CONDITION
Atlantoaxial Instability
Atlantoaxial instability describes excessive movement between the first two bones of the neck: the atlas and the axis. In a stable joint, ligaments and the shape of the bones themselves hold these vertebrae in alignment; when that support is absent or insufficient, the bones can shift out of position, and the spinal cord that runs between them may be compressed. The condition is seen most often in young toy-breed dogs, and less commonly in larger dogs and cats, though it can also develop after injury in animals of any size. Owners may notice neck pain, a reluctance to move the head, an unsteady gait, or weakness in one or more limbs. In some animals the signs appear suddenly after minor trauma, such as a jump or a stumble, while in others they build gradually over weeks or months. The severity varies widely; some animals show only discomfort, while others lose the ability to walk. This page explores what might be observed in an affected animal, the anatomical and developmental factors that underlie the instability, how the condition is investigated through imaging, and the range of approaches, both surgical and non-surgical, that exist to address it.
Why this matters now
Atlantoaxial instability most often appears in toy-breed dogs during the first year of life, reflecting an underlying developmental abnormality in which the bony peg that normally projects from the axis either fails to form properly or remains poorly anchored by ligaments. Yorkshire Terriers, Chihuahuas, Toy and Miniature Poodles, Pomeranians and Pekingese are among the breeds in which this congenital form is reported most frequently. In larger dogs and cats the condition is less common and tends to arise after direct trauma to the neck, such as a road traffic accident or a fall, which can damage the ligaments or fracture the bone. Some dogs with a congenital abnormality do not show signs until later in life, sometimes after a minor knock.
The onset of signs can be abrupt or gradual. In some animals a minor event, such as a jump from furniture, a sudden turn of the head, or rough play, may be enough to shift the vertebrae and compress the spinal cord, leading to deterioration within minutes, hours or days. In others the instability exists for months or years without provoking obvious signs, until cumulative stress or a small additional injury crosses a threshold. Once spinal compression begins, the severity of neurological signs often correlates with the degree of cord damage; some animals remain at a mild level of discomfort for a time, while others progress to complete loss of voluntary movement in all four limbs if the compression remains unrelieved. Severe compression at this level can also stop breathing, and breathing failure is a recognised cause of sudden death in this condition. It can follow a minor knock, a jump or handling that bends the neck, in an animal whose previous signs were mild. How unstable the joint is, and how close the spinal cord is to serious injury, cannot be judged from how well the animal appears to be walking.
Signals & patterns
Early signals
Neck pain or stiffness
The dog or cat may hold the head in a lowered or rigid position, resist gentle movements of the neck, or cry out when picked up under the chest. This pain reflects irritation of the membranes surrounding the spinal cord or strain on the ligaments and muscles attempting to stabilise the unstable joint.
Reluctance to look up or down
An owner may notice that the animal avoids lowering the head to a food or water bowl on the floor, or lifting it to look at a toy held above eye level. This guarding behaviour suggests that certain neck positions increase discomfort or a sensation of instability.
Subtle gait changes
The animal may walk with a slightly shortened stride, place the feet more carefully than usual, or show a mild wobble when turning. These changes can reflect early interference with the nerve signals travelling through the compressed segment of spinal cord.
Episodes of wobbliness or weakness in all four legs
Compression in the upper neck usually affects all four legs rather than the front legs alone, and the hind legs are often the more obviously wobbly or weak. Episodes may come and go as the vertebrae shift, with the animal stumbling, crossing its legs or briefly sinking down before appearing normal again. Improvement between episodes does not mean that the joint has become stable.
Later signals
Inability to rise or walk
As spinal cord compression becomes more severe or sustained, the animal may lose voluntary control of one or more limbs and remain lying down, unable to support weight. This reflects significant disruption of the nerve pathways that co-ordinate movement and maintain posture.
Changes in breathing
Severe compression at the level of the first two neck vertebrae can disrupt the pathways that carry breathing signals from the brainstem down the spinal cord to the diaphragm and chest muscles. Breathing may become shallow, laboured or irregular, and it can stop altogether. Breathing failure is a recognised cause of sudden death in atlantoaxial instability, and it can occur in an animal that was still walking and could still feel its feet. Laboured, shallow or irregular breathing in an affected animal reflects compression severe enough that breathing can fail completely, and how close that point is cannot be judged at home.
