Facial palsy is a devastating event in a patient’s life and can result in disfiguring permanent flaccid paralysis or synkinesis, a long-lasting muscle discoordination caused by aberrant facial nerve regeneration. Synkinesis generates both hypertonicity of recovered muscles and also involuntary, discoordinated facial movements that may significantly interfere with facial functions such as blinking, chewing, speaking, nasal breathing, and, often most disturbing to patients, smiling.
Patients presenting with chronic symptoms of paralysis require a thorough history to be taken and careful physical examination to differentiate between hypofunction, hyperfunction, or mixed recovery between different facial muscles. Those who develop synkinesis will often have reported recovery of function only to later develop symptoms of hypertonicity of recovered muscles and involuntary facial movements. Patients typically report the initial phase of synkinesis as a deterioration of their recovery as volitional movements become restricted.
Relevant Muscle and Facial Nerve Anatomy
Muscles of the facial expression relevant to the smile include the perioral muscles as well as elevators and depressors of the lips and oral commissure ( Fig. 5.1 ). The orbicularis oris (OO) muscle is responsible for closure of the lips and puckering is innervated by the buccal and marginal mandibular branches of the facial nerve and is in close proximity to the depressor anguli oris (DAO), depressor labii inferioris (DLI), and mentalis (MEN) muscles (also innervated by the marginal mandibular nerve branches). The DAO depresses the oral commissure, whereas the DLI depresses the lower lip and is key in showing the lower teeth in a full denture smile. The MEN counteracts the DLI and elevates the central portion of the lower lip helping the pouting of lips. Platysma (PLA) muscle fibers of the neck innervated by the cervical branch interdigitate with the lower facial muscles, playing a role in downward action of the oral commissure and lower lip. The upper lip and oral commissure elevators are innervated by the buccal and zygomatic branches and include the zygomaticus major (ZYJ), zygomaticus minor (ZYN), levator anguli oris (LAO), levator labii superioris (LLS), and levator labii superioris alaeque nasi (LLA). Risorius (RIS) and buccinator (BUC) muscles innervated by the buccal branch provide lateral pull of the oral commissure.
Smile Analysis and Examination
Evaluation of symmetry at rest and examination of dynamic facial movements will determine the muscle function status in chronic facial paralysis. In patients with synkinesis, the nasolabial fold may be deepened and the oral commissure may actually be upturned on the side of involvement at rest due to hyperfunction of the facial muscles involved in lip elevation including the LLS, LLA, ZYJ, and ZYN muscles. This may be associated with a deepened nasolabial fold. Synkinesis of the large PLA muscle and DAO can generate downward pull of the oral commissure at rest as well as with animation, antagonizing the elevators and preventing the superolateral rise of the oral commissure to create a smile. Therefore, patients with synkinesis could have an upturned or downturned oral commissure at rest, both with limited smile excursion. In contrast, patients with flaccid paralysis will show a downturned oral commissure, often associated with ptotic soft tissue of the lower face, and an inability to smile ( Fig. 5.2 ).