Atlantoaxial Instability in Down Syndrome

Atlantoaxial instability (AAI) is a common co-morbidity among patients with Down syndrome/trisomy 21. The main discussion in the literature involves pediatric patients. About 10%–30% of patients having trisomy 21 will traditionally develop AAI. Yet, some reports mark the risk to be more than 60%. ^{[1, 2, 3]}  

The occiput, the atlas (C1), and the axis (C2) form a functional unit, the occipitocervical articulation, which is characterized by a high degree of mobility and little intrinsic bony stability. Strong ligaments facilitate movement and keep these structures in place. The head rotation occurs primarily at this junction, and the odontoid bone is the axis that allows this rotation. ^{[4, 5, 6, 7]}

The atlantooccipital joints allow movement in extension and flexion. In flexion, an anterior translation of C1 on C2, which normally does not exceed 3 mm in adults, exists. In children younger than 8 years, this translation can be as wide as 5 mm. The measurement of the movement of C1 over C2 is mainly measured by the gap between the anterior wall of the dens the posterior wall of the anterior rim of C1 (atlanto-dental interval, ADI). In pathologic conditions (eg, abnormalities of the odontoid bone or in the ligaments that keep these joints together), this displacement increases, and bone structures can pressure the spinal cord, producing clinical symptoms. According to the most published literature ADI greater than 4.5 mm is considered pathological. ^[8]

In most instances, the radiologic findings are not associated with clinical symptoms. ^[9]  Symptoms present at a rate of 1%–2% and  include upper motor neuron signs, syncope/pre-syncope, motor or sensory deficits, and even lower brain stem involvement affecting the ability to swallow, speak, and even breathe. 

No racial predilection is known for AAI in Down syndrome, and the role of sex is unclear. Most studies suggest a female preponderance; ^{[1, 2, 10]}  however, some reports noted a male preponderance, ^[11] and others found no difference between men and women. ^[12]

Most cases of AAI have been described in children. ^{[13, 14]} Longitudinal studies of children and adults show a high degree of stability both clinically and radiologically. ^{[1, 9]} In some individuals with radiologic evidence of AAI at the beginning of the study, radiographic findings normalized in subsequent evaluations.

A negative correlation exists between atlanto-odontoid distance and age. ^[15]

The following radiographs are provided for comparison between an image with a normal atlantoaxial relationship and that of a mild abnormality in a patient with Down syndrome.

This radiograph shows the normal relationships between the anterior arch of C1 and the odontoid bone, as well as the odontoid bone and the posterior arch of C1.

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Routine lateral cervical radiograph of a 28-year-old woman with Down syndrome confirmed by chromosomal analysis. The radiograph shows a mild degree of subluxation. The patient had no clinical signs of cord compression.

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See also the following:

• Atlantoaxial Instability

• Atlantoaxial Injury and Dysfunction

• Cervical Radiculopathy

• Os Odontoideum

• Down Syndrome

• Alzheimer Disease in Down Syndrome

• Ophthalmologic Manifestations of Down Syndrome

• Dermatologic Manifestations of Down Syndrome

• Pathology of Myeloid Proliferations Related to Down Syndrome

Many genetic and congenital developmental abnormalities affect the craniocervical junction, ^{[4, 12, 16, 17, 18, 19]} and a wide spectrum of congenital and acquired lesions can result in atlantoaxial instability (AAI). ^{[20, 21]}

In persons with Down syndrome, congenital absence or laxity of the transverse atlas ligament (which may be associated with congenital anomalies of the odontoid bone) must be considered. The excessive laxity of the posterior transverse ligament, which attaches the odontoid bone to C1, is considered an important factor in AAI, and might be an important factor for the greater prevalence of AAI in Down syndrome. ^[13] When the transverse atlas ligament abnormality is present, trivial trauma to the cervical area or acute infections of the nasopharyngeal area may also produce subluxation. ^[22]

Other conditions are also known to present with generalized increased laxity of the ligaments (eg, Marfan syndrome, Ehlers-Danlos syndrome), and AAI might be a complication in these patients. ^[21] Additional factors may also be important; for example, malformations of the odontoid bone can be a factor in some cases. ^[22] Odontoid hypoplasia might be a predisposing factor by facilitating a slippage of the transverse ligament.

