History

A 6-year-old Thoroughbred mare presented to a tertiary referral center for evaluation of a neck injury sustained 5 days prior. The mare had been evaluated by her primary veterinarian on the farm the day of injury. Severe painful soft tissue swelling and edema of her left dorsocranial neck and head were noted, and the mare had an extremely limited range of motion in the neck. On day 3, lateral radiographs were taken of C1 and C2, but a clear diagnosis was not immediately identified. On-farm treatment with NSAIDs (flunixin meglumine once then firocoxib twice daily for 4 days) and dexamethasone for the first 2 days was elected. The soft tissue swelling and edema improved over the next few days prior to referral, but a lateral deviation in the neck contour on the left side, behind the ear, remained. The mare was referred due to persistent neck pain and abnormal head carriage.

On presentation, the mare was quiet, alert, and responsive. The patient exhibited signs of pain with full-body muscle fasciculations and tachycardia (heart rate, 60 beats/min) and tachypnea (respiratory rate, 20 breaths/min). There was marked cranial-cervical, left lateralized firm swelling at the level of the atlas (C1). Palpation suggested lateral displacement of the left wing of the C1. The mare exhibited a moderate pain response upon palpation of the cranial and midcervical neck. Neurologic examination revealed normal mentation and cranial nerve function as well as spinal ataxia of all 4 limbs, worse in the pelvic limbs (Mayhew grade 3/5) than the thoracic limbs (grade 2/5). The mare had a narrow-based stance in the pelvic limbs with intermittent hypermetria and inappropriate placement. The mare had neck rigidity, carried her neck in a lower-than-normal position, and was unable to flex in lateral or dorsoventral planes when stationary or walking.

Four-view cranial-cervical radiographs were acquired and were assessed (Figure 1).

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Cervical spinal radiographs (dorsal-ventral [DV], right lateral [R Lat], left-ventral right-dorsal oblique [LVRDO], and right-ventral left-dorsal oblique [RVLDO] views) of a 6-year-old Thoroughbred mare evaluated for an unknown neck injury sustained 5 days prior.

Citation: Journal of the American Veterinary Medical Association 261, 12; 10.2460/javma.23.05.0256

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Formulate differential diagnoses, then continue reading.

Diagnostic Imaging Findings and Interpretation

In the dorsal-ventral projection, there is diffuse, moderately increased soft tissue opacity of the cranial cervical region, and a left lateral displacement of the left wing of C1 is appreciated. Also noted is an asymmetry and misalignment of the perpendicular axes of the head with the first and second cervical vertebrae (Figure 2). The right lateral demonstrates a rotation of C1 with a widening between the dorsal arch of C1 and the occipital condyles (Figure 2). There is a moderate widening between the dorsal arch of cervical vertebra 1 (C1) and the occipital condyles appreciated in the left-ventral right-dorsal oblique (Figure 2). The cranial articular fovea summates with the ventral aspect of the vertebral canal on the left-ventral right-dorsal oblique. Final diagnosis is a suspected acute, traumatic atlanto-occipital (AO) luxation.

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Same images as in Figure 1. DV—Moderate soft tissue opaque swelling of the cranial cervical region is appreciated. A left lateral and rotational displacement of the left wing of C1 is appreciated (curved arrow). Also noted is an asymmetry and misalignment of the perpendicular axes of the head, C1, and C2 (dotted lines). R Lat—C1 is rotated with a widening between the arch of C1 and the occipital condyles (double-headed arrow). LVRDO—There is a moderate widening between the dorsal arch of C1 and the occipital condyles (double-headed arrow). The cranial articular fovea summates with the ventral aspect of the vertebral canal (asterisk). There is also axial luxation of the left dorsal arch of C1 relative to the left occipital condyle (single-headed arrow) appreciated on the LVRDO when compared with the RVLDO. This finding is definitive for complete luxation.

Citation: Journal of the American Veterinary Medical Association 261, 12; 10.2460/javma.23.05.0256

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Treatment and Outcome

Due to the poor prognosis for return to full function, euthanasia was elected. A postmortem CT scan was performed for educational purposes. Multiplanar reconstruction and a 3-D reconstruction of the cranial cervical spine show complete luxation of the AO joint with several small bone fragments, presumed to be avulsion fractures of the AO lateral ligaments, located between the occiput and C1 (Figure 3). Importantly, the extent of the luxation was difficult to interpret via lateral digital radiography in this case; however, physical examination revealed a palpable lateral deviation of the wing of C1. Conventional 2-D imaging is limited in this region due to anatomic complexity, resulting in superimposition and difficulty in positioning for orthogonal views. As CT is not always feasible in large animals, this highlights the importance of combining the physical examination findings and palpation with radiographic findings to substantiate your diagnosis. Furthermore, multiple oblique radiographic views are critical to the evaluation in areas of complex anatomy such as the craniovertebral joints.

