Case presentation
A 9 year old male neutered Rottweiler presented with a history of unilateral left sided masticatory muscle atrophy. There was no history of difficulties with prehension, or pain on opening the jaw. Physical examination revealed a dramatic left sided loss of masticatory muscle(fig 1). The right sided masticatory muscle was normal on palpation. Neurological examination revealed that the corneal reflex and nasal mucosal sensation were absent on the left but normal on the right. Menace and palpebral reflexes were normal. No proprioceptive deficits were found.

Question: What is your neurolocalisation?
Question: What are your most likely differentials?

For further investigations, discussion and the answers to these questions, please scroll down the page.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Further investigations
Routine haematology and biochemistry were unremarkable. MRI of the brain was performed (Figs 2-5 below)

Fig 2: T1 weighted dorsal view of the brain stem. The trigeminal nerve on the left (arrowed) is thicker than the right.

Fig 3: T1 weighted dorsal view of the brain stem post contrast. The trigeminal nerve on the left (arrowed) has taken up more contrast than the right.

 
Fig 4: T1 weighted transverse MRI scan of brain, showing mass adjacent to brain stem (arrowed) in region of trigeminal nerve. The dramatic left sided masticatory muscle wasting is also evident in this image.

 
Fig 5: T1 weighted transverse MRI scan of brain, post contrast, showing mass adjacent to brain stem (arrowed) in region of trigeminal nerve.

Diagnosis
Malignant nerve sheath tumour of the trigeminal nerve.

Follow up
No specific treatment was given. The dog was euthanased 12 months later due to heart failure secondary to dilated cardiomyopathy.

Discussion
The slow onset of signs in this case, together with the unilateral masticatory muscle wasting and the signs attributable to deficits of the trigeminal nerve made a presumptive diagnosis of trigeminal nerve sheath tumour likely. Imaging findings were supportive of a trigeminal nerve sheath tumour. Malignant nerve sheath tumours are slowly progressive, and although surgery is reported as possible, the survival times without surgery can be substantial (range of 5 to 21 months in one study, Bagley et al 1998). Another differential for these imaging findings is trigeminal neuritis. This condition usually presents as a relatively acute, idiopathic, bilateral condition, causing a dropped jaw, and is usually self-limiting. However, in one study of the imaging findings of 6 cases of acquired trigeminal lesions (Schulz et al 2007), 4 cases were found histologically to be malignant nerve sheath tumours, but two were trigeminal neuritis. In both disease processes, masticatory muscle atrophy was seen, with hyperintensitiy of the atrophied muscle, and in both diseases there was increased contrast uptake in the affected nerves. The trigeminal nerve was diffusely thickened in cases of trigeminal neuritis, but had a lobulated or mass-like lesion in trigeminal nerve sheath tumour (as can be seen in figure 5 above). Trigeminal nerve sheath tumour was considered most likely in this case, but histology would be required to differentiate definitively between these two conditions.

References:
Bagley et al (1998), JAAHA, 34, 19
Schulz et al (2007), Vet Rad & Ult, 48, 101

 

Question. What is your neurolocalistion? Answer. The corneal sensation and nasal mucosal sensation travel via the trigeminal nerve. The trigeminal nerve supplies motor innervation to the masticatory muscles. The neurolocalisation is therefore the left trigeminal nerve or the left brain stem. The lack of proprioceptive deficits make brainstem disease less likely. Question. What are your most likely differentials? Answer. The chronic and progressive nature of the disease make neoplasia the most likely differential, including malignant nerve sheath tumour and lymphoma. Trigeminal neuritis could also fit with the findings, see discussion. Masticatory myopathy is another possible cause of masticatory muscle atrophy. This is usually bilateral, but can be unilateral. However, it is possible that prior to the availability of MRI, cases of trigeminal nerve sheath tumours were misdiagnosed as unilateral masticatory myopathy. In this case, masticatory myopathy would not explain the sensory deficits in the cranial nerves.

First published Fri, Mar 18 2011