Lymphatic drainage of the craniocervical junction is the same as that of the cervico-occipital junction, and retrograde infections may play a role in the instability of the craniocervical joint. ^[22] The syndrome has been described after infections or surgical procedures (eg, mastoidectomies, tonsillectomies) in the mastoid, middle ear, and tonsils. ^[10]

Some pure cases of AAI are associated with degenerative arthritis or rheumatoid arthritis of the cervical spine. ^[23] Degenerative changes of the cervical spine are often overlooked, mostly because the majority of the studies of AAI are performed in children; however, these changes are frequently seen in adults with Down syndrome and might play a role in the development of clinical symptoms. ^[12]

Spinal canal stenosis due to C1 hypoplasia have been described in several children with Down syndrome and symptomatic AAI, which was minimal (only 6 mm) in some of them. ^[24] This paper reported that the cross-sectional area of C1 on computed tomography (CT) scan was significantly smaller in children with Down syndrome (505 mm²) when compared with children without trisomy 21 (602 mm²). This study showed that the space for the spinal cord can be reduced and a minimal degree of subluxation might be enough in some children to produce compression of the spinal cord. ^[24]

The early symptoms of subluxation are often subjective and difficult to elucidate in patients with intellectual disability. This may result in a delay of diagnosis. In some cases, clinical symptoms must be inferred. For example, changes in behavior, refusal to participate in usual activities, changing hand preference, and urinary incontinence in previously continent individuals can be associated with atlantoaxial instability (AAI). No good longitudinal studies describe the natural evolution of asymptomatic persons with atlantoaxial instability; however, this condition is accepted as rarely symptomatic.

Symptoms

Suspect AAI in an individual with Down syndrome who presents with decreased motor skills or who develops torticollis, a gait disorder, or any form of progressive paralysis. Although the compression is in the high cervical spine, the first motor symptoms or signs may be discovered in the legs.

Signs and symptoms often result from mechanical compression of the cervical nerve roots and/or spinal cord. ^{[25, 26]} In some cases, the vertebral arteries may be distorted. Root compression at C1 and C2 levels produces pain in the upper cervical spine, the neck, and the occipital area that can extend to the head, eyes, ears, and/or throat.

Degenerative disease of the cervical spine, often seen in adults with Down syndrome, may complicate the clinical picture. ^{[12, 14, 27, 28, 29]}

Constant or intermittent vertebrobasilar insufficiency may produce dizziness, vertigo, tinnitus, diplopia, and/or syncope. Unilateral or bilateral tingling and numbness may be present.

In many instances, infections of the pharynx, the middle ear, and/or the upper respiratory tract precede symptoms of atlantoaxial instability. Clinical deterioration following any of these conditions justifies a complete evaluation. ^{[22, 10, 30]}

Signs

Motor system abnormalities are the most common findings at clinical presentation (primarily gait disorders and weakness).

Babinski sign and clonus in the lower extremities are not seen in healthy children with Down syndrome. If these signs are present, they should be considered abnormal. Progressive spasticity in the legs (characterized by increased muscle tone, muscle weakness, ataxic gait, increased deep tendon reflexes, Babinski sign, and clonus) can be a presenting sign. ^{[26, 16, 31]} In adults with Down syndrome who develop Alzheimer disease (also characterized by long-tract signs), these signs have less clinical value. (See Alzheimer Disease in Down Syndrome.)

In most instances, the neurologic signs and symptoms progress slowly in a matter of weeks. New-onset focal weakness might be an early sign of myelopathy due to atlantoaxial instability. ^[32]

Children with Down syndrome are hypotonic, and may remain hypotonic even with compression of the spinal cord.

Torticollis may be a presenting sign; the diagnosis of torticollis in a person with Down syndrome indicates atlantoaxial instability until proven otherwise.

Any pathology in the upper cervical spine can mimic symptoms of atlantoaxial instability (AAI). AAI has been found to be associated with other pathologies of the craniovertebral junction such as Chiari malformation. ^[33]  In addition, brainstem syndromes and cervical disk syndromes should be considered. Given that the atlantoaxial joint (C1-C2) is one of the most mobile joints in the spine that is mainly held together by a complex ligamentous structure that allows the versatile mobility of this joint,16-19 pathology of this joint which can present as AAI is common in many disorders that affect connective tissues, ^[34]  such as Down syndrome, ^{[35, 36]}  Ehlers-Danlos syndrome (EDS), ^[37] , Goldenhar syndrome, ^[38]  and rheumatoid arthritis. ^{[39, 40]}  Connective tissue disorders are not primarily a Caucasian phenomenon; epidemiological studies suggest that hypermobility is more prevalent in African and Asian groups than in the white population. ^[41]

