Facial nerve neurectomy as palliative care in a rabbit with mandibular osteosarcoma

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 rabbit with mandibular osteosarcoma

While osteosarcoma is well characterised in canines, it is poorly studied and reported in many exotic pet species. As veterinary management of exotic pets continues to advance, improvements in husbandry, management of chronic disease, and nutrition are extending the lifespan of exotic companion mammals. Improvements in client education are also helping exotic pet owners recognise signs of illness, and thus present their pets at earlier stages of disease. 

As opportunities for earlier diagnosis increases, diseases previously thought of as rare may start to become more commonly seen. Osteosarcoma in the mandible of a rabbit has been reported before, but treatment options remain more conservative and limited than in dogs and cats. 

Even though chemotherapy is still not well characterised for exotic pets, and radical resection may not be possible in some areas—such as the mandible—palliative care can still produce good quality of life. In the author’s knowledge, this is the first incidence of facial nerve neurectomy in a rabbit as analgesia for a terminal condition. 

A five-year-old male neutered mini lop rabbit was presented in December 2021 for reduced appetite of a week’s duration. In this patient, initial presentation was complicated by the presence of concurrent chronic otitis media and externa, and a left-sided lateral ear canal resection had been performed in August 2020. On initial physical examination, a 1cm x 1cm firm piriform mass was found on the right mandible over the diastema, just mesial to the right mandibular premolar. As the initial presentation was non-specific, it was unclear if the mass or the otitis was the primary concern. 

Diagnostics

Initial diagnostic testing consisted of a computed tomography (CT) scan of the skull, a complete blood count (CBC), biochemistry, electrolytes, urinalysis, and urine protein-creatinine ratio. Initial clinical pathology results were unremarkable. The CT scan indicated that the mass was not wholly soft tissue in origin, consisting of a centre of thin, poorly organised bony and soft tissue proliferation covered in a layer of oral mucosa. The mandible did not have appreciable change at that time. The left tympanic bulla was normal, but mild otitis media was present in the right, and unlikely to be significant enough to produce inappetence of this duration. 

The second stage of diagnostic testing consisted of a surgical wedge biopsy and histopathology of the mass under general anaesthesia, at which time the diagnosis of osteosarcoma was confirmed with classic multinucleated cells and several mitotic figures. A repeat CT scan was performed of the head and chest in January 2022 to screen for metastases, and monitor the primary lesion. While the primary lesion had progressed as expected, no appreciable metastases were found in the chest. 

Medical treatment

The patient was already on meloxicam at 0.5 mg/kg once every 12 hours after initial presentation for presumed oral, dental, or otic pain. The risks of ongoing meloxicam were discussed with the owner with regards to renal function, and they elected to proceed while monitoring renal function with biochemistry and urine protein-creatinine ratios at regular intervals. Gabapentin at 11 mg/kg once every 12 hours was added for neuropathic pain. 

At home, the rabbit’s owner monitored food intake, comfort, and modified the diet for ease of oral manipulation and chewing, including supplementary feeding with Oxbow Critical Care. These modifications were sufficient until March 2022, when the owner reported that the rabbit was starting to decline in food intake and oral comfort. 

Surgical treatment

Neurectomies are performed in horses for cases of lameness where the primary problem cannot be satisfactorily resolved, and are most commonly performed on the palmar or plantar digital nerve, and the deep branch of the lateral plantar nerve. They are a moderately successful option for palliative care and reduction of discomfort. Due to the location of the mass and the arrangement of the facial nerves of the rabbit, it was proposed to try to perform a neurectomy on the ventral buccal branch of the facial nerve where it transverses over the masseter muscle, which would reduce or stop sensation to the right mandible while sparing sensation to the right maxilla. 

Intra-operative views of the ventral buccal branch of the facial nerve prior to transection

As no report of this procedure could be found, a frank discussion was had with the rabbit’s owner about potential outcomes including no change in quality of life, a decrease in quality of life, or the patient becoming less able to eat and orally manipulate food. Exacerbation of neuropathic pain was also discussed as a potential complication of this procedure. 

To get a better estimate of potential effects of the procedure, a 2 mg/kg lignocaine dose was used to block the dorsal and ventral buccal branch of the facial nerve. Due to their proximity, it was not possible to block just the ventral branch. While the evaluation was completely subjective, it did appear that masticatory function was improved, and no deterioration was seen. 

A 1cm incision over the ventral buccal branch of the facial nerve was made under general anaesthesia, with the nerve identified, splash blocked with 2 mg/kg lignocaine, then severed by removing a 5mm section. A Lone Star Retractor was used to achieve visualisation. 

The subcutaneous layer was closed with 3-0 PDS in a horizontal buried fashion, and the skin closed with 3-0 PDS intradermal sutures. Anaesthesia and recovery were uneventful.

Outcome

Although the mandibular lesion continues to enlarge and distort the mandibular anatomy, the patient is still alive and continuing to have good quality of life. At the six-day post-operative revisit, the rabbit’s owner reported that after the initial 48 hours, the patient had shown marked improvement in general, and reported that he appeared more comfortable, and was eating soft food enthusiastically. Hypersalivation was also noticed, which is still present but reduced in severity, and managed well using hydrophobic creams on affected areas. The patient had two bleeding episodes from the lesion which may be related to loss of sensation and accidental self-trauma, or a sequelae of neoplastic growth causing necrosis and aberrant vascularisation. 

As of the last visit in May 2022, the patient was eating soft foods well and without discomfort, and was on twice daily gabapentin and once daily meloxicam.


References:

Alhayani, W., Ma, M., & Hamoodi, O. (2020). Surgical Anatomy of Facial Nerve in Rabbits. International Journal of Psychosocial Rehabilitation, 24(1), 6732-6735

Elce, Y. (2021). Complications of Peripheral Nerve Surgery. Complications In Equine Surgery, 843-854. doi: 10.1002/9781119190332.ch60

Izquierdo, J., Campos, Á., Romero, C., & Romero, G. (2014). Predegenerated Great Auricular Nerve Graft in Facial Nerve Defects. Otology &Amp; Neurotology, 35(2), e64-e68. doi: 10.1097/mao.0b013e31829e1680

Matthews, S., Dart, A., & Dowling, B. (2003). Palmar digital neurectomy in 24 horses using the guillotine technique. Australian Veterinary Journal, 81(7), 402-405. doi: 10.1111/j.1751-0813.2003.tb11545.x

Miwa, Y., Nakata, M., Takimoto, H., Chambers, J., & Uchida, K. (2019). Spontaneous oral tumours in 18 rabbits (2005–2015). Journal Of Small Animal Practice, 62(2), 156-160. doi: 10.1111/jsap.13082

van Zeeland, Y. (2017). Rabbit Oncology. Veterinary Clinics of North America: Exotic Animal Practice, 20(1), 135-182. doi: 10.1016/j.cvex.2016.07.005


Dr Nicole Su BSc 

 rabbit with mandibular osteosarcoma

BVMS MANZCVS (Unusual Pets)

Dr Nicole Su graduated from Murdoch University in 2014 and has worked in exotic animal veterinary practice ever since. 

While her main interest is rabbit medicine and surgery, she has also seen every other exotic pet species kept in Australia, including venomous snakes, peacocks, and axolotls. 

Dr Su is committed to providing gold-standard care for exotic pets, encouraging client education, and continuing to develop the understanding of welfare and husbandry of exotic companion animals.

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