Olfactory Nerve (I)

The olfactory nerve carries special sensory afferents. The receptor neurons of the olfactory nerve are located in the upper part of the nasal cavity. These neurons continue to grow throughout life and get stimulated by gas molecules. The signal is transduced in the olfactory bulbs that lie in the cribriform plate. Anterior cranial base lesions such as olfactory neuroblastomas require resection of the cribriform plate and hence, the olfactory bulbs. Such patients need to be counseled about permanent postoperative anosmia. They are educated about using cooking gas at home because gas can leak and cause carbon dioxide/monoxide poisoning and can even be an explosion hazard. Therefore, these individuals need to have carbon dioxide and carbon monoxide detectors installed at home.

Optic Nerve (II)

The optic nerve has special sensory afferents. The optic nerve is resected during orbital exenteration or extensive tumor resection around the optic chiasm. In either situation, the nerve cannot be regenerated or grafted. The defect from the orbital exenteration, though, can be filled with prosthetics or obliterated with a vascularized local flap such as the temporoparietal fascial flap. Extensive defects that result in dural exposure are closed with vascularized free tissue transplantation.

Cavernous Sinus Nerves

The cavernous sinus contents include the oculomotor (III), the abducens (VI), and the first and second divisions of the trigeminal nerve (V1 and V2). Tumors involving this region can result in resection of these nerves.

The oculomotor nerve provides motor innervation to four extraocular muscles of the orbit: superior rectus, medial rectus, inferior oblique, and superior oblique. It also provides motor supply to the levator palpebrae superioris muscle and parasympathetic innervation to the ciliary muscles that constrict the pupil. Injury to this nerve results in inability to move the eye in all directions except laterally (abducens nerve) and inferomedially (trochlear nerve), upper eyelid ptosis, and the papillary dilation due to unopposed sympathetic action. The key rehabilitation in this case is of eyelid ptosis, which can be accomplished by levator tightening procedures to allow for the eye to open better and to improve vision.

The loss of V1 results in loss of sensation over the ipsilateral scalp, forehead, upper eyelid, cornea, nasal mucosa (frontal sinus), and nose. V2 loss results in loss of sensation from the ala, upper lip, maxillary dentition and gingiva, nasal mucosa (maxillary, ethmoid, and sphenoid sinuses), and hard and soft palate. Rehabilitation involves reassurance and avoidance of hot creams or emollients to prevent skin burns. The loss of sensation in the hard palate may put some people at risk for oral dysphagia because of the lack of palate sensation that triggers the tongue to propel a food bolus into the oropharynx.

The abducens (VI) nerve controls the lateral rectus and is particularly susceptible to injury during cavernous sinus surgery. It can be injured with tight packing of the cavernous sinus to control bleeding, despite the nerve being intact. Initially, treatment entails observation with the use of refractive prism glasses. If the nerve is paralyzed, then transposition of superior and inferior rectus with Botulinum toxin injection in the ipsilateral medial rectus can help improve diplopia.

Trigeminal Nerve (V3)

The third division of the trigeminal nerve has sensory and motor branches. The sensory branches include the buccal, inferior alveolar, lingual, auriculotemporal, and meningeal nerves. The last two cause skin sensory deficits and patients need to be instructed to be careful with grooming, including the use of hairdryers and curling irons because they can cause burns in the facial region because of lack of sensation. The buccal, inferior alveolar, and lingual nerves provide sensation to the inside of the mouth and can cause difficulty with swallowing. Hence, patients need to be prepared for this and may need swallowing therapy to allow for compensation. Grafting of these nerves, if possible, has been recommended.

The motor division of V3 supplies the following muscles: tensor veli palatini, tensor tympani, medial pterygoid, lateral pterygoid, temporalis, and masseter. Therapy for tensor veli palatini and tensor tympani loss is not needed because the function of palatal elevation (by tensor veli palatine) is compensated for by the levator veli palatini. The loss of tensor tympani results in reduced sound attenuation but is compensated for by the stapedius muscle. The rest of the muscles aid in mastication and if the contralateral muscles are intact, then the patient has minimum trismus or chewing problems. The issue of scarring and trismus can occur with extensive resection of the infratemporal fossa muscles. Such patients need to be placed on aggressive mandible physical therapy to keep the resultant fibrosis, and hence trismus, to a minimum. A stack of tongue depressors works well as self-directed physical therapy for patients. Temporalis and masseter wasting can occur by 1 year after sacrifice of V3 and this can be cosmetically corrected with silicone implants in the face and head.