Adults with Down syndrome frequently develop Alzheimer disease. This disorder is associated with long-tract signs that may mask or mimic spinal cord compression. ^[1]

No specific laboratory tests are indicated in atlantoaxial instability (AAI). However, given the high frequency of hypothyroidism in individuals with Down syndrome, a full thyroid workup may be indicated. Hypothyroidism may play a role in this condition or be a separate cause of behavioral changes. ^[42]

Degenerative disease of the cervical spine is common in individuals with Down syndrome. ^[12]  Hence, full evaluation of the cervical spine and the rest of the spinal column is needed. ^[15]

Posterior atlanto-occipital subluxation with or without atlantoaxial instability has been described, including malformations of the odontoid bone, intervertebral disk abnormalities, and changes at the base of the skull.

Other conditions to consider include the following:

• Alzheimer Disease

• Ankylosing Spondylitis

• Ataxia with Identified Genetic and Biochemical Defects

• Intellectual Disability

• Spinal Cord Hemorrhage

• Spinal Cord Infarction

• Spinal Epidural Abscess

• Torticollis

• Vitamin B-12 Associated Neurological Diseases

Screening

Recommendations for radiologic evaluation and surveillance are still in debate. Previously, the American Academy of Pediatrics (AAP) recommended routine surveillance, but the recommendation in regard to asymptomatic patients was removed in the 2011 guidelines. ^[8]  In most individuals with symptomatic atlantoaxial instability (AAI), the neurologic signs develop slowly, in a matter of weeks, so a neurologic examination before sports activities might be more predictive of atlantoaxial instability than radiologic screening. ^[10] Yet, in some patients with occult AAI the presentation can be acute with severe neurological deficit. Lateral radiographs of the neck are recommended by the Special Olympics Committee for participation in many sports and activities. ^[8] Neuroimaging studies of the cervical spine are the most important diagnostic tests. The need for screening (proactive evaluation including imaging in asymptomatic patients) is still debatable, although most of these patients will have at least one imaging screening. Patients with occult AAI (asymptomatic AAI) may risk spinal cord injury, especially when participating in activities that are associated with forcible head movements, such as gymnastics, martial arts, football, diving, and more. Therefore, physicians face a challenge when deciding how and when to pursue radiographic evaluation or surgical intervention. ^[8] Furthermore, it is not completely clear what is the risk of evolution of AAI in the face of initial normal or almost normal imaging in an asymptomatic patient.

As discussed, the AAP removed its recommendation for routine screening imaging for asymptomatic patients with Down syndrome, while the Special Olympics, Inc. replaced its screening guidelines in 2015 for all athletes in regards to AAI screening to now include neurological evaluation focused on the possible signs for AAI including myelopathy. ^{[9, 43, 44]}  One of the concerning questions is in regard to the possible evolution of either instability or stenosis in a patient that previously had normal imaging studies. The question of the possible evolution of AAI on a previous normal x-ray study was recently questioned by different studies. Hengartner et al. tested the efficacy and safety of the guidelines over the course of 8 years in their facility and found that only 0.19% of patients would have potentially benefited from routine screening tests regardless of their clinical status. They also gave an example of a patient that had initial normal lateral x-rays and then developed AAI later on, which in times can lead to false reassurance regarding possible spinal cord injury in this population.

A different group published different cutoffs measurements for the diagnosis of AAI. They diagnosed patients with AAI as those that have ADI that is equal to or greater than 6 mm or those whose space available for the spinal cord (SAC) is 14 mm or less. When checking the risk for the late development of AAI after initial normal screening the researchers found that only 1.6% of patients in their cohort that were initially stable went on to become unstable. ^[45]  In the same study, they found that the risk of progression for asymptomatic patients with normal dens anatomy is quite low unless the patient has another anatomical anomaly such as os odontoideum. The group recommended initial surveillance imaging for all children older than 3 years of age with trisomy 21, and those with os odontoideum cervical flexion and extension films should be added to the regular neutral lateral position films.

In summary, the usefulness of lateral cervical radiographs in asymptomatic children has not been fully resolved. Until more evidence-based information is available, the author recommends that patients with Down syndrome around the age of 3–4 years obtain full neurological evaluation and screening x-rays that include neutral and dynamic flexion and extension radiographs. In case the radiographic images reveal instability or other pathology such as os odontoideum, further workup with MRI as well as further followup is recommended. It is also recommended to consider screening imaging before the patient starts any contact sport or other activity with possible high-velocity head or neck injury.