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Multiplanar reconstruction and a 3-D reconstruction of a postmortem CT revealing complete luxation of the atlanto-occipital joint with a left lateral and slight rotational displacement of C1 and overriding of the occipital condyles (OCs) and C1. There are several small bone fragments (arrow), presumed avulsion fractures of the AO lateral ligaments, located between the OC and C1, which can be appreciated in the transverse view multiplanar reconstruction.

Citation: Journal of the American Veterinary Medical Association 261, 12; 10.2460/javma.23.05.0256

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Traumatic luxation of the AO joint is rare. The equine AO joint is stabilized by the dorsal and ventral AO membranes and the lateral AO ligaments.¹ The dorsal AO membrane extends from the dorsal border of the foramen magnum and the occipital condyles to the cranial border of the dorsal arch of the C1.¹ The ventral AO membrane extends from the ventral arch of C1 to the ventral border of the foramen magnum.¹ Both the dorsal and ventral AO membranes are fused with the joint capsule. The lateral AO ligaments provide further stability and extend from the base of the jugular processes and part of the paracondylar processes of the occipital bone toward the craniolateral border of the dorsal arch of C1 where it fuses with the joint membrane.¹ These structures, in addition to the extent of soft tissue support in this region in equids, provide excellent anatomic stability. AO subluxations may be seen in foals with congenital occipitoatlantoaxial malformation.² Traumatic luxations of the cervical spine may be seen in young horses secondary to falling or colliding with large stationary objects, cervical hyperflexion, and falling over backward.³ However, traumatic luxation of the AO joint is relatively rare in the veterinary literature, and most patients are expected to present as nonambulatory and tetraparetic. Successful treatment of small animals (canine and feline) and large animals (alpaca and goat) have been reported via both closed reduction and surgical stabilization.^4,5 To date, there have been no reports of AO luxation treatment in adult horses. Postmortem treatment via closed reduction has been attempted; however, the authors cautioned of potential neurologic injury secondary to traction and manipulation with unpredictable results.¹ Selection of treatment options in horses is complicated by the paucity of reported cases, difficulty in AO joint reduction with associated exacerbation of neurologic injury, limited cervical stabilization methods, patient compliance, prognosis for treatment when offering options for owner review, and permanence or incomplete recovery from existing neurologic deficits and pain syndromes, which can be permanently disabling for the horse and result in safety concerns for handlers.

In the present case, lateral radiographs performed in the field did not readily identify AO luxation. Evaluation of symmetry on dorsal-ventral projection is critical to evaluate lateral and rotational cervical spinal luxation. Orthogonal radiographs of the cervical spine are generally sufficient diagnostically; however, in this case, the oblique radiographs acquired allowed definitive diagnosis of the luxation. The authors suggest using these additional views in these cases to help interpret the orthogonal radiographs. Ultrasound may also be useful in diagnosis in the field. This case was not evaluated with ultrasound. Furthermore, myelography or CT myelography may be recommended prior to consideration of any treatment to better prognosticate. As induction, manipulation of the horse into the CT, and then recovery from general anesthesia pose a significant risk of worsening the neurological signs in these horses, sufficient evaluation of the luxation may be beneficial prior to anesthesia if standing, sedated CT is not available. Prognosis for recovery in horses with AO luxation is presumed to be grave.¹ Early diagnosis and stabilization would be necessary for survival and return to function. The authors recognize that external coaptation (ie, fiberglass cast) is unlikely to be feasible in the field, and therefore, shipping in a free box stall without tying the horse’s head is recommended. If conservative management were to be pursued, complications would likely include persistent pain, development of post-traumatic osteoarthritis, reduced cervical range of motion, vertebral artery hemorrhage or hematoma leading to stroke or ischemic brain injury, and persistent lameness or neurologic deficits. Furthermore, any persistent instability of the AO joint and compression of the spinal cord pose an important safety risk for those handling a horse.

Disclosures

The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.

Funding

The authors have nothing to disclose.