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Case Report: Atlantoaxial subluxation with concurrent quadrigeminal cyst in a five-year-old Yorkshire Terrier.

A 2.5kg, male, entire Yorkshire Terrier was referred for investigation of a month-long history of cervical pain preceding peracute-onset tetraparesis.

fig.1—sagittal T2W MR-scan of brain & cranial cervical spine showing quadrigeminal cyst and oedema of the spinal cord at the level of the atlantoaxial joint

There had been no relevant previous history and initial assessment by the referring vet had revealed no abnormalities other than medial luxation of the patellae. .

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Examination

General physical examination revealed a thin body condition, but no abnormalities other than guarding of the cervical spine.  There was voluntary movement, but marked reduction of conscious proprioception in all limbs, with the patient being unable to stand. Neurological assessment revealed increased muscle tone and brisk segmental spinal reflexes in all limbs.  There was no evidence of intracranial disease, which was consistent with a CNS lesion within the Ce1-5 segments of the spinal cord..

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Further Investigations

fig.2 — lateral radiograph of cervical spine with moderate flexion of the atlantoaxial joint:- note increased distance between the dorsal arch of Ce1 and the cranial limit of the dorsal spine of Ce2—compare with fig 3.

Routine blood-testing, including coagulation studies, was unremarkable.  MR-scans of the cranial cervical spine (fig.1) revealed a large cystic structure rostral to the cerebellum, consistent with a quadrigeminal cyst.  In addition, there was increased T2W signal within the spinal cord at the level of the atlantoaxial joint.  This joint appeared stable on the scans but dynamic flexion radiographs (figs. 2 & 3) revealed subtle instability of the joint.

Treatment

fig. 3 — lateral radiograph of cervical spine following screw fixation of the atlanto-axial joint:- note normal distance between the dorsal arch of Ce1 and the cranial limit of the dorsal spine of Ce2—compare with fig 2, above.

fig.4 — ventrodorsal postoperative view of cranial cervical spine:- note divergence of screws to avoid neural canal.

Ventral screw fixation of the atlantoaxial joint was performed via a standard, midline, ventral approach (see figs. 3 & 4), using two divergent 1.5mm lag-screws to stabilise the affected joint1....

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Outcome

The patient made an unremarkable recovery from surgery, and regained the ability to walk within a few days, with slight residual ataxia which persisted for several months.

Discussion

The discovery of a quadrigeminal cyst was an unexpected finding; these are normally found in brachycephalic breeds and are often associated with overcrowding of the caudal fossa.  Given the clinical signs and neurolocalisation, the cyst was thought to be an incidental finding.  Conversely, the changes present within the cord parenchyma at the level of the atlantoaxial joint did fit with the clinical findings, and this precipitated further imaging (dynamic radiology) with a positive diagnostic yield.

Flexion studies of the cervical spine should be performed with extreme caution in cases where atlantoaxial subluxation is suspected; excessive force may compress the cord enough to cause respiratory arrest– An initial lateral view of the cervical spine should be performed with the neck in extension, before mild flexion is applied; any increase in the distance between the dorsal arch of Ce1 and the cranial limit of the dorsal spine of Ce2 is pathognomonic for instability and further degrees of flexion should be avoided.

The lack of any previous cervical signs and relatively late onset is unusual for atlantoaxial subluxation and it is normally seen in immature animals.  However, this condition must be ruled out as a possible differential with unexplained cervical spinal pain or acute onset paresis in miniature dog breeds of any age where disc-disease has been excluded.

Atlantoaxial subluxation may be managed conservatively using a neck brace, but excellent results can be achieved with internal fixation, but bearing in mind the risks of placing implants so close to the cervical spine..

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Reference:

1. Sharp NJ & Wheeler SJ (2005) Atlantoaxial subluxation in Small Animal Spinal Disorders.  Diagnosis and Surgery p.161-180 Elsevier Mosby, London.