Abstract

A captive adult female southern hairy-nosed wombat (Lasiorhinus latifrons) was discovered recumbent and unable to rise. Small patches of blood were present in her pelage and around her muzzle. She had appeared completely normal when last seen the previous evening. Physical examination revealed mild scrapes and lacerations, predominantly down the left side, consistent with convulsive or struggling self-trauma. She also displayed rigid paralysis, an inability to raise herself or stand (with stimulation producing only uncontrollable rolling to the left), evenly dilated pupils which were responsive only to intense light, continuous nystagmus, tachypnoea, and anxiety. Urine and CSF samples were normal and serial testing for toxoplasmosis proved negative. Blood samples revealed lymphocytosis and azotaemia. Supportive care was provided for the next 17 days during which the lymphocytosis and azotaemia resolved. Her neurologic condition improved slightly; however, her physical condition deteriorated over this period.

Detailed neurologic examination at this point revealed mildly rigid paresis with significant muscle wastage; nearly absent postural reactions; depression (strong stimulation producing some mild paddling only); constant chewing; right head tilt; significant struggling if placed on left side; right side of head and ears cooler than left; profound blindness and deafness; and absent pupillary, menace, and startle responses (to light, pain, movement, and arousal) although pupils were even. Ophthalmoscopic examination, nystagmus, strabismus, cranial motor and sensory functions, olfaction, and spinal reflexes all appeared normal. The problem could then be categorized as left-central vestibular disturbance with possible partial involvement of cranial nerves II and VIII (producing visual and auditory deficits). However, there was still no indication of the cause of the disturbance and, thus, no way to determine treatment or prognosis.

It was decided to attempt magnetic resonance imaging (MRI) as this technique produces excellent soft tissue definition, particularly in the cranial region where soft tissue is difficult to visualise. Prior to transport to the MRI unit, the wombat was briefly anaesthetized (isoflurane in oxygen via a face mask) to place a catheter in each cephalic vein. At the unit, anaesthesia was induced using IV propofol. The animal was intubated and anaesthesia maintained using isoflurane in oxygen. As no metallic objects (or people) can remain inside the shielded MRI room during imaging, anaesthesia was maintained remotely using manual IPPV. The wombat was strapped into the MRI machine in dorsal recumbency with anaesthetic and oesophageal stethoscope extension tubing running into the next room. There was no visual contact with the wombat, so the anaesthetist relied on the oesophageal stethoscope alone to judge anaesthetic depth. A special MRI pulse oximeter, present in the MRI room, was used periodically to support the stethoscopic monitoring. A 20-minute regular MRI series was followed by a further 20-minute contrast MRI series. Had either of these series been interrupted due to anaesthetic difficulties it would have been necessary to repeat them from the beginning. The procedure and recovery went extremely smoothly.

The MRI clearly revealed a focal oedematous lesion in the upper aspect of the cerebellum on the left side with a small area of extension to the left dorsolateral aspect of the brain stem. The lesion strictly respected the midline and vascular boundaries and was consistent with an area of subacute infarction in the region of the left superior cerebellar artery. The nature of the embolus was unknown. Nor was any embolic source ascertained on later cardiac ultrasound or blood culture. No other cranial abnormalities were detected and no cause for the deafness or blindness could be found. In humans and domestic species, it is not uncommon that cerebellar infarction can occur spontaneously, unassociated with any event or underlying/pre-existing illness. As the wombat appeared normal the day before she became ill, this spontaneous sequence seemed quite feasible in her case.

Prognostically, many humans and domestic animals with spontaneous cerebellar infarction eventually recover to almost normal function. Once a plateau is reached during convalescence, it is likely that this will be the permanent state of the patient. There is no treatment other than supportive care. In the case of the wombat, it was decided to continue supportive care and monitor progression. After a further two weeks, the neurologic examination was repeated by a human neurologist producing findings consistent with multifocal CNS disease, bilateral optic atrophy, and profound bilateral deafness. At this time, her condition had plateaued at a level that was considered unacceptable in terms of quality-of-life and she was euthanatized.

Upon sectioning the fixed brain, a large malacic focus was identified within the left caudal cerebellum extending into the left dorsal region of the posterior colliculus. Histologically, there was multifocally extensive encephalomalacia in the cerebellum and posterior colliculus and mild, non-suppurative, focally eosinophilic encephalitis. The foci of malacia found within the cerebellum and posterior colliculus were consistent with the lesions seen on the MRI.

There is one further curious point that may or may not have had any bearing on part or all of this case. The wombat had recently recovered from a two-month period of profound sedation following administration of 20 mg of fluphenazine (1 mg/kg) some three months prior to her illness.

MRI has very rarely been used on non-domestic species in Australia due to prohibitive costs, restraint/anaesthetic difficulties, lack of suitable facilities, and ethical concerns regarding human waiting lists (there are no purely veterinary CT or MRI machines in Australia). As a result, cranial infarctions (and other cranial soft tissue lesions) have not previously been able to be confirmed antemortem in these species. To our knowledge, this was the first wombat to undergo advanced imaging in this country. The diagnosis provided by MRI and the subsequent progression provided a solid foundation for treatment decisions and enough prognostic information to ultimately indicate the need for euthanasia.