Atlantoaxial Instability


Article Author:
Jordan Lacy


Article Editor:
Christopher Gillis


Editors In Chief:
Kranthi Sitammagari
Mayank Singhal


Managing Editors:
Avais Raja
Orawan Chaigasame
Khalid Alsayouri
Kyle Blair
Radia Jamil
Erin Hughes
Patrick Le
Anoosh Zafar Gondal
Saad Nazir
William Gossman
Hassam Zulfiqar
Navid Mahabadi
Hussain Sajjad
Steve Bhimji
Muhammad Hashmi
John Shell
Matthew Varacallo
Heba Mahdy
Ahmad Malik
Abbey Smiley
Sarosh Vaqar
Mark Pellegrini
James Hughes
Beenish Sohail
Hajira Basit
Phillip Hynes
Sandeep Sekhon


Updated:
11/6/2019 1:56:47 PM

Introduction

The atlantoaxial segment consists of the atlas (C1) and axis (C2) and forms a complex transitional structure bridging the occiput and cervical spine. The functional result of the joint is two-fold: (1) providing support for the occiput and (2) providing the greatest range of motion and flexibility possible while still maintaining stability. The instability in this joint is usually congenital, but in adults, it may be due to an acute traumatic event or degenerative disease.[1],[2],[3]

Etiology

The atlas is a ring-shaped structure that is devoid of a vertebral body, consisting of anterior and posterior arches bridged by two lateral masses. The lateral masses of C1 consist of superior and inferior articulating facets. The superior facets are medially angled, paired structures that articulate with the occipital condyles. The inferior facets articulate with the axis. Immediately posterior to the anterior arch of C1 is the odontoid process of C2. This configuration allows for a significant degree of flexion, extension, and rotation at the craniocervical junction. Several ligaments bridging the occipitoatlantoaxial complex produce the stability required to prevent devastating neurologic injury; the most important of which is the cruciate ligament. The cruciate ligament is composed of a transverse axial ligament that runs horizontally between the lateral masses of C1 (considered to the be the strongest ligamentous attachment in the cervical spine), posterior to the odontoid process, and a vertical ligament extending from the basion to the C2 vertebral body. Additional stability is provided by paired alar ligaments running from the tip of the odontoid process to the foramen magnum and the apical ligament connecting the tip of the odontoid process to the basion. Finally, anterior longitudinal ligament, rostral extension of the posterior longitudinal ligament (tectorial membrane), facet joint capsules, and the posterior myoligamentous complex complete the cohesive occipitoatlantoaxial junction. The list of common disorders associated with atlantoaxial instability includes the following:

  • Achondroplasia
  • Congenital scoliosis
  • Down syndrome
  • Morquio syndrome
  • Neurofibromatosis
  • Osteogenesis imperfecta
  • Rheumatoid arthritis

Epidemiology

In individuals without any predisposing factors, atlantoaxial instability is extremely rare.

Radiographic atlantoaxial instability is seen in up to 30% of patients with Down syndrome (DS), but only 1% of patients with DS have symptomatic atlantoaxial instability. Patients with rheumatoid arthritis (RA) are also susceptible to cervical spinal instability and atlantoaxial instability. Ranges of 25% to 80% have been seen, and newer disease-modifying RA drugs may change the course of the disease and decrease the incidence of atlantoaxial instability.

Atlantoaxial instability can occur equally in both genders and can present at any age. A high risk of atlantoaxial instability is seen in Down syndrome and older patients with rheumatoid arthritis.[4]

Pathophysiology

Atlantoaxial instability can be classified into three generalized categories: inflammatory, congenital, and traumatic. RA is the most common inflammatory disease to affect the craniovertebral junction. This is due to a large number of synovial joints and ligaments contributing to the overall stability of the upper cervical spine. The chronic inflammation of RA leads to laxity and stretching of the transverse ligament, development of granulation tissue, and erosion of the bony structures and results in atlantoaxial subluxation and instability. Several congenital disorders are associated with atlantoaxial instability. The most well known of these is DS. Patients with DS are more prone to atlantoaxial instability due to ligamentous laxity and osseous abnormalities. Os odontoideum is the separation of the odontoid process from the body of C2. Previously it was thought to be a congenital lesion, but it is more likely due to a traumatic insult early in life. Patients with os odontoideum require dynamic studies to accurately demonstrate the degree of movement and canal compromise. Trauma as the sole cause of atlantoaxial instability is a unique entity and usually the result of a disruption of the transverse, alar, or apical ligaments. This type of injury is commonly associated with head trauma. Fractures of C1 or C2 also are traumatic causes of atlantoaxial instability.

History and Physical

Patients presenting with atlantoaxial instability can suffer from a wide variety and severity of complaints.[5] Initial presentation can be with complaints of mild axial neck pain, paresthesias, weakness or signs of myelopathy on examination. Additional signs of lower cranial nerve dysfunction may also be seen in more advanced cases. Rarely, patients present with severe spinal cord injuries, causing quadriplegia or even death.

Evaluation

The "Rule of Spence" has classically determined stability of C1 fractures and is determined by measuring the lateral overhang of the lateral masses of C1 on C2 when viewing an AP radiograph, if the sum of both lateral masses of C1 on C2 is greater than 7mm the fracture is considered unstable; this measurement tool is also generally used in assessment of CT scan images. The atlantodens interval (ADI) is an additional method of determining atlantoaxial instability. The ADI is determined by measuring the distance between the posterior edge of the anterior arch of C1 and the anterior edge of the dens. A value less than three mm is considered normal in adults and a value less than 4.5 mm is considered normal in children. Numbers greater than the maximal normal values listed above suggest disruption of the transverse ligament. There are numerous proposed measurements for the assessment of atlantoaxial instability specific to patients with RA.

Treatment / Management

Treatment of atlantoaxial instability varies widely, and intervention is usually tailored to patients on an individualized basis. Asymptomatic patients can be monitored over time with dynamic imaging and MRI to monitor disease progression. Patients that develop symptoms or present with symptoms are usually encouraged to undergo surgical fixation and fusion to limit the chance of future neurologic injury. Nonsurgical intervention can consist of rigid cervical collar or halo immobilization. Surgical intervention may require, placing patients in traction preoperatively and both anterior and posterior approaches to the spine may be employed, depending on the patient's pathology.[1],[6],[7]

Based on the staging system, the general treatment guidelines are as follows:

  • Type I - Stable subluxations manage conservatively with a collar
  • Type II - Potentially unstable and if asymptomatic, the neurosurgeon should be involved in making the treatment decision
  • Type III - Unstable joint, requires surgical stabilization
  • Type IV - Unstable joint, requires surgical stabilization

Differential Diagnosis

  • Cervical strain, trauma, fracture
  • Occipital headache
  • Degenerative disease of the cervical spine

Staging

A four-stage classification scheme based on rotatory displacement has been developed:

  • Type I - Simple rotatory displacement with an intact transverse ligament
  • Type II - Anterior displacement of C1 on C2 of 3 to 5 mm with one lateral mass serving as a pivot point and a deficiency of the transverse ligament
  • Type III - Anterior displacement exceeding 5 mm
  • Type IV - Posterior displacement of C1 on C2

Type III and type IV are highly unstable, and emergent treatment is recommended.[8]

Prognosis

The prognosis in symptomatic patients who are treated is good. Posterior spinal fusion can help restore function and reverse symptoms like pain and myelopathy.[9] However, in some patients, pain is a common symptom that can be disabling. Other individuals may develop the following neurological deficits:

  • Lack of coordination
  • Clumsiness
  • Difficulty with gait
  • Sensory deficits
  • Neurogenic bladder
  • Spasticity, clonus, hyperreflexia
  • Paraplegia, quadriplegia

In many cases, these neurological deficits are not always completely restored even with surgery.

Complications

The biggest risk of atlantoaxial instability is spinal cord compression which can present with the following complications:

  • Neck pain
  • Myelopathy
  • Spasticity
  • Radicular symptoms

Postoperative and Rehabilitation Care

Before any physical activity, a thorough neurological exam is recommended to determine the presence of any deficits. Premature exercise can result in paraplegia or quadriplegia.

Consultations

Once the diagnosis of atlantoaxial instability is made, one should consult the neurologist, neurosurgeon, and a geneticist if the patient is a child.

Pearls and Other Issues

The atlantoaxial segment consists of the atlas (C1) and axis (C2) and forms a complex transitional structure bridging the occiput and cervical spine.

The functional result of the joint is two-fold: to provide support for the occiput and the greatest range of motion and flexibility possible while still maintaining stability.

Instability in this joint is usually congenital, but in adults, it may be due to an acute traumatic event or degenerative disease.

Atlantoaxial instability can be classified into three generalized categories: inflammatory, congenital, and traumatic.

The "Rule of Spence" classically determines the stability of C1 fractures by measuring the lateral overhang of the lateral masses of C1 on C2 when viewing an AP radiograph. If the sum of both lateral masses of C1 on C2 is greater than 7mm, the fracture is considered unstable. This measurement tool is also generally used to assess CT scan images.

Treatment of atlantoaxial instability varies widely, and intervention is usually tailored to patients on an individualized basis. Asymptomatic patients can be monitored over time with dynamic imaging and MRI to monitor disease progression.

The prognosis in symptomatic patients who are treated is good. Posterior spinal fusion can help restore function and reverse symptoms like pain and myelopathy.

Enhancing Healthcare Team Outcomes

The management of patients with atlantoaxial instability is usually by the neurosurgeon. However, these patients are usually followed in the clinic by the primary care provider or nurse practitioner. The treatment is tailored to each patient based on symptoms. The majority of asymptomatic patients need regular monitoring with imaging studies. Symptomatic patients usually undergo fusion with good outcomes. However, the surgery is associated with neurological injury which can be incapacitating.

 

 


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Atlantoaxial Instability - Questions

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In which of the following conditions is atlantoaxial instability least likely to occur?



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A 67-year-old patient with rheumatoid arthritis develops vertigo, occipital pain, tinnitus, and dysphagia. Which of the following may be the cause?



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In which of the following sports may a patient with instability of the atlantoaxial joint participate?



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A 26-year-old patient with Down syndrome works full-time washing dishes. She has started falling when she walks long distances and has developed daytime encopresis. Which of the following is the most likely the cause of her new problems?



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Atlantoaxial Instability - References

References

Scholz C,Klingler JH,Scheiwe C,Naseri Y,Masalha W,Hubbe U, Atlantoaxial Instability in Patients Older Than 70 Years: What Is the Outcome When Further Conservative Treatment Is Not an Option? Journal of neurological surgery. Part A, Central European neurosurgery. 2018 Jul 4     [PubMed]
Cummings KR,Vilaplana Grosso F,Moore GE,Rochat M,Thomovsky SA,Bentley RT, Objective measurements of the atlantoaxial joint on radiographs performed without flexion can increase the confidence of diagnosis of atlantoaxial instability in toy breed dogs. Veterinary radiology & ultrasound : the official journal of the American College of Veterinary Radiology and the International Veterinary Radiology Association. 2018 Jul 16     [PubMed]
Kothe R, [Rheumatoid instability in the cervical spine : Diagnostic and therapeutic strategies]. Der Orthopade. 2018 Jun     [PubMed]
Lyons C,Ross M,Elliott R,Tall M, Atlantoaxial Instability in a Patient with Neck Pain and Ankylosing Spondylitis. Military medicine. 2018 Mar 26     [PubMed]
Robinson LC,Anderson RCE,Brockmeyer DL,Torok MR,Hankinson TC, Comparison of Fusion Rates Based on Graft Material Following Occipitocervical and Atlantoaxial Arthrodesis in Adults and Children. Operative neurosurgery (Hagerstown, Md.). 2018 Mar 15     [PubMed]
Joaquim AF,Tedeschi H,Chandra PS, Controversies in the surgical management of congenital craniocervical junction disorders - A critical review. Neurology India. 2018 Jul-Aug     [PubMed]
Harati A,Schultheiß R, New technique for C1-C2 fixation. Surgical neurology international. 2018     [PubMed]
Scollan JP,Alhammoud A,Tretiakov M,Hollern DA,Bloom LR,Shah NV,Naziri Q,Diebo BG,Yoshihara H,Scott CB,Hesham K,Paulino CB, The Outcomes of Posterior Arthrodesis for Atlantoaxial Subluxation in Down Syndrome Patients: A Meta-Analysis. Clinical spine surgery. 2018 Aug     [PubMed]
Macovei LA,Rezuş E, CERVICAL SPINE LESIONS IN RHEUMATOID ARTHRITIS PATIENTS. Revista medico-chirurgicala a Societatii de Medici si Naturalisti din Iasi. 2016 Jan-Mar     [PubMed]

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