mandatory.
| Finding | Right sided facet subluxation | Left sided facet subluxation |
|---|---|---|
| Chin rotation | Rotated to the left | Rotated to the right |
| Muscle spasm | Left SCM muscle is spasmed | Right SCM muscle is spasmed |
Imaging
Radiographs: Recommended radiographic views include an open-mouth odontoid, a lateral view, and flexion-extension views. Radiographs are commonly normal in AARS.
- The open-mouth view can show variations in the size and distance of the C2 lateral masses from the midline, indicating rotation.
- On a lateral view, the facet joint may appear anterior and wedge-shaped instead of its normal oval shape.
- Flexion-extension views can help exclude instability but may be difficult to obtain due to pain and restricted motion.
CT: A dynamic CT scan is the gold standard for diagnosing AARD. The protocol involves taking scans with the head held straight, as well as in maximal rotation to both the right and left. This will reveal a fixed rotation of C1 on C2 that does not change with dynamic rotation. However, its use should be weighed against the significant radiation dose to the neck of a growing child.
MRI: An MRI is generally of little value in diagnosing AARD unless the patient presents with neurologic symptoms or if there is suspicion of an underlying infectious process such as retropharyngeal bascess.
Blood tests: If an infection is suspected, lab studies including a FBC, CRP, and ESR.
Treatment
Treatment for AARD can be either non-operative or operative, with the choice of management largely dependent on the duration of symptoms. The goal is to reduce the subluxated atlantoaxial joints and allow the inflamed tissues to heal.
Non-Operative Management
- For subluxation present for less than one to two weeks: A soft collar, non-steroidal anti-inflammatory drugs (NSAIDs), and an exercise program are indicated. Many of these cases may resolve spontaneously in early presentations.
- For subluxation persisting for more than one to two weeks: Head halter traction with a small amount of weight (around five pounds), NSAIDs, and benzodiazepines, followed by a hard collar for three months, is the recommended treatment. This is also indicated for persistent torticollis despite two weeks of soft collar use. Muscle relaxants and analgesics may also be necessary.
- For subluxation persisting for more than one month or failed halter traction: Halo traction, followed by a halo vest for three months, is indicated.
Operative Management
Surgical intervention in the form of a posterior C1-C2 fusion is indicated in the following scenarios:
- Subluxation that persists for more than three months.
- The presence of neurologic deficits.
- Failure of halo traction for two weeks.
- Recurrent subluxation.
Studies have shown that conservative management fails in a significant percentage of patients, particularly those with a longer duration of subluxation before initial treatment, who are more likely to require surgery.
Complications
A significant complication of AARD is a missed or delayed diagnosis. Given the often subtle initial presentation, a high index of suspicion is required for timely and appropriate management.
Resources
References
- ā Bourghli, Anouar; Al Araki, Ahmad; Konbaz, Faisal; Almusrea, Khaled; BoissiĆØre, Louis; Obeid, Ibrahim (2023-02-13). "Delayed atlantoaxial rotatory dislocation in a child with Crohn's disease: illustrative case". Journal of Neurosurgery: Case Lessons. 5 (7): CASE22515. doi:10.3171/CASE22515. ISSN 2694-1902. PMC 10550607. PMID 36794742.CS1 maint: PMC format (link)
