An adequate understanding of the pertinent anatomy of the head and neck is necessary to successfully diagnose and treat injuries of the facial and trigeminal nerves.
There is a constantly evolving and improving collection of diagnostic tools that one must be familiar with and use to advantage to provide the best care for patients.
There is a wide range of therapeutic modalities when it comes to managing nerve injuries, anywhere from simple observation to complex grafting. No two nerve injuries are the same; it is the responsibility of clinicians to be familiar with the treatment options and present them to patients.
Despite the wide range of scope of practice of oral and maxillofacial surgeons, one pertinent aspect of our training that we must stay up to date on is an adequate knowledge of the pertinent anatomy of the head and neck, and management of these vital structures, especially in the setting of trauma. This article provides a better understanding of the anatomy of the facial and trigeminal (specifically mandibular branch) nerves, and a method of diagnosis and management in the setting of trauma.
There are three main components of the pertinent neural anatomy, composed of multiple layers that surround the central axonal fibers (endoneurium, perineurium, and epineurium), a knowledge of which is crucial in understanding the varying degrees of inferior alveolar nerve (IAN) and facial nerve injury.
Nerve injury classification
Inferior alveolar nerve
Despite multiple peripheral injury classification systems in existence, the most widely used and accepted in describing injury to the IAN are the Seddon and Sunderland systems. The Seddon system, first described in 1943, consists of neurapraxia, axonotmesis, and neurotmesis ( Table 1 ). The Sunderland system (1951) includes first through fifth degrees of peripheral nerve injury, which are based on anatomic continuity defects ( Fig. 1 ).
|I||Normal||Normal facial function in all areas|
|II||Mild dysfunction||Slight weakness noticeable on close inspection, may have slight synkinesis|
|III||Moderate dysfunction||Obvious, but not disfiguring, difference between 2 sides, noticeable but not severe synkinesis, contracture, or hemifacial spasm, complete eye closure with effort|
|IV||Moderately severe dysfunction||Obvious weakness or disfiguring asymmetry; normal symmetry and tone at rest; incomplete eye closure|
|V||Severe dysfunction||Only barely perceptible motion, asymmetry at rest|
|VI||Total paralysis||No movement|
When describing injury to the facial nerve, the Sunderland classification and the House-Brackmann system are used. The Sunderland system describes nerve injury by degree of injury (total of 5°), starting with neuropraxia and ending with neurotmesis as the most severe. The House-Brackmann system grades facial nerve injury on a scale of one through six based on functional limitation, with one being normal symmetric function and six being total facial paralysis (see Table 1 for details).
Diagnostic algorithms, tools
Although there have been significant advances in the objective assessment of peripheral nerve injury (including MRI, discussed later), they are not required to conduct a reproducible and accurate clinical examination.
The first step involves obtaining a detailed history from the patient in their own words, and whether they describe symptoms of paraesthesia, dysesthesia, anesthesia, or a combination. Because sometimes neuropathic symptoms are difficult to describe for some patients, it is helpful to have the patient complete a preprinted questionnaire that breaks down in detail exactly what the patient has been experiencing. It is important to differentiate frequency, spontaneity, and duration, because these can all be helpful diagnostic factors in determining the time frame and severity of the nerve injury. Often there is an obvious cause for the patient’s symptoms (eg, trauma, iatrogenic); however, practitioners must be wary of spontaneous symptoms because these are associated with some form of pathology and/or malignancy. The time/onset of injury is also critical because the success of nerve repair is directly proportional to the time from injury (after 3 months the chances of full recovery begin to decrease because of Wallerian degeneration).
There are many instruments/methods described in the literature that are used for neurosensory testing. Some commonly used methods involve a caliper, vitalometer (pulp tester), algometer, thermal disks, Semmes-Weinstein monofilaments, or as simple as a cotton swab. The examination should start with a typical head and neck examination, ruling out any evidence of trauma (acute vs chronic), evidence of previous surgery, or possible underlying pathology. Palpation of the regions of typical nerve anatomy (below the infraorbital rim, retromolar pad, and mental foramen) may illicit an atypical response in the setting of nerve injury (pain, tingling, itching). A painful response, or a nonpainful response that radiates, is known as Tinel sign (an indication of nerve regeneration or possible neuroma).
When completing neurosensory testing, it is recommended that the evaluation be completed in a quiet room with the patient in a seated position. Have the patient close their eyes, with their lips slightly apart so that any stimulus/vibration is not transferred to the opposite side. If simply wanting to assess for altered sensation, the marching needle technique is completed using a 27-gauge needle to contact the skin lightly until the patient indicates sensation by raising their hand. This technique is used to map areas of normal sensation, hypoesthesia, or complete anesthesia.
The three main levels of neurosensory testing include levels A, B, C, which is used to assess the degree of impairment. Level A (testing myelinated A-alpha sensory fibers) is tested using two-point discrimination, directional (brush stroke), or stimulus localization. For two-point discrimination (using either a caliper or Boley gauge), start by contacting the skin in the region of interest with the caliper tips together (zero distance). Gradually increase the distance by 1 mm, until the patient is able to detect two separate points. Normal values vary based on the region ( Table 2 ). Stimulus localization is done by contacting the skin with the wooden end of a cotton tip applicator, then asking the patient to touch the exact same location with a separate applicator. Normal response is within 1 to 3 mm of the initial point of contact. Two-directional response (brush stroke) is completed using a camel hair brush, Semmes-Weinstein fibers, or cotton wisp to lightly contact the skin in a series of directional movements, then asking the patient to verify the direction. If a patient passes level A testing this correlates with a Sunderland first-degree injury, if a patient fails level A but passes level B this correlates with a Sunderland second-degree injury, if a patient fails levels A and B that leaves level C. Level C testing includes a positive normal response, abnormal response, and no response correlating with Sunderland third, fourth, and fifth degree, respectively.
|Scheme of discrimination sensation|
|Ending of tongue||1.1 mm|
|Tips of tongue||2.2 mm|
|Red part of the lips||4.5 mm|
|Back of tongue||9.0 mm|
|Skin of cheek||11.2 mm|
|Back of neck||67.5 mm|
For all of these tests, start with the normal side first to establish a control. Level B (testing myelinated A-beta sensory fibers) measures static touch, and is completed using the wooden end of a cotton tip applicator and lightly contact the patient’s skin without indentation. On perception of the contact, the patient is to raise their hand. Another technique involves using either von Frey or Semmes-Weinstein fibers, which are monofilaments that are labeled according to the amount of force in grams of applied pressure required for them to bend. Both the normal side and site of interest are tested using the fibers, looking to rule out any significant difference between the two required levels of force needed for touch perception. Lastly, level C (testing poorly myelinate A-delta of unmyelinated C fibers) is evaluated using a 27-gauge needle to test for detection of painful stimuli. Have the patient raise their hand once they are able to feel the needle as you lightly contact the skin/region of interest (without indentation). It is under analysis of level C where some practitioners may also use algometers, thermal disks, and vitalometers to further clarify magnitude of stimulus required for sensation.
When assessing for possible nerve injury, at minimum some form of two-dimensional imaging should be completed (panorex, PAs) to look for obvious cause for patient’s symptoms (eg, retained root tip, poorly placed dental implant, pathology). If available, it is recommended to also obtain computed tomography (CT; either cone beam or medical grade), to further assess the pertinent anatomy. Ideally magnetic resonance neurography is used, which is a modification of MRI that uses specific water properties of neural tissue to optimize visualization of the nerve itself (rather than adjacent soft tissue). Because the source of the image is the nerve itself, diagnoses, such as compression, irritation, or nerve edema, are made using magnetic resonance neurography. Other resources include magnetic source imaging, which measures electric brain activity using magnetic fields to evaluate nerve response; high-resolution MRI; and ultrasound.
Assessment of nerve injury
Similar to the initial evaluation of an IAN injury, the first step of facial nerve injury evaluation involves an extensive history and physical. This includes not only any pertinent medical history (CN VII palsy is seen in multiple medical conditions including multiple sclerosis, myasthenia gravis, opercular syndrome, and many others), but also surgical history (recent mastoid or parotid), or even recent travel (scuba diving) or trauma (skull base fracture, penetrating injury).
Once an adequate history of present illness has been obtained, there are multiple topognostic studies available (can help determine the site of injury). These include salivary flow test, stapedial reflex, lacrimal flow (Schirmer), and taste sensation (specifically anterior two-thirds of the tongue). The next set of diagnostic tools includes electrodiagnostic testing, orthodromic conduction (nerve stimulated proximally and distal muscle response recorded), and antidromic conduction (nerve stimulated in a retrograde manner). These tests can be used as facial nerve recovery prognostic indicators. Electromyography is used to detect subclinical early evidence of neural regeneration by measuring electrical response during needle insertion at rest and elective movements. Nerve conduction time uses electromyography technology to stimulate the facial nerve at the stylomastoid foramen and then record the latency between the stimulus and nerve response at a specific muscle group (eg, frontalis, mentalis). Nerve excitability test compares the amount of electrical current needed to illicit a response when stimulating the nerve at the stylomastoid foramen (using the unaffected side as a control). Maximal stimulation testing involves similar technique as nerve excitability test; however, it uses maximal stimulation rather than minimal when stimulating the nerve. Magnetic stimulation works by using magnetic fields to stimulate the motor cortex at the nerve’s root entry zone, the response of which is measured by surface electrodes on specific muscle groups. Electroneurography (ENoG), which has been shown to be the most accurate prognostic indicator, uses bipolar electrodes to deliver an electrical stimulus at the stylomastoid foramen. The degree of response is compared between the normal and injured side, a decrease in response amplitude correlating with axonal injury.
Similar to radiologic assessment of the IAN, CT and MRI are the modalities of choice for evaluation of the facial nerve. High-resolution CT is most effective for intratemporal evaluation of the nerve, which is important because 90% of facial nerve disorders originate within the temporal bone. Gadolinium (intravenous administration)-enhanced MRIs are frequently used for their soft tissue detail, which typically allows easier detection of pathology, such as neuromas.