Unilateral atrophy of the masticatory muscle

67 Unilateral atrophy of the masticatory muscle




INITIAL PRESENTATION


Asymmetry of head.



SIGNALMENT


A 6-year-old neutered female Labrador retriever dog.



CASE PRESENTING SIGNS


Skin wounds to the right face, altered facial symmetry.



CASE HISTORY


The dog was initially presented for a right upper lip wound and abrasions to the right zygomatic area. One week later the right face suddenly ‘caved in’. The dog wasn’t catching thrown items as skilfully as usual.



CLINICAL EXAMINATION


Clinical examination showed atrophy of the right temporalis and masseter muscles. The jaw could be voluntarily closed by the dog. Sensation to the right side of the face (canthus, cornea, nostril, upper and lower lips) was absent. Right enophthalmos was present but no anisocoria was noted in either a light or darkened room. Schirmer Tear Test (STT) was 0 mm/minute on the right and 9 mm/minute on the left. The wound on the right upper lip pierced the lip and the hole corresponded to the position of the canine tooth. Observation found normal voluntary movement of the eyelids, nostrils and ears bilaterally. The rest of the neurological examination was normal (Fig. 67.1).


image

Figure 67.1 Facial wounds (arrow), atrophy of the right temporalis and masseter muscles.



NEUROANATOMICAL LOCALIZATION


The lesion was localized to the right trigeminal nerve (CN V), affecting all three branches as indicated by lack of sensation to the eye (ophthalmic branch), eyelids (ophthalmic/maxillary branches), upper lip (maxillary branch), lower lip (mandibular branch), and atrophy of masticatory muscles (mandibular branch) on the right side.


Enophthalmos can result from masticatory muscle atrophy or from Horner’s syndrome. No miosis was present in this dog so the former is the most likely cause.


Dry eye (STT = 0 mm/minute) can result from trigeminal nerve lesions.


When facial muscles are paralysed (CN VII) the upper lip is flaccid and can become trapped between teeth when eating. The dog typically drops food rather than bite through its own lip. The lip wound in this dog has resulted from lack of sensation to the face and resulting self-trauma.


Lack of proprioceptive deficits or paresis in the limbs indicates that the CN V lesion may be peripheral to the brainstem, rather than affecting the nucleus within the brainstem. Compression of the brainstem may be present but clinical signs were not detected.

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Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on Unilateral atrophy of the masticatory muscle

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