April 2020 HSC Section 4 - Plastic and Reconstructive Problems

Joseph & Kim

the sural nerve, antebrachial cutaneous nerve, and the great auricular nerve. Epineural repair is then performed in an end-to-end fashion.

Nerve substitution procedures Most facial reanimation procedures performed for patients with flaccid facial paralysis less than 2 years in duration and without a functional (or accessible) proximal facial nerve are nerve substitution procedures. These are also known as reinnervation tech- niques, in which the motor input from the native facial nerve nucleus is replaced by motor nerve axons from another cranial nerve or from the contralateral functional facial nerve. The following section reviews modern nerve substitution procedures. Hypoglossal nerve (XII–VII transfer) In many institutions, the hypoglossal nerve is the most common cranial nerve used to reinnervate the facial nerve. There are several distinct advantages in using this cranial nerve, including close proximity to the surgical site, a consistent location making surgical identification relatively uncomplicated, a robust supply of donor motor axons, and a relatively minimal donor organ deficit when using modern techniques. 9 The hypoglossal nerve has been used as a donor nerve since the early twentieth century. 10 The original descriptions of hypoglossal to facial nerve transfers involved use of the entire hypoglossal nerve. In the classic procedure, the hypoglossal nerve was identified and then fully transected so that it could be transposed into the prox- imity of the distal facial nerve stump and sutured in an end-to-end fashion (crossover procedure). As might be expected, this approach results in hemilingual atrophy, result- ing in dysarthria and dysphagia. 11,12 To avoid functional deficits associated with the classic technique, several newer approaches were developed. The first was developed by May and colleagues. 12 In their technique, a perineural window was created by making an incision in the hypo- glossal nerve to 50% of its diameter. Following creation of the perineural window, an interposition free nerve graft was coapted in an end-to-side fashion to the hypoglos- sal nerve. The distal aspect of the interposition nerve graft may then be sutured to the affected facial nerve. The use of only 50% of donor hypoglossal axons is sup- ported by histologic studies that suggest that the hypoglossal nerve contains up to twice as many axons as the injured facial nerve. 13 Therefore, use of only half of the available hypoglossal axons is presumably appropriate. The procedure as described by May and colleagues 12 resulted in a drastically reduced risk of devel- oping symptomatic tongue-related deficits (4%) but showed equivalent dynamic re- sults with less synkinesis when compared with the classic technique. Some surgeons have raised the concern that the use of an interposition nerve graft may have worse outcomes due to axonal loss across 2 neurorrhaphy sites. Nonetheless, numerous reports have shown dynamic results that are equivalent to the classic end-to-end technique. 14,15 To avoid use of an interposition nerve graft as well as to circumvent drawbacks to longitudinal nerve section, an additional technique has been described that involves rerouting of the temporal portion of the proximal facial nerve. 11,16,17 With this tech- nique, the mastoid portion of the facial nerve distal to the second genu of the facial nerve is isolated and mobilized from the temporal bone. This technique allows enough facial nerve length to be mobilized so that a direct end-to-side neurorrhaphy between the proximal facial nerve and the hypoglossal nerve can be achieved. This technique shows efficacy similar to direct end-to-end hypoglossal to facial anastomosis but with morbidity similar to the technique by Samii and colleagues. 18 The main drawback to this technique is the requirement for mastoidectomy and facial nerve drillout.

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