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Case Report: Sub-arachnoid Cyst in a Seven-month-old Tamaskan.

 

History

A seven-month-old, entire, male Tamaskan presented with a two-month history of progressive ataxia of all limbs. The owner also reported difficulty in getting the dog to gain weight, but a select-protein diet had helped in this regard.  In the few weeks prior to presentation, he had begun to drag the left thoracic limb and frequently fell over during free running.  He had remained bright and comfortable throughout.

Clinical Examination

General physical examination was found to be unremarkable other than a thin body condition (score: 2/5)  and an ataxic, high-stepping gait.  Neurological examination revealed normal mentation.  Cranial nerve examination was within normal limits.  There was reduced conscious proprioception and increased muscle tone, with brisk segmental spinal reflexes in all limbs, indicative of a spinal lesion cranial to the Ce5 cord segment.  There was no evidence of cervical spinal discomfort.

Further Investigations

fig.1 Parasagittal T2W MR-scan of subarachnoid cyst within spinal cord parenchyma, just cranial to the Ce2-3 disc space

MR scans of the brain were normal.  Cervical spinal scans (fig.1; click to enlarge) revealed focal enlargement of the spinal cord just cranial to the level of the Ce2-3 disc space.  There was increased T2W signal intensity within the swelling suggestive of fluid accumulation.  There was no evidence of gadolinium uptake on contrast series.  CSF analysis, including serology for protozoal infection, thyroid function and  coagulation testing were all found to be within normal limits.  The above findings were consistent with a subarachnoid cyst.  Other differentials include syringomyelia secondary to local obstruction of CSF flow, traumatic/ vascular insult and lymphoma.  These could not be completely excluded on the basis of the scans alone, but the CSF findings and history were consistent with congenital cyst formation.

Treatment

fig.2 - Intraoperative photograph of durotomy over cyst to marsupialise and effect drainage

The affected region of the cervical spine was approached via a dorsal laminectomy and a durotomy was performed (fig. 2).  This confirmed the presence of a cyst within the cord substance, which took the form of a network of small spaces, or trabeculae arranged as a mesh across the subarachnoid space.  The overlying dura was marsupialised to the surrounding tissues and the epaxial muscle layers were closed, over an active drain.  The drain was removed at 48 hours post –op.  The dog made an unremarkable recovery from surgery, and was discharged the day after the drain was removed.  In the six months since surgery, the dog has made a gradual improvement, with ataxia no longer apparent.

Discussion

Sub-arachnoid cysts are relatively uncommon.  They lack an epithelial lining, so are not true cysts and may present in two forms; congenital defects, along with dysraphism in juveniles and acquired (c.f. syringomyelia) in older patients.  They may be found as an incidental finding in survey MR-scans or result in disease by way of an accumulation of CSF/ interstitial tissue fluid, resulting in focal ischaemia of cord substance.  Signs usually include progressive ataxia and paresis of limbs caudal to the cyst, (C2-3 and T13-L1 being common sites) with signs being steadily progressive over several months.  MRI is very useful to assess the integrity of the cord parenchyma but supplementary myelography may also be used to help guide the surgical approach.  However, diagnosis is usually confirmed at surgery or necropsy.  The condition has a poor prognosis with conservative management, but appropriate surgical management, may have a successful outcome.