Conduction Studies 21 Facial Nerve

2.1.1. Relevant Anatomy

Of all cranial nerves, the facial nerve (Fig. 1) is the one most frequently studied in EMG labs. It emerges from the pons, coursing across the cerebello-pontine angle to the internal

From: The Clinical Neurophysiology Primer Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ

Facial Nerve b—

Temporal

Buccal

Cervical

Temporal Nerve Forehead

FRONTALIS

NASALIS

ORBICULARIS ORIS

MENTALIS

Fig. 1. Major branches of the facial nerve and muscles that they innervate.

Facial Muscles

Temporal

FRONTALIS

ORBICULARIS OCULI

Zygomatic

NASALIS

Buccal

ORBICULARIS ORIS

Mandibular

MENTALIS

Cervical

Fig. 1. Major branches of the facial nerve and muscles that they innervate.

acoustic meatus, where it enters the facial canal in the petrous bone. Branches given off within the petrous bone innervate the stapedius muscle, salivary, and lacrimal glands as well as taste buds on the anterior tongue. Soon after it exits the skull through the stylomastoid foramen, the nerve sends small branches to the digastric, stylohyoid, and occipital scalp muscles. The main nerve trunk courses through the parotid gland and then divides into five branches innervating facial muscles:

1. The temporal branch supplying the frontalis.

2. The zygomatic branch supplying the orbicularis oculi and nasalis.

3. The buccal branch supplying the orbicularis oris.

4. The mandibular branch supplying the mentalis.

5. The cervical branch supplying the platysma.

2.1.2. Conduction Studies

Motor conduction studies can be performed for any of the facial muscles mentioned in Subheading 2.1.1. Percutaneous stimulation underneath the ear at the tip of the mastoid process excites all five facial branches. A more distal site, just anterior to the tragus, stimulates the zygomatic and temporal branches. The nasalis muscle is a particularly attractive recording site because it tends to produce a compound muscle action potential (CMAP) with a sharp initial negative deflection. Motor conduction studies to the orbic-ularis oculi are convenient because the same recording site is shared with the blink reflex. However, flat circular muscles, such as the orbicularis oculi and oris may produce CMAPs that are more dispersed, with initial positive deflections. Furthermore, artifact from direct co-stimulation of the masseter is more likely to complicate recording from these muscles. Facial muscles normally produce CMAPs with amplitudes ranging between 2 and 4 mA, although a value less than 50% of the normal contralateral is the most widely accepted criterion for abnormality.

2.2. Trigeminal Nerve

2.2.1. Relevant Anatomy

Unlike the facial nerve, the trigeminal nerve contains a large number of somatosensory afferent fibers in addition to motor axons. Primary neurons relaying pain/temperature and touch modalities reside within the gasserian ganglion. Their distal axons leave the ganglion in the three sensory divisions: ophthalmic (V1), maxillary (V2), or mandibular (V3). Proximal axons of touch fibers project to the principal trigeminal nucleus of the pons, whereas pain/temperature fibers send their proximal axons to the spinal trigeminal nuclei (extending from the pons to the cervical spinal cord). The cell bodies of trigeminal muscle spindle afferents are thought to occupy the mesencephalic trigeminal nucleus. This nucleus extends rostrally to the level of the colliculi, serving as a relay center for proprioceptive reflexes.

Motor fibers of the trigeminal nerve originate from their nucleus in the mid pons. They pass through Meckel's cave at the tip of the petrous bone and then under the Gasserian ganglion to emerge from the skull via the foramen ovale. The motor fibers join the mandibular division of the nerve to supply the masticatory muscles (masseter, temporalis, and pterygoids) as well as palatal, mylohyoid, and the anterior belly of the digastric muscle.

2.2.2. Conduction Studies

The trigeminal nerve's extracranial course is not suitable for the performance of motor conduction studies. Branches to masticatory muscles originate from the mandibular division soon after its exit from the foramen ovale and take short, deep trajectories to their targets. Many sensory branches are also inaccessible, but orthodromic sensory conduction studies of the supraorbital nerve have been described. Stimulation at the upper forehead elicits a sensory nerve action potential recordable at the supraorbital notch. The latency of this response is usually shorter than 1 ms, which makes it prone to distortion from stimulus artifact.