Cervical spine imaging

Neutral and dynamic cervical radiographs (flexion and extension of the cervical spine) are often used to diagnose atlantoaxial instability and dislocation. Yet, the diagnostic sensitivity is low. The additive value of magnetic resonance imaging (MRI) in the diagnosis of AAI has been discussed in the literature. MRI most likely adds benefit in evaluating the severity of the condition in relation to the spinal cord as well as gives information on soft tissues, joints, and the spinal cord with high sensitivity and specificity. It can help to point out those cases that have early signs of instability with some ligamentous changes or T2 changes at the joints as well as evaluate sequelae of repetitive spinal cord injury by instability, as can be seen with T2 signal changes in the cord as well as atrophic changes. ^{[46, 47]} Another study to consider is the use of cervical computed tomography (CT) for diagnosis given its greater specificity. ^[48]  

Radiographic findings

In cooperative individuals, an open-mouth view of the upper cervical spine can assess the odontoid bone.

Normally, neck flexion shows minimal separation between the odontoid and the atlas (ADI). This separation should be less than 3 mm in adults (see the image below). ^[2] In children younger than 15 years, up to 4 mm can be considered normal. ^{[21, 49, 50]}  Some recent works in the pediatric population describe ADI of 6 mm and greater as pathological. ^[51] As previously mentioned, another important parameter to consider is the space available for the spinal canal, which reflects the space available for the spinal cord and is measured between the posterior rim of the odontoid process and the anterior rim of the posterior arch of C1. This space, according to different publications, is considered pathological if it is equal to or less than 14 mm. ^[51]

Screening radiographs, including dynamic radiographs, on a 13-year-old patient with Down syndrome before he begins an active contact sport.

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A 6-year-old boy who became almost paralyzed after a fall a few years ago, but was never treated. He started to rapidly lose function in his upper limbs. Flexion, neutral, and extension radiographs are shown with severe instability and severe ADI. Patient was put emergently in a hard collar and received surgical treatment soon after.

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The same patient as above with advanced imaging. Pathology also indicates os odontoideum. The initial preoperative reduction with neuromonitoring failed and the patient received occipital-cervical fusion.

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Lateral cervical radiograph in a female with Down syndrome due to chromosomal translocation. The patient developed progressive gait deterioration, weakness, loss of muscle tone in the legs, and increased deep tendon reflexes in both arms and legs. Radiographs of the cervical spine documented a marked subluxation. She underwent fusion of the cervical spine.

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CT scanning and MRI

Computed tomography (CT) scanning and magnetic resonance imaging (MRI) are the best neuroradiologic tests to view the various structures of the cervical spine. ^[11] MRI is superior to CT scanning for evaluating the spinal cord, thus MRI should be used when spinal cord compression is suspected. However, CT scans are useful as an adjunct to MRI for the evaluation of bony abnormalities.

These imaging studies are indicated in all patients who show some clinical symptoms suggesting complications of atlantoaxial instability (AAI). They are optional in asymptomatic individuals when the cervical spine plain radiograph reveals subluxation greater than 5–6 mm. These studies are not indicated for routine screening. In specific cases, the use of dynamic images (flexion and extension) can be used in the MRI or the CT scan. In a recent study, the use of dynamic MRI was suggested. ^[52]  Since MRI is a different modality, different criteria are needed in order to define the pathological condition. In the same study, the following measurements on dynamic MRI were found to be worrisome and demand further workup: SAC less than 9 mm or ADI greater than 4 mm on neutral MRI, changes in ADI greater than 3 mm or SAC greater than 5 mm on dynamic MRI, or the presence of any bony abnormality. ^[52]  Yet, further validation is needed.

Myelography is rarely indicated, as MRI resolution is sufficient in most clinical situations.

Medical (ie, nonsurgical) care for atlantoaxial instability (AAI) is primarily symptomatic to treat pain or discomfort. A cervical collar and traction can be used as a bridge for the definite treatment, which is surgical. ^{[20, 21]}  In most cases, patients with Down syndrome and AAI do not need emergency surgery. Yet, in cases where there is a neurological deficit that is either acute, subacute, or actively progressing, urgent surgery is needed to prevent further deterioration. In cases where MRI was done and already showing radiographic damage to the cord (eg, T2 changes), surgical treatment should be considered as soon as possible. 