Facial Nerve (VII)

The facial nerve provides movement to all the facial muscles of expression, the stapedius muscle, and the posterior belly of the digastric muscle. In addition, it provides taste to the anterior two thirds of the tongue by way of the chorda tympani nerve, and the palate by way of the pterygoid nerve and the greater superficial petrosal nerve. The facial nerve is saved in most cranial base procedures by way of facial nerve rerouting. In the event that the facial nerve is sacrificed, the resulting deficits are primarily due to motor dysfunction of the nerve. The most catastrophic of these events is the inability to close the eye and hence, risk for exposure keratitis and blindness. Lip droop can cause difficulty with swallowing and drooling because the oral commissure cannot form a complete seal to propel the food bolus posteriorly. Surgical rehabilitation methods to correct facial paralysis after cranial base surgery involve prostheses, neural anastomoses, and muscle flaps.

The most basic prosthetic device is a gold weight implant in the upper eyelid with the goal of allowing closure of the eyelid and hence, preventing exposure keratitis. A lateral tarsal strip procedure or tarsorrhaphy can also be performed to shorten the eyelid, correct the ectropion, and improve eye closure and lubrication. Hypoglossal–facial nerve anastomosis has been described, which allows the use of hypoglossal proximal fibers to help trigger the distal facial nerve branches. We have to be careful when considering hypoglossal–facial nerve transposition because many patients undergoing extensive skull base resections will have their vagus nerve injured, and a deficit in the hypoglossal nerve may be catastrophic for the patient’s swallowing. The asymmetry of the mouth can be corrected with temporalis or masseter muscle transpositions, which act as dynamic facial slings. If the trigeminal nerve has been sacrificed, then the neural supply to the temporalis and masseter muscle is compromised and these flaps will undergo atrophy with time. In general, for rehabilitation of the cranial nerve deficits, static slings for the face work more predictably. This procedure involves the placement of a sheet of acellular dermis or tensor fascia lata to pull the oral commissure superiorly in a more normal location, which allows for cosmetic improvement of the lips at rest. The gracilis free tissue transplant has been a good alternative for restoring tone to the facial muscles. Overall, many options exist for the rehabilitation of the facial nerve. All procedures are aimed at improving cosmesis and function of the eye and oral commissure.

Vestibulocochlear Nerve (VIII)

The vestibulocochlear nerve provides sound and balance to the body. The auditory nerve transmits sound from the inner ear to the auditory cortex in the temporal lobe. If a translabyrinthine approach to the internal auditory canal is performed to remove a cerebellopontine angle tumor, then hearing and balance are destroyed. In most situations in which the patient undergoes extensive cranial base surgery, both branches of the VIII cranial nerve are affected. In this situation, the patient can undergo vestibular rehabilitation to improve balance. The patient may be able to be fitted with a contralateral routing of sound (CROS) hearing aid if he/she desires bilateral hearing and at least one ear has serviceable hearing.

Glossopharyngeal Nerve (IX)

The glossopharyngeal nerve has multiple functions that are related to swallowing. It has a visceral motor component that provides parasympathetic supply to the parotid gland. This supply reaches the parotid gland by way of the tympanic plexus in the middle ear and V3 by way of the otic ganglion. Injury to these branches may result in decreased salivary flow and hence, parotitis.

The general sensory component has afferent supply from the base of the tongue and pharynx, external ear, and external surface of the tympanic membrane. Damage to these fibers results in significant oropharyngeal dysphagia because of delay in oropharyngeal swallow. Isolated glossopharyngeal nerve deficit can be rehabilitated by aggressive swallow therapy, which focuses on trying to place the food bolus voluntarily on the sensate side of the pharynx.

The branchial motor component provides motor innervation to the stylopharyngeus muscle, which allows for elevation of the palate. Isolated loss to these fibers produces little difficulty in swallowing but when combined with the loss of the other fibers, severe dysphagia and velopharyngeal insufficiency can result.

Finally, the glossopharyngeal nerve carries visceral sensory information from the carotid body and sinus by way of the Hering nerve. Disruption of this nerve can result in carotid sinus syndrome. Ipsilateral loss usually does not cause problems with blood pressure, probably because of an intact system on the nonaffected side. If an individual has had contralateral surgery, then the possibility of carotid sinus syndrome should be entertained, with administration of beta-blockers and nitrates as needed.

Vagus Nerve (X)

The vagus nerve occupies the most central position in control of swallowing and the airway. Therefore, injury to the vagus nerve can have the most debilitating effects on the cranial base patient. The vagus nerve carries branchial motor, general sensory, visceral sensory, and visceral motor fibers.