2.2.3. Blink Reflex

Reflex studies provide an alternative method of assessing sensory conduction in the trigeminal nerve. Of these, the blink reflex is the most commonly used. It represents the elec-trophysiological correlate of the glabellar tap reflex. The technique involves recording responses bilaterally from orbicularis oculi after electrical stimulation of the supraorbital trigeminal branch. This produces two separate responses. The earlier response (termed R1) appears with a latency of 8 to 13 ms and is normally limited to the ipsilateral orbicularis oculi. Appearing later, and bilaterally, is the R2 response. It is temporally more dispersed, with a normal onset latency ranging from 24 to 41 ms. R2 responses recorded contralateral to stimulation have a slightly longer average latency, but the side-to-side latency difference for any single stimulation should not exceed 5 ms.

Paired stimuli at 3 to 5 ms intervals will, at times, evoke an R1 response when single stimuli fail. The blink reflex can also be elicited by a mechanical glabellar tap delivered using a reflex hammer with a microswitch to trigger the trace. This technique stimulates both sides, leading to bilateral R1 responses with slightly longer latencies than those evoked electrically. Electrical stimulation of the infraorbital or mental nerves often elicits blink responses, which can assess sensory transmission through these branches.

Blink reflexes can delineate pathology in the trigeminal nerve, the facial nerve, and brainstem pathways connecting trigeminal and facial nuclei (Fig. 2). Interpretation requires comparison

Schematic Diagram Trigeminal Nerve

Fig. 2. Schematic diagram illustrating the neural pathways of the blink reflex. Large-caliber fibers form a reflex arc through the main trigeminal nucleus to the ipsilateral facial nucleus, producing the R1 response. Thinner fibers form multisynaptic pathways through the spinal trigeminal nucleus to produce bilateral R2 responses.

Fig. 2. Schematic diagram illustrating the neural pathways of the blink reflex. Large-caliber fibers form a reflex arc through the main trigeminal nucleus to the ipsilateral facial nucleus, producing the R1 response. Thinner fibers form multisynaptic pathways through the spinal trigeminal nucleus to produce bilateral R2 responses.

of responses from both right- and left-sided stimulation. Further localization involves an understanding of the divergent pathways underlying the R1 and R2 components. The R1 response reflects conduction through the main trigeminal nucleus and is a good indictor of pontine function. The R2 responses are mediated by the spinal trigeminal nuclei and can identify pathology at pontine or medullary levels. Table 1 summarizes the patterns of abnormality used to localize various lesions.

2.2.4. Masseter Reflex

The masseter reflex is an electrophysiological analog of the jaw jerk. It is used less commonly than the blink reflex for evaluation of trigeminal sensory function. However, it can provide complementary information, in that it assesses muscle spindle afferents and their connections through the mesencephalic trigeminal nucleus. Additionally, it is one of the few tests assessing motor conduction within the trigeminal nerve.

The reflex is elicited by a reflex hammer with a microswitch that triggers recording on percussion. Response to a midline tap of the chin is recorded bilaterally from percutaneous electrodes over the masseter muscles. Because the response can be difficult to obtain in normal subjects, the only meaningful abnormality is a consistent unilateral absence or delay.

2.2.5. Masseter Silent Period

This reflex, also known as the masseter inhibitory reflex, is mediated through both sensory and motor divisions of the trigeminal nerve. Stimulation of the mental or infraorbital branch is performed during tonic, voluntary closure of the jaw. Surface EMG activity recorded over the masseter muscle is typically interrupted by two silent periods. The first, termed SP1, has an onset latency of 10 to 15 ms. The later SP2 silent period normally occurs at 40 to 50 ms. Both SP1 and SP2 occur bilaterally after unilateral stimulation. Successful recording of this reflex requires forceful dental occlusion for 2 to 3 s while relaxing other facial musculature. Background activity from

Table 1

Interpretation of Blink Reflex Studies

R supraorbital stim L supraorbital stim

Table 1

Interpretation of Blink Reflex Studies

R supraorbital stim L supraorbital stim

Record

R1

R2

R1

R2

Normal

Right

nl

nl

nl

Left

nl

nl

nl

R CN VII lesion

Right

Ab

Ab

Ab

Left

nl

nl

nl

R CN V lesion

Right

Ab

Ab

nl

Left

Ab

nl

nl

R pons lesion

Right

Ab

±

±

Left

±

nl

nl

R medulla lesion

Right

nl

Ab

nl

Left

Ab

nl

nl

Expected blink reflex results for trigeminal and facial neuropathies as well as brainstem lesions. R, right; stim, stimulus; L, left; nl, normal; CN, cranial nerve; ab, abnormal or absent; ±, possibly abnormal depending on precise location of lesion within the pons.

