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All dermatologic surgeons have to manage facial nerve injuries and subsequent facial nerve paralysis, whether temporary or permanent, when performing facial surgery. Avoiding facial nerve injury is the best route and this can be best achieved by adequate knowledge of the anatomy as well as understanding tumor type and size.
"There are basically two parts of facial nerves, namely the sensory and the motor branches. Damage to the sensory nerves are much more common.
"Oftentimes, they do regenerate and improve over time, but some injury may be permanent," says Peter K. Lee, M.D., associate professor of the department of dermatology and director of Dermatologic Surgery, University of Minnesota.
"Not every facial nerve is located in the exact anatomical location in every patient, and inadvertent facial nerve injury during the course of reconstruction could definitely be due to surgeon error. It is not necessarily an error, but sometimes an unforeseen event," Dr. Lee tells Dermatology Times.
Dr. Lee says prior to surgery, it is important that the surgeon assess the patient correctly in terms of age and degree of atrophy of subcutaneous tissues, position the head and neck properly before surgery and check to see if there is pre-existing facial nerve paralysis.
The three major sensory nerve branches of the trigeminal nerve are the ophthalmic, maxillary and mandibular branches.
Dr. Lee says if these sensory branches are distally traumatized during tumor excision or repair, the damage is usually temporary. The patient can expect approximately six to eight weeks of a tingling or itchy sensation and a return of normal sensation between three to six months, but this may take up to 12 months.
However, if the nerve is injured proximally, such as near the infra-orbital foramen, the damage is most likely permanent.
The facial nerve and its five branches innervate the muscles of facial expression and include the temporal, zygomatic, buccal, marginal-mandibular and cervical branches.
The temporal branch typically has only one main branch and innervates the frontalis, upper orbicularis and corrugator supercilii muscles. Any damage here will cause a flattening of the forehead and a drooping of the eyebrow, which can result in a visual field loss.
Dr. Lee says the temporal branch is the most vulnerable branch on the face and it is covered only by dermis, subcutaneous fat and SMAS at the temple. He says that the elderly are particularly vulnerable to injury here, due to the continuing atrophy of the overlying structures.
The zygomatic branch innervates the orbicularis oculi, elevators of the lips and nasal muscles. Dr. Lee says if the zygomatic branch is injured, the patient will not be able to completely close the eye or move the nose.
"Fortunately, this branch is usually more likely temporarily paralyzed from anesthesia, rather than traumatized surgically. This branch is less vulnerable due to its deeper location, its more than one branches as well as its anastomosis with fibers from the buccal branch," Dr. Lee says.
The marginal-mandibular branch innervates the depressors of the mouth, and if it is traumatized, the result will be a functional and cosmetic deficit, allowing lateral and upward pull on the opposite side of the mouth. Dr. Lee says on the ipsilateral side, the patient will be frozen in a persistent grimace due to the lack of apposing downward muscle contraction.
"The marginal-mandibular branch has one, or sometimes two, branches as well as anastomosis with buccal branches. Surgeons must be very wary with this branch, as it can have variability in its location. If it is located posterior to the facial artery, 20 percent are located 1 cm or more below the inferior rim of the mandible, and 80 percent at or above the lower level of the mandible.