The branchial motor fibers provide motor innervation by way of three distinct branches to the pharynx, palate, and larynx, except for the stylopharyngeus muscle (IX) and the tensor veli palatini (V3). The three branches are the pharyngeal branch, the external branch of the superior laryngeal nerve, and the recurrent laryngeal nerve. Injury to the pharyngeal branch results in paralysis of the ipsilateral palate and pharynx, leading to incomplete closure of the nasopharynx and hence, velopharyngeal insufficiency. The pharyngeal dysfunction responds well to swallowing therapy. Other modalities to improve velopharyngeal insufficiency include palatal obturators and palatal lift prostheses. Surgical approaches include pharyngeal augmentation (superiorly based) and pharyngoplasty. Another simple approach to treating velopharyngeal insufficiency is a palatal adhesion. The palatal adhesion technique has become the procedure of choice in the authors’ practice to correct hypernasality and nasal regurgitation, mainly because it does not require long flaps or alter the pharyngeal anatomy. They perform this procedure several months after the initial skull base surgery to allow for swallowing to stabilize and compensate with the swallowing therapy that the patient receives. The procedure is described later in this article, in the summary.

The paralysis of the constrictor muscles is more devastating than that of the palate. Normally, food would reach the oropharynx and then a downward constriction would push the food toward the hypopharynx while the larynx closes. With paralysis of the constrictors, the food is pushed toward the paralyzed side by the nonparalyzed side, resulting in bulging of the pharynx and delayed movement of the food bolus to the hypopharynx. Because the food stays in the hypopharynx and the larynx reopens, aspiration occurs. Treatment of this problem is centered on an intensive swallowing retraining program. Such a program emphasizes head positioning techniques that obliterate the paralyzed pharyngeal surface. In addition, avoidance of a tracheotomy can greatly help in swallowing therapy because it prevents the larynx from being “pegged” by a tube and allows for it to elevate normally (which is needed to prevent aspiration).

The external branch of the superior laryngeal nerve provides motor innervation to the cricothyroid muscle, which results in changes in vocal pitch. Because cranial base surgery usually results in high vagal transection or injury, the recurrent laryngeal nerve, which provides innervation to the intrinsic muscles of the larynx and the cricopharyngeus, can also be affected. The result is ipsilateral vocal cord paralysis and dysfunction of the cricopharyngeal muscle, which results in glottal incompetence with hoarseness and aspiration. In addition, the failure of cricopharyngeus relaxation also results in dysphagia and aspiration. Generally, a tracheotomy is performed in such cases for pulmonary toilet. In the authors’ experience, most patients tolerate unilateral vocal cord paralysis well without tracheotomy. Initially, a Gelfoam medialization thyroplasty is performed (which can last up to 6 weeks) and then a permanent silastic medialization thyroplasty is performed at least 6 weeks after resection of the cranial base tumor. Silastic medialization thyroplasty with arytenoid adduction has been the authors’ procedure of choice to address vocal fold paralysis in such patients. Cricopharyngeal myotomy can be performed and does allow for improved aspiration and assistance with swallowing therapy. The procedure for vocal fold medialization is described in the summary of this article.

The general sensory fibers of the vagus nerve provide sensation from the supraglottic larynx, lateral pharyngeal wall, external auditory canal, and tympanic membrane. The sensory fibers from the supraglottis form the superior laryngeal nerve and pass deep to the carotid arteries as they synapse to the inferior (nodose) ganglion. Loss of the superior laryngeal nerve can result in swallowing difficulties because of loss of sensation. Swallowing therapy is needed to rehabilitate this defect.

The visceral sensory fibers provide sensory and parasympathetic tone to the pharynx, larynx, esophagus, trachea, and thoracic and abdominal viscera down to the splenic flexure of the colon. Hence, unilateral loss of the vagus can result in reduced motility of the gastrointestinal tract, reduced tone of the lower esophageal sphincter, and delayed gastric emptying. These issues are dealt with by promotility pharmacologic agents, and a jejunal feeding tube may be needed in the early rehabilitation phase to prevent emesis. If bilateral vagal injuries occur, then a Nissen fundoplication may be needed to counter the esophageal sphincter dysfunction.

Spinal Accessory Nerve (XI)

The spinal accessory nerve is a motor nerve that innervates the sternocleidomastoid and trapezius muscles. It courses superficial to (70%), posterior to (27%), and through (3%) the internal jugular vein. The resulting deficit is weakness in turning the head away from the operated side (sternocleidomastoid) and shoulder droop and pain with lateral rotation of the scapula (trapezius). In general, grafting of the spinal accessory nerve does not show much promise but such patients need to get enrolled in an aggressive physical therapy program for shoulder strengthening. This program may be needed indefinitely. Some patients undergo steroid injections for shoulder pain.

Hypoglossal Nerve (XII)

The hypoglossal nerve provides innervation to all intrinsic and extrinsic muscles of the tongue. It exits the skull base at the hypoglossal canal and as it curves laterally, it shares some fibers with the vagus nerve at the inferior (nodose) ganglion. Injury in this region can result in hypoglossal and vagal deficits, which can be morbid for the patient. The main issue with hypoglossal injury patients is that the food bolus cannot be propelled adequately during the oral phase of swallowing. Swallowing therapy is imperative in these situations.