Regarding the indication of elective surgery for asymptomatic patients, there is still much controversy. As previously mentioned, the occurrence of AAI among patients with Down syndrome is 7%–29% according to different publications, but only 1%–2% of them will eventually evolve to symptomatic AAI. Hence, it is not clear if there is any role for preventive surgery in the scenario of the asymptomatic patient. This question aggravates before allowing a patient diagnosed with Down syndrome to participate in sport, especially in activities that are labeled as high risk for head and neck injury. In a review from the UK, Tomlinson et al. described the guidelines for sports preparticipation screening when the candidate has Down syndrome and no symptoms indicating potential AAI. ^[53]  They concluded that the use of plain radiographs alone is not enough as a screening tool and that the combination is a dedicated neurological and well-being evaluation as well as plain neutral and dynamic cervical radiographs. The physical examination should include evaluation for hyperlaxity and instability searching for signs of myelopathy, for poor head/neck muscular control, and signs for hyperlaxity. Also, the history and exam should look for easy fatigability, change in bowel and bladder function, motor deficits, difficulties in walking, abnormal gait, neck pain, torticollis or head tilt, incoordination and clumsiness, sensory deficits, spasticity, signs of UMN pathology (hyperreflexia, clonus, pathological reflexes), proprioception, and deep sensation deficits. If any of these signs and symptoms are positive then there is high suspicion for possible AAI and further investigation is warranted. 

In a work published in 2005, Pizzutillo et al. summarized their experience and the data that we have into relatively clear general guidelines. ^[54]  According to their work, whenever the ADI is 9.9 mm or greater than 4.5 mm with neurological deficit immediate workup is needed and the next step is either further investigation with MRI to rule out cord changes or surgery for cervical fusion. For those with ADI greater than 4.5 mm but less than 10 mm, and no neurological deficit, close followup and restriction of high-risk activity is warranted. These general guidelines state that if there is pathological ADI on films, most neurosurgeons and orthopedic surgeons should consider MRI to rule out cord changes. In any case of acute or progressive neurological changes, surgery is warranted.

Persons with Down syndrome have a greater incidence of congenital cardiac conditions, hepatitis, and hypothyroidism, and should be evaluated for these conditions before surgery. Emotional and behavioral issues could be complicating factors, mostly in the postoperative period. Explaining the inconveniences associated with major surgery to people with developmental disabilities is always difficult.

The anesthetist’s management of a person with atlantoaxial instability requires great care to avoid unnecessary mobilization of the neck that can occur to have access to an airway during the operation. ^{[55, 56]}

Consultations

Consider consultation with a neurosurgeon or orthopedic surgeon specializing in the spine when the symptoms suggest AAI and diagnostic workup is needed. Evaluation by a neurologist can be beneficial as well.

Red flags (warning signs/symptoms) include any of the signs and symptoms mentioned above.

Activity restrictions

In individuals with radiologic evidence of atlantoaxial instability without clinical signs or symptoms, activities that might produce injury to the cervical spine should be restricted.

Without evidence of AAI or occipitoatlantial hypermobility, physical activity restriction is unnecessary.

Symptomatic individuals need full restriction of physical activities until surgery is performed (use of a rigid collar).

Patient education

Explain the following to parents and/or legal guardians:

• Potential problems that could occur

• The need to impose certain limitations and restrictions

• The associated ethical issues

Special considerations for anesthesia should be taken. Relaxation during anesthesia might allow extreme movements of the cervical spine. This should be a concern in any person with Down syndrome who needs general anesthesia; however, children with normal radiographic studies of the cervical spine can tolerate flexion and extension of the cervical spine during anesthesia. ^[57]  Patients undergoing surgery for AAI will need to be considered for fiberoptic anesthesia with possible use of neuromonitoring in some cases if neck manipulation is still needed. 

