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Atlantoaxial Instability: Definition, Causes, Risk Factors and Complications

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Atlantoaxial Instability Definition

Atlantoaxial Instability is a condition that is characterized by an excess in movement between the vertebrae in the neck known as the atlas and the axis. Speaking about Atlantoaxial instability definition, this condition is considered as a result of an abnormality in the bones or the ligaments. If the nerve roots or the spinal cord are affected, neurologic symptoms can result as a complication of Atlantoaxial Instability.

Atlantoaxial Instability can be due to congenital conditions, but when the condition occurs in adults, it is usually due to degenerative changes related to Rheumatoid Arthritis or due to acute trauma. The infection has also been found to cause Atlantoaxial Instability. This is because the area has a rich supply of blood which provides a route for complications of infections to develop.

There are many congenital causes for Atlantoaxial Instability. These include:

  • Down Syndrome
  • Larsen Syndrome
  • Osteogenesis Imperfecta
  • Morquio Syndrome
  • Spondylotic-Epiphyseal Dysplasia (SED)
  • Neurofibromatosis
  • Meta tropic Dysplasia
  • Kniest Syndrome
  • Chondrodysplasia Punctata

Other diseases that can result in Atlantoaxial Instability include diseases that involve bone structure or metabolism or the characteristics of the ligaments.


Atlantoaxial Instability can occur due to abnormalities in the bones or can be the result of abnormal development, or an ossification or fracture of the odontoid process (peg) of the axis, without or with remodeling.

It is not known what happens in the transverse ligament that leads to Atlantoaxial Instability. It could be that an abnormal protein in the connective tissue causes the lack of tension in the ligament that is seen in people with Down syndrome. Ligament inflammation that occurs in people with Rheumatoid Arthritis or infections who have Atlantoaxial Instability, can weaken the joint and this leads to subluxation (dislocation).

Conditions associated with atlantoaxial instability include:

  • Down Syndrome
  • Larsen Syndrome
  • Congenital Scoliosis
  • Neurofibromatosis
  • Osteogenesis Imperfecta
  • SED Congenita
  • Morquio Syndrome
  • Chondrodysplasia Punctata
  • Kniest Syndrome
  • Metatropic Dysplasia
  • Os Odontoideum
  • Odontoid Abnormalities
  • Third Condyle
  • Ossiculum Terminale
  • Hypoplasia or absence of the dens
  • Pseudoachondroplasia
  • Achondroplasia
  • Rheumatoid arthritis
  • Cartilage-hair hyperplasia
  • Ankylosing Spondylitis
  • Steroid therapy
  • Scott Syndrome
  • Trauma
  • Tumors
  • Cerebral Palsy
  • Infections of the head and neck


United States Statistics

Atlantoaxial Instability is extremely rare in people who do not have factors that predispose them to the development of the condition. There is no data in existence related to the prevalence of the condition when risk factors are absent. Among people who have Down Syndrome, Asymptomatic Atlantoaxial Instability occurs at a frequency of approximately 13% based on some studies. The symptomatic type of the condition was found to occur at a frequency of approximately 1.5% in Down Syndrome patients.

In people who have Rheumatoid Arthritis, the rate of Atlantoaxial Instability or AAS has been estimated at approximately 20%, but it occurs in up to 49% of older individuals who have RA according to some other research studies.

Other demographics

Atlantoaxial Instability does not appear strongly preponderant in any specific age group. Individuals more at risk are those younger people who have Down Syndrome and older people who have Rheumatoid Arthritis. Also, Grisel Syndrome occurs more frequently in children, however, AAS can develop after any infection of the nose and throat. Most of these conditions resolve without the patient ever being seen for medical treatment.

No association with gender has been observed with Idiopathic Atlantoaxial Instability, but other disorders that could lead to developing the condition may be more prevalent in one sex than the other. For instance, Rheumatoid Arthritis occurs in women nearly five times more often than it does in men.

There is no recognized racial predilection for Atlantoaxial Instability.


Prognosis is typically good for patients who have Symptomatic Atlantoaxial Instability when they have a posterior spinal fusion and their function returns. Surgery may be able to decrease symptoms of Myelopathy, relieve pain, or accomplish both in many people. Results and outcome are dependent on the cause of the instability and the severity of symptoms.

Extreme ranges of motion can make patients susceptible to an arising or even worsening spinal cord compression. There are symptoms of Neurologic Dysfunction progression such as:

  • Myelopathy
  • Spasticity
  • Radicular symptoms
  • Neck and shoulder pain

Mortality and Morbidity

For most people, pain is the most common symptom experienced in patients with Atlantoaxial Instability. Pain is typically either a headache or vague pain in the neck pain. Pain is typically nonspecific and many times, patients require further evaluation to determine the exact source. Due to the location of the spinal cord and the blood vessels to the vertebrae in this area, additional serious complications like Vascular Occlusion or Myelopathy are possible.

Neurologic signs and symptoms include loss of coordination, clumsiness, difficulty walking, abnormal gait, easy fatigue, limited neck mobility and pain, torticollis, neurogenic bladder, sensory deficits, upper motor neuron signs (Hyperreflexia, Spasticity, Babinski sign, Clonus), Hemiplegia, Paraplegia, and Quadriplegia. Any person who is at risk for Atlantoaxial Instability and develops these symptoms should be urgently evaluated.

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