Facial Nerve Paralysis
Facial Nerve Paralysis Across Species
Disease that involves the facial nerve (cranial nerve VII) is a common problem in clinical practice. CN VII supplies innervation to the muscles of facial expression, to the lacrimal and salivary glands, to the middle ear and the blood vessels of the head, and to the palate and the rostral two-thirds of the tongue associated with branches of the trigeminal nerve. These latter two functions are not as clinically apparent or important.
As the facial nerve is responsible for innervation to the muscles of facial expression, astute observation of the animal’s head and face may reveal abnormalities of symmetry and facial muscle movement. Animals with facial nerve disease often have abnormalities of position and movement of the lips, ears, and eyelids. Clinical signs of a lesion in the facial nerve may include paresis or paralysis of the facial muscles which results in abnormal movement or function of these muscles.
Facial nerve motor function is most easily evaluated by first assessing the palpebral reflex. When the medial aspect of the palpebral fissure is touched (with a finger or other non-injurious instrument, the animal should blink rapidly (within milliseconds of the stimulus) and completely (the margins of the palpebral fissure should touch). The stimulus (i.e. finger touching the medial canthus) is sensed through the trigeminal nerve (CN V, ophthalmic branch) and the muscle contraction is elicited through the motor functions of CN VII resulting in contraction of the orbicularis oculi muscle. If the more lateral aspects of the periorbital region are touched, the afferent stimulus may be projected in the maxillary branch of CN V fibers.
Animals with an abnormality of CN VII function will not be able to close the palpebral fissure completely when this reflex is attempted. The menace response will also be decreased or absent; however, if the animal has only a facial nerve abnormality and no other ocular complications, the animal’s vision should be normal. When testing the menace response, it may be noticed that the eye can be retracted in the orbit away from the menacing gesture suggesting the afferent components of this response are normal. Often, when the eye is retracted in the globe in this situation, the nictitating membrane will be prolapsed up and over the eye in a rapid movement. Similarly, when the palpebral area is touched, the animal may retract the eye but not be able to close the lids. The nictitating membrane will often be rapidly prolapsed concurrently.
Clinical evidence of facial nerve dysfunction is often suggestive of a significant pathology at the nerve origin (brainstem) or anywhere along its course, warranting further investigation of an underlying etiology.
Ocular Complications from Facial Nerve Paralysis: Clinical Tips