Surgery of the face

24.1

Introduction

In 1829, the surgeon and artist Sir Charles Bell stated that the facial nerve (FN) is the nerve of “anatomy expression” .

The FN has two entirely distinct functions in humans and high primates. The first comprises essential somatomotor functions such as eye protection through the blink reflex, keeping the food inside the mouth cavity, assisting in speech. The second function is the facial expression of emotions, which is particularly critical in humans. Palsy of the FN creates an easily recognizable facial deficit and most importantly alters the perception of oneself, having lifelong implications for human psychology. Contemporary FN-related surgery requires a comprehensive approach with particular attention on protecting all functions of the FN, even in the most challenging surgeries.

Since the 1970s the intraoperative monitoring of the FN has continuously evolved. Today, the monitoring of the FN is exhaustive and includes a collection of different methodologies with specially dedicated devices and tools. The contemporary neurophysiologist must have a deep understanding of all type of FN monitoring methodologies available and must choose the most appropriate methodology for each surgical procedure.

Delgado et al. proposed the very first methodology for monitoring the FN during surgery of acoustic neurinoma of internal meatus . He anticipated that by electrically stimulating the FN with an electrode placed proximally to the tumor, he could continuously monitor the compound muscle action potential (CMAP) of the FN throughout the surgery. Even though this was theoretically the most reliable methodology for monitoring the function of the FN, it was not widely adopted. At that time, CMAP monitoring lacked advanced monitoring devices and competed with a more accessible and more practical form of the FN monitoring that was the free-running EMG monitoring technique, which then drew all the attention. Unfortunately, although intraoperative neurophysiologists know very well the shortcomings of the free-running EMG, many surgeons still prefer this methodology over the eliciting CMAP for monitoring surgeries where either the intra- or the extracranial segment of the FN is at risk of injury. However, we believe that this concept must be reevaluated. Continuous CMAP monitoring has profound limitations for monitoring the FN in intracranial posterior fossa surgery, because the proximal segment of the nerve, relative to the lesion, is frequently inaccessible for nerve stimulation (e.g., too close to the brainstem). However, this is not the case for the extracranial FN monitoring, because access to the trunk of the FN for placing continuous nerve stimulating probes (i.e., needles) is always attainable.

Use of free-running EMG methodology shows strong evidence that it could be useful for monitoring the intracranial segment of the FN, but there is no definitive scientific evidence to back up that this methodology is for monitoring surgeries where the extracranial segment of the FN is at risk of injury. This false presumption is tied to several historical facts: first, the same group of surgeons (particularly the ENT group) usually operate all spectrum of surgeries (intracranial, temporal, and extracranial FN disorders) using the same monitoring tools. Second, the free-running EMG methodology does not require the active assistance of a neurophysiologist or surgeon, being easy to implement in the operating room. Third, the free-running EMG shortcomings are extensively compensated by intraoperative nerve mapping. Nevertheless, Delgado’s outstanding peripheral FN CMAP methodology , fully developed 40 years ago, was left behind and was almost forgotten in FN surgery, although his methodology deserves acknowledgment as an ideal method for monitoring the function of the FN.

The monitoring of the FN in the 21st century has almost reached the most ideal and complete form with new methodologies and dedicated advanced equipment. Contemporary FN monitoring of the intra- and extracranial segments comprises several methodologies that can be used alone or in a combination, according to the monitoring requirement. These methodologies are

• 1.

  FN free-running EMG,

• 2.

  intraoperative FN mapping,

• 3.

  continuous FN CMAP monitoring,

• 4.

  continuous FN corticobulbar motor-evoked potential monitoring (FN-coMEP), and

• 5.

  blink reflex.

Utility and reliability of these methodologies for preserving the FN function in surgeries involving the intracranial and extracranial segments of the nerve have been previously established .

In contrast to the past, today’s monitoring machines can perform all methodological modalities of FN monitoring. Still, free-running EMG is the predominant methodology preferred among surgeons. However, there is substantial evidence against the reliability of the free-running EMG when used for monitoring the extracranial segment of the FN. In a retrospective study of extracranial FN surgeries, Meier et al. reported that only 16% of patients with postoperative injury of the FN presented with abnormal patterns of free-running EMG during surgery, and those who had abnormal patterns did not significantly correlated with permanent or temporary postoperative FN dysfunction. Similarly, Grosheva et al. in a prospective controlled study gave evidence that free-running EMG had low reliability when used for extracranial surgery of the FN and did not predict the postoperative FN outcome . These poor results contrast with the reliability of the free-running EMG when used for monitoring the intracranial segment of the FN .

Intraoperative neurophysiologists and surgeons must understand equally the uniqueness of the FN regarding anatomy, physiology, pathological conditions, and monitoring abilities. The surgeon must intricately involve in the monitoring of the FN. A mindset change will allow a contemporary surgeon to develop the necessary skills to effectively protect the FN either alone or assisted by an intraoperative neurophysiologist in the operating room.

In this chapter, we will present an overview of the monitoring methodologies involving the extracranial segment of the FN.

Surgery for excision or sclerotherapy treatment of head and neck facial vascular malformations (FVM) is among the most challenging surgeries for preserving the extracranial segment of the FN. The risk of FN injury in FVM is high in every single surgery and exponentially increases with multiple-staged operations. These patients must undergo multiple surgeries due to the proliferative-type of the lesion. Keeping the function of the FN within normal limits throughout this long and arduous treatment process is the most critical concern.

Figs. 24.1 and 24.2 frame the problem on hands and depict the conditions at which surgeons demand precise, accurate, and real-time FN monitoring from neurophysiologist. Over the last 20 years, we have been improving the monitoring of the extracranial segment of the FN . Here, we present the results of our efforts in this comprehensive CMAP-based monitoring methodology that integrates percutaneous preoperative mapping, intraoperating field mapping, and a continuous CMAP monitoring of the FN functional integrity.

24.2

Anatomy, physiology, and surgery of the facial nerve

Understanding the complicated course of the FN is fundamental for deciding on the treatment of FVM. From the monitoring perspective, the first gross inquiry should be what anatomical segment (intracranial or extracranial) of the FN is at risk of injury.

Anatomically, the segment of FN from brainstem to the internal meatus (defined as intracranial FN) does not contain a full sheath of connective tissue, being only covered by a thin layer of pia mater and directly bathed in cerebrospinal fluid. This segment of the FN is typically exposed during cerebellopontine angle surgeries, is more vulnerable to surgical manipulation , and is successfully monitored by free-running EMG methodology. On the contrary, a full sheath of connective tissue that comprises three layers of tissue (epineurium, perineurium, and endoneurium) covers the extracranial segment of the FN, protecting it against surgical manipulation. This anatomical feature has a significant impact on the decreased capability for free-running EMG to detect injury during surgeries where the extracranial segment of the FN is at risk .

Another important feature relates to the anatomical position of the FN trunk according to age and is particularly critical for CMAP-based monitoring methodology. In children until 4 years, the mastoid process is not fully formed, and the FN remains openly exposed behind the ear, very close to the surface of the skin. In contrast, the mastoid process after reaching maturation in the adulthood pushes the exit of the FN (the stylomastoid foramen) medially, under the cranial base . The FN can be 5 cm deeper from the skin surface in adults and is better protected by a mature mastoid tip when compared to young children.

Finally, anatomical branching of the FN is one of the most common misperceptions about the FN. Expecting the topography of the branching to be five separate tree-like branches is a convenient simplification rarely found in cadaveric studies. In almost 90% of the specimens, the temporal, zygomatic, and buccal branches of the FN have an exclusive and complex interconnection. This anatomic detail is particularly important when evaluating branches of the FN during the intraoperative monitoring because significant contributions contributing innervation may pass through these interconnections rather than from a single main branch. Furthermore, FVM often result in further distortion and displacement of normal FN anatomy, resulting in an atypical topography and confusing results during the intraoperative mapping.

Being familiar with the most common topographical branching patterns of the FN will be essential before applying this methodology to avoid confusion during intraoperative mapping and monitoring ( Fig. 24.3 ) .

Neuropathy of the FN due to compression by the FVM is common according to our experience on more than 400 patients being operated. However, to our knowledge, it has never been previously documented ( Fig. 24.4 ). The severity of the neuropathy relates to the degree of mass effect of the lesion over the nerve, the time of evolution, and afterward with the treatment procedure and postrecovery process (fibrosis). Of note, a neuropathic nerve has a higher risk of conduction block and even permanent injury during moderate surgical maneuvers and should not be overlooked to prevent permanent FN deficit.

24.3

Methodology