Sudden collapse after a minor event
A jump from furniture, a fall, rough play, a minor knock or a movement that bends the head down can shift the vertebrae abruptly. Some animals become unable to move all four legs within minutes, and some die from breathing failure, including animals whose earlier signs were only mild neck pain or a slightly unsteady gait. The absence of severe signs beforehand is not a sign that the joint is stable.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with a clinical history and systematic neurological examination to establish the pattern and severity of signs, followed by imaging to confirm the diagnosis and assess the degree of instability. Radiography may be performed first, often with the neck in a neutral position or with gentle flexion, though care is taken to avoid worsening compression during positioning. Advanced imaging with computed tomography or magnetic resonance imaging can clarify the anatomical abnormality, the extent of spinal cord compression, and the presence of secondary changes such as swelling or signal change within the cord itself.
Physical examination
Radiography
Computed tomography (CT)
Magnetic resonance imaging (MRI)
Options & trade-offs
Management can involve surgical stabilisation of the joint, conservative approaches with restricted activity and external support, or a combination of both depending on the severity of signs and the owner's circumstances. Different animals and different owners find different combinations workable. The choice often reflects the degree of neurological impairment, the age and size of the animal, and the practical constraints of prolonged confinement or anaesthetic risk.
Surgical stabilisation
Surgery aims to fix the atlas and axis in a stable alignment, either by placing implants from below the vertebrae (ventral fixation) or from above (dorsal fixation), often using screws, pins, wire, and bone cement or plates. The procedure aims to relieve pressure on the spinal cord and prevent further movement at the joint. Recovery typically involves several weeks of restricted activity while the fusion matures.
Trade-offs: Surgery carries the risks of general anaesthesia and the technical challenges of working in a small space near vital structures; complications can include implant loosening, infection, or inadvertent injury to the spinal cord or blood vessels. Some animals improve within days, while others take months, and some do not regain full function despite successful stabilisation. Published case series report death or euthanasia around the time of surgery in a minority of dogs, ranging from none in some recent series to between a quarter and a third of dogs in one older series, and breathing complications are among the reported causes. The procedure is usually performed at a referral centre and involves the costs associated with specialist intervention.
Conservative management with external support
This approach involves strict cage rest and the use of a neck brace or similar device to limit movement at the atlantoaxial joint for a period of six to eight weeks, during which time scar tissue may form and provide some degree of natural stabilisation. Pain relief and anti-inflammatory medication may be used concurrently. The intention is to allow healing without surgical intervention.
Trade-offs: Conservative management gave a good long-term outcome in 10 of the 16 dogs followed up long term in one series of 19, regardless of how severe the neurological signs were at the time, though results were less favourable when signs had been present for more than a month. Recurrence of instability is common once the external support is removed and activity resumes. Many animals managed in this way experience a return of pain or worsening neurological signs, and some eventually require surgery. Prolonged confinement can be difficult to maintain in active young dogs, and the scar tissue that forms is often insufficient to hold the joint stable long term.
Medical support and monitoring
For animals in which neither surgery nor prolonged external coaptation is pursued, perhaps because of owner preference, concurrent illness, or very mild intermittent signs, management may focus on controlling pain with analgesics and anti-inflammatory drugs, minimising activities that involve jumping or sudden head movements, and monitoring for changes in neurological status. This is not curative and does not address the underlying instability.
Trade-offs: This approach does not prevent progression of spinal cord injury and carries the risk that a minor trauma or sudden movement will precipitate acute deterioration. It may be appropriate for animals with minimal clinical signs or those for whom anaesthesia poses unacceptable risk, but it requires careful observation and a willingness to reassess if signs worsen. Quality of life and the potential for sudden catastrophic injury are ongoing considerations.
Common misconceptions
"A neck brace reliably fixes atlantoaxial instability without surgery."
In one series, about six in ten dogs managed with a neck brace had a good long-term outcome. The brace restricts movement while scar tissue forms, but the scar tissue is often not strong enough to hold the joint stable once the brace is removed and normal activity resumes, so pain or neurological signs can return and some dogs go on to have surgery.
"If an animal with atlantoaxial instability has only mild symptoms, the condition will not progress."
The severity of signs can fluctuate, and an animal with mild discomfort or subtle gait changes can deteriorate suddenly after a seemingly trivial event such as a jump or a knock. The underlying instability remains unless the joint is stabilised, and the spinal cord is at ongoing risk of compression. Mild signs do not guarantee a benign course.
"Surgery always restores full neurological function."
Surgical stabilisation aims to relieve ongoing compression and protect the cord from further injury, but it cannot reverse damage that has already occurred within the spinal cord. Some animals regain near-normal function, while others show partial improvement or remain with persistent deficits. The degree of recovery depends on the severity and duration of spinal cord injury before surgery, and outcomes vary widely between individuals.
Related conditions
Discospondylitis
Discospondylitis can produce neck pain and reluctance to move the head in a way that may resemble atlantoaxial instability, though the underlying cause, infection of the vertebrae and disc rather than ligamentous laxity, differs. Both conditions may require imaging to distinguish spinal cord compression from infection-related inflammation.
Fibrocartilaginous Embolism (FCE)
Fibrocartilaginous embolism can cause sudden weakness or paralysis, sometimes during exercise or after a minor knock, which may resemble the acute deterioration seen in atlantoaxial instability. Signs of fibrocartilaginous embolism typically stop worsening within the first 24 hours and are usually not painful once the event has passed, whereas atlantoaxial instability is often painful and remains liable to worsen. Imaging is needed to separate the two.
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Cauda Equina Syndrome
Cauda equina syndrome involves compression of the nerves at the base of the spine, mostly in middle-aged and older medium to large dogs, and affects the hind legs, tail, bladder or bowel while the front legs remain normal. A lesion in the upper neck, as in atlantoaxial instability, typically affects all four legs, so the pattern of signs on neurological examination helps place a problem at the correct level of the spine.
Intervertebral Disc Disease (IVDD)
Disc extrusion in the neck can cause neck pain, a low head carriage and weakness or incoordination in all four legs, closely resembling atlantoaxial instability in a small-breed dog. Disc disease is more typical of adult dogs, while congenital atlantoaxial instability usually appears in the first year of life, but imaging is needed to tell them apart.
Syringomyelia (CM/SM)
Chiari-like malformation and syringomyelia affect many of the same small breeds and can cause neck pain, scratching at the neck or shoulder, and incoordination. The malformation is also reported alongside atlantoaxial instability in some dogs, and MRI is typically needed to establish which abnormality accounts for the signs.
Granulomatous Meningoencephalomyelitis (GME)
Inflammatory diseases of the brain and spinal cord of unknown cause, including granulomatous meningoencephalomyelitis and necrotising meningoencephalitis, are seen in young to middle-aged small and toy breeds and can cause neck pain, incoordination and weakness. They are distinguished from atlantoaxial instability with MRI and analysis of spinal fluid.
Steroid-Responsive Meningitis-Arteritis (SRMA)
Steroid-responsive meningitis-arteritis causes marked neck pain, a stiff gait, reluctance to lower the head and often a fever, typically in dogs between six and eighteen months old. It is reported most often in breeds such as Beagles, Boxers, Border Collies and Weimaraners, and in its typical form it tends to cause pain without the incoordination or weakness that atlantoaxial instability often produces.
Hydrocephalus
Congenital hydrocephalus is seen in many of the same toy breeds, often in the first months of life, and can cause a domed head, altered behaviour, visual problems, circling or seizures. Some of its signs, such as incoordination, overlap with those of a high neck lesion, and imaging of the brain and neck helps separate the two.
In atlantoaxial instability, the main risk is often not a gradual decline but sudden injury to the spinal cord at a level that controls breathing. While the joint remains unstabilised, ordinary events such as jumping down from furniture, rough play, a minor knock, or handling that lets the head drop forward can shift the vertebrae without warning. Laboured, shallow or irregular breathing, a sudden inability to stand, or collapse in an affected animal reflects compression severe enough that breathing can fail, and none of these can be assessed or reversed at home. Moving the neck to look for pain can itself worsen compression, which is why examination and imaging are carried out with the neck supported. For animals under investigation or awaiting a decision about treatment, the relevant conversations tend to cover how the neck is protected in the meantime (including whether a harness is used in place of a neck collar), the risks and benefits of surgical and non-surgical paths, and the timeframes over which improvement might be expected.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS