Discussion
Diseases affecting the atlanto-occipital joint have been rarely documented in the horse and their diagnosis can be a considerable challenge. Craniovertebral junction (CVJ) trauma is a difficult clinical condition. Being a highly mobile functional unit at the junction of the skull and the vertebral column, traumatic events in this area may produce devastating neurological complications and death. Additionally, many of the CVJ traumatic injuries can be left undiagnosed or even raise difficult treatment dilemmas .
In human literature it is considered that the combination of clinical and radiographic examination when indicated (using the National emergency X-Radiography Utilization study criteria) is inadequate for fracture detection of the cervical spine following blunt trauma compared to CT imaging .
Again in human literature, when comparing radiography to CT imaging the lateral view radiograph alone had only a 73% sensitivity (95% CI, 50–89%) and 92% specificity (95% CI, 87–95%) for cervical spine abnormalities compared with MDCT findings. The addition of other radiographic views did not change the sensitivity of radiography but rather marginally decreased its specificity to 91% (95% CI, 86–94%).
Although radiographic examination of the entire cervical vertebral column had been performed at the time of initial onset of clinical signs, the fracture had not been identified. Retrospective review of the radiographic examination allowed identification of the ventrally located linear osseous fragment, however definitive diagnosis of the extent of the atlas and occiput fractures was not possible, nor was the identification of any compression of the spinal cord.
Definitive diagnosis was achieved by CT myelography of the cervical vertebral column, performed under general anaesthesia. This allowed detailed analysis of the fracture configuration and visualisation of extradural compression caused by soft tissue swelling associated with the atlanto-occiptal fracture site. In addition, general anaesthesia allowed CT imaging of the entire cervical spine providing the ability to rule out the presence of injury in the rest of the neck. Magnetic resonance imaging would have theoretically allowed improved soft tissue assessment and more accurate determination of the nature of the soft tissue material within the vertebral canal however the practical application of Magnetic Resonance Imaging for imaging the neck in the live horse is limited due to patient size and practical application. CT imaging was chosen due to availability of the imaging modality. Whilst general anaesthesia was an increased risk to the patient, this was considered justifiable in order to determine a diagnosis.
General anaesthesia is not always possible if horses are displaying severe neurological signs or for economical reasons. Practical application of CT imaging a neurological case under standing sedation is based upon level of neurological deficits and risk assessment would be performed on a case by case basis due to the increased difficulties of positioning a neurological patient for standing sedation. In neurological patients, initial treatment and monitoring has been performed prior to anaesthestic in order to reduce risk to the patient during recovery. In this case, general anaesthesia allowed positive myelogram acquisition and also to assess the entirety of the neck however standing CT could be performed for lesion monitoring once the primary diagnosis had been made.
Considered treatment options for this case included (1) conservative management, (2) fracture fixation, (3) dorsal hemilaminectomy or (4) local infiltration of steroids into the cerebrospinal fluid and left atlanto-occipital joint.
Continued conservative management was considered inappropriate by the owner due to ongoing clinical signs, pain and neurological deficits displayed by the patient. Both clinician and owner felt interventional therapy was justified.
Option (2) fracture fixation was considered. Atlantoaxial arthrodesis is described in humans and is a technically difficult and invasive procedure. Internal fixation of a fractured axis in an adult horse has been successfully performed but fixation of an atlas has not been described in the horse. Given the comminuted conformation and chronic nature of the fracture site, it was considered this would be challenging surgery with a high risk recovery for the patient from general anaesthesia.
Option (3) dorsal hemilaminectomy in order to release the pressure within the spinal column and improve ataxia. Nixon and Stashak (1988) have described successful laminectomy for relief of compression caused by atlantoaxial subluxation in four horses. Two horses recovered fully, one horse had residual grade 1 neurologic deficits and the fourth horse was euthanised due to limb fracture 6 weeks after surgery. Laminectomy and hemilaminectomy has been reported as a successful treatment for relieving spinal compression in dogs. In canines this procedure is commonly associated with postoperative neurological deterioration (. Given the increased risk to the patient this option was not chosen.
Finally, option (4) infiltration of corticosteroids into the cerebrospinal fluid and left atlantooccipital joint medication. Intra-thecal steroid use is widespread for a variety of reasons in humans but to the authors knowledge, has not been reported in equines. Personal communication with colleagues had suggested positive response to intrathecal corticosteroids following traumatic spinal injury in equines. It was thought this could be successfully performed in the standing patient using the technique described by and therefore eliminate any further anaesthetic risk for the patient. Medication of both the atlantoccipital joint and the cerebrospinal fluid was performed sequentially under the same sedation, therefore it is not possible to definitively determine which of these procedures provided a greater contribution to improvement in the clinical signs.
The technique for medication of the cerebrospinal fluid and the atlanto-occiptal joint was performed under ultrasound guidance. This technique was a quick procedure to undertake and repeatable without side effect. However, this case report describes only one case whereby no adverse side effects were found following subarachnoid space injection of corticosteroids under standing sedation. A larger number of cases is required in order to ascertain the safety of this procedure.
This case report describes a fracture to the atlanto-occiptial joint which had been refractory to conservative management and presented with chronic remodelling of the fracture site. Local medication of the atlanto-occiptal joint and cerebrospinal fluid resulted in significant improvement in ataxia and clinical signs almost immediately post treatment.