The preferred surgical option for AAI, in the presence of symptoms or radiographic signs for cord injury from instability or stenosis, is a fusion of C1 to C2, which will eliminate the subluxation of C1 over C2. ^{[21, 58]}  Yet, in some cases some surgeons will defer performing occipital-cervical fusion from the occipital bone down to C2 and sometimes C3 (OC-C2 or OC-C3). The latter option should be considered especially when there are signs other than AAI that suggest occipital-atlantal instability. Surgery is more effective in individuals in whom AAI has been diagnosed recently. Hence, the importance of urgent and sometimes emergent care once AAI is diagnosed in symptomatic patients. In those with long-standing symptoms, surgery may prevent further deterioration, but chronic symptoms don't tend to improve significantly. ^[2]  Whenever feasible, the instability is reducible (alignment of C1 and C2 is possible) and the bone quality allows it, a fusion of C1-C2 either by wires that prevent the movement of the two vertebral elements on top of each other or by rods and screws (lateral mass or pedicle or both when needed) is preferable. In small children when the bone quality is not good enough to hold the screws or the instability is not reducible OC-C2 fusion is recommended. ^[59]  In many cases a bone graft is needed and it can be harvested from a donor site in the body (eg, iliac crest or ribs). In cases where there is no good bone to harvest (small kids), allograft frozen bone was found to be effective and safe. ^[60]  A reappraisal of known technique, using contoured rod and wire construct, was found to be as effective as more current techniques using an occipital plate and C1,2 screws. ^[61]  This is a cheaper technique that eliminates the need for complex screw insertion in C1 and C2, especially in young children.

In some cases, especially when the causative pathology is related to the dens, such as os odontoideum, an anterior  transoral or endonasal approach can be considered. ^[62]  However, some advocate for the posterior approach even in the setting of os odontoideum and Down syndrome. ^[63]  Preoperative installation of a Halo brace or traction system can be considered in some cases to reduce  instability and gradually fix the spinal alignment. In many cases, the reduction is done in awake conditions when the patient's neurological status can be monitored. In cases when awake monitoring is not possible, for example, in young children, the use of neuromonitoring is important. The use of somatosensory and motor evoked potentials is an appropriate indication in these cases even for anesthesia induction or any procedure that potentially includes neck movement. ^[64]

Pain should be expected in the postoperative period. In most instances, because of their intellectual disability, patients with Down syndrome are not able to verbalize their pain, and behavioral changes might be the only indication of pain. Hence, proactive pain management is recommended, and not just as needed. Caregivers should be involved in the care of these patients regardless of their age, since many have some degree of intellectual disability and need support, and sometimes even someone that knows them well that will convey their feelings. In many cases, post-surgical use of a hard collar is warranted for 1–3 months, especially in children when the bone quality is not good enough to hold the screws and the secondary bony fusion is the main key for stability. The use of a collar or Halo postoperatively is not mandatory and depends on the surgical technique, bone quality, and other components.

Complications

The main complication following surgery for atlantoaxial instability (AAI) is spinal cord injury secondary to a temporary contusion to the cord or lower medulla or to severe stenosis. In addition, involvement of the lower cranial nerves or dorsal root compression in the cervical area might be associated with other neurological deficits as well as post-operative posterior neck and occipital pain. Infection is a serious hazard since in most cases a construct is inserted and acts as a foreign body. A broken construct or failed fusion might demand further surgical intervention. 

Patients are encouraged to ambulate immediately, and there is the usual use of physical therapy and occupational therapy either immediately or a day after surgery. 

Other postoperative complications can occur as in other surgical interventions. 

Following surgery, patients are likely to remain in the hospital for 3–7 days. Many patients will need postoperative rehabilitation service, especially if the reason they underwent surgery to begin with was an acute neurological deficit. Hence, in many cases preparing rehabilitation postoperatively is recommended and discussion with the family should be held accordingly. 

Radiologic evaluations

Individuals with radiologic evidence of atlantoaxial instability (AAI) but no symptoms should have at least a yearly clinical evaluation. Immediate re-evaluation is needed if they present with new symptoms. MRI should be considered from time to time or as the main tool for followup in order to avoid radiation exposure.

For patients that had normal imaging at some point, clinical followup is warranted without the need for routine imaging. They and their caregivers should be educated to pursue immediate care in case of new neurological symptoms, even if mild. 

For patients that had surgery for AAI, lifetime followup should be considered. Postoperatively, many centers will advocate for repeat images, especially x-rays or CT in 3–6 months to evaluate for bone fusion. Future MRI should include sequences that allow for reduction of artifacts and hence evaluation of the spinal cord. 

Atlantoaxial instability (AAI) does not usually produce clinical symptoms. In symptomatic individuals, treatment in the acute phase may reverse the symptoms. In long-standing cases, the surgical procedure is not associated with clinical improvement.

Several studies have shown that serious complications are rare. ^[42] The primary complications result from spinal cord compression. In most cases, signs and symptoms progress slowly. The diagnosis can be made, therefore, before the advanced stages of the disease.

Death is unusual but may occur in cases of acute decompensation as a result of respiratory arrest related to compression of the high cervical spinal cord. ^[65]