Expected blink reflex results for trigeminal and facial neuropathies as well as brainstem lesions. R, right; stim, stimulus; L, left; nl, normal; CN, cranial nerve; ab, abnormal or absent; ±, possibly abnormal depending on precise location of lesion within the pons.

surrounding muscles may mask the silent periods and, if this is the case, recording the EMG activity with a concentric needle in the masseter may prove more successful.

The SP1 silent period likely reflects conduction through the sensory trigeminal branch to inhibitory interneurons projecting bilaterally to the motor trigeminal nucleus. This arc lies within the midpontine level. The SP2 is mediated through the spinal trigeminal nucleus and involves upper medullary as well as pontine levels. Similar to the masseter (jaw jerk) reflex, the masseter inhibitory reflex provides an assessment of both trigeminal sensory and motor conduction, but it supplies complementary information regarding smaller caliber sensory fibers and pontomedullary connections that are more caudal.

2.3. Spinal Accessory Nerve

2.3.1. Relevant Anatomy

This largely motor nerve emerges from the brainstem and cervical spine as a long, linear array of rootlets. The spinal portion of the nerve ascends through the foramen magnum to merge with the cranial portion and then exits the skull through the jugular foramen. Coursing down the neck between the carotid artery and jugular vein to innervate the sternocleidomas-toid, the nerve penetrates that muscle before traversing the posterior triangle of the neck. It then terminates by innervating the trapezius. Communications from cervical roots C3 and C4 join the spinal accessory nerve along its distal course. These innervate the trapezius to a variable degree. The lower portion of the trapezius more commonly receives direct innervation from the C3 and C4 roots, but cervical innervation of the entire trapezius has been reported.

2.3.2. Conduction Studies

Motor conduction studies of the spinal accessory nerve most often evaluate the distal segment innervating the trapezius. The nerve is stimulated percutaneously at the posterior border of the sternocleidomastoid muscle, with CMAP recorded from the upper portion of the trapezius. Conduction studies assessing portions of the nerve that are more proximal can be performed. These involve stimulation anterior to the mastoid process with recording via needle electrodes in the sternocleidomastoid and trapezius.

2.4. Hypoglossal Nerve

2.4.1. Relevant Anatomy

The hypoglossal nerve derives from 10 to 15 rootlets exiting the ventral medulla. These form two bundles that pass through the hypoglossal canal and merge into a single nerve on leaving the skull. The nerve descends toward the angle of the mandible, receiving communications from C1 and C2 ventral rami and giving off a branch to the ansa cervicalis that supplies the infrahyoid muscles. The hypoglossal nerve proper turns medially past the hyoid bone and divides into branches supplying the extrinsic (styloglossus, hypoglossus, genioglos-sus, and geniohyoid) muscles as well as the intrinsic muscles of the tongue.

2.4.2. Conduction Studies

Hypoglossal nerve stimulation is achieved with a bipolar surface electrode, placing the cathode 1 cm medial to the inner aspect of the mandible, at a point one-third the distance from the angle to the apex. CMAPs are recorded from the surface of the tongue using disc electrodes affixed to a bite bar or tongue depressor. The electrodes are placed along the midline, with the active electrode 1 cm behind the incisors and the reference electrode 2 cm more posterior. The amplitude of the CMAP may be variable because of tongue movement, but latencies are reportedly quite stable. As with other cranial nerve conduction studies, clear asymmetries on bilateral stimulation provide the most convincing abnormalities.

2.5. Recurrent Laryngeal Nerve

2.5.1. Relevant Anatomy

This branch of the vagus nerve originates at the base of the neck and takes a different proximal course on the two sides. On the left, it loops under the aortic arch. On the right, it loops under the subclavian artery before ascending to the larynx in the sulcus between the trachea and esophagus. All laryngeal muscles except the cricothyroid receive innervation from the recurrent laryngeal nerve.

2.5.2. Conduction Studies

Conduction studies of this nerve require specialized techniques. Recording from laryngeal muscles is most often accomplished with a concentric needle placed under laryngoscopic guidance. Needle electrodes are used to stimulate two sites: the descending vagus trunk at the posterior border of the sternocleidomastoid and the recurrent laryngeal branch lateral to the trachea. Conduction velocity can be estimated using measurements of nerve length between these two sites, derived from cadavers.

The needle exam of cranial musculature involves largely the same principles and technique used in studying limb muscles. However, a few points deserve special attention. Because facial muscles are small and located in sensitive areas, many examiners prefer to use very thin recording needles (0.3-mm diameter or 30-gauge concentric needles, often referred to as "facial needles"). Their impedance is higher than standard concentric needles, raising the amplitude of both signal and noise. Other signal characteristics, most importantly the duration and complexity of MUPs, are not altered by the use of facial needles.

In general, motor units in cranial muscles tend to be brief and of low amplitude, making assessment of myopathies difficult. Relaxation of cranial muscles may be hard to achieve but is most important for detection of fibrillation potentials, which can be confused with small brief motor units. Furthermore, activity from surrounding muscles may contaminate the recording unless selective activation of a given muscle is attempted on a background of complete relaxation. This is particularly relevant to examination of the orbicularis oris and orbic-ularis oculi, thin muscles in close proximity to the masseter. Every effort should be made to relax the jaw when recording from these muscles.

The cranial muscles most commonly studied by EMG include the temporalis and masseter for the trigeminal nerve; the frontalis, orbicularis oculi, and orbicularis oris for the facial nerve; the trapezius and sternocleidomastoid for the spinal accessory nerve; and the tongue for the hypoglossal nerve. One approach to hypoglossal muscles involves needle insertion through the surface of the tongue while holding it protruded with a gauze pad. An alternative is to assess the genioglossus through the under surface of the chin, 2 cm back from the mental apex. With either approach, it is important to allow the tongue to settle back within the floor of the mouth to achieve relaxation, which can often be very difficult to accomplish.

EMG of extraocular muscles is performed rarely and is usually carried out by ophthalmologists familiar with the details of the anatomy. The approach is through the eyelid, sometimes with the use of a topical anesthetic. Assessment for spontaneous activity is difficult because extraocular muscles maintain some baseline motor unit activation even in primary position.

4. CLINICAL CORRELATIONS 4.1. Bell's Palsy—Initial Evaluation

The diagnosis of Bell's Palsy is usually obvious from its clinical features. Electrodiagnostic studies serve mainly to quantify the severity and determine prognosis. Assessing the relative contribution of demyelinating and axonal components is crucial to prognosis. Because Wallerian degeneration does not fully develop until 5 to 8 d after axonal injury, motor conduction studies provide accurate prognostic data only after that time. CMAP amplitude is the most useful prognostic parameter. If the CMAP amplitude is less than 10% of that on the healthy side, maximum recovery will be delayed 6 to 12 mo, usually leaving moderate-to-severe weakness. If the amplitude is 10 to 30% of the healthy side, recovery may take 2 to 8 mo, with mild-to-moderate residua. If the CMAP amplitude is greater than 30% of normal, there is usually full recovery within 2 mo. CMAP amplitude is most accurate in predicting a good prognosis, and less accurate in those patients with greater than 90% loss of amplitude, because a substantial number (up to 47%) will still have a good recovery. The CMAP amplitude method is of limited use in bilateral facial neuropathies.

The blink reflex does not add much to prognosis in Bell's palsy. Although it evaluates conduction along the entire nerve, its assessment of axonal degeneration offers little beyond direct facial nerve studies and is limited by the same time constraints. Absence of the R1 component within 2 wk of onset indicates a 45% chance of satisfactory recovery, compared with 94% in patients with normal R1. A prolonged R1 latency only reduces the chances of satisfactory recovery to 87%.

Needle EMG will show abnormalities in motor unit recruitment from the outset of Bell's palsy, but this finding does not help differentiate demyelinating from axonal lesions and, consequently, has little prognostic usefulness. However, the presence of even a few voluntary MUPs in a patient with complete clinical paralysis indicates the nerve remains in continuity and is consistent with a better prognosis than those patients with no MUPs. This finding needs to be interpreted cautiously in the orbicularis oris because there may be some degree of crossed innervation in this midline muscle. Fibrillation potentials and positive sharp waves indicate the presence of axonal degeneration but are difficult to quantify and do not necessarily imply poor recovery. They usually do not appear earlier than 1 to 2 wk after onset and may not be observed for up to 3 wk.

4.2. Bell's Palsy—Later Studies

Regeneration after Bell's palsy can lead to a number of electrophysiological changes. Findings on needle exam include low-amplitude, polyphasic MUPs typical of newly regenerated motor units (nascent motor unit potentials). In addition to abnormal volitional recruitment, these motor units may also generate spontaneous discharges, either single or grouped (myokymic). This presumably reflects a hyperexcitable state of facial motor neurons or their axons.

Synkinetic movements often develop after regeneration of the facial nerve, manifesting as jaw winking and other clinical phenomena. An electrophysiological counterpart can be observed in the blink reflex, in which supraorbital nerve stimulation elicits R1 and R2 responses not only from the orbicularis oculi but also from the orbicularis oris, platysma, or other lower facial muscles. A combination of aberrant axonal branching, ephaptic transmission between axons, and neuronal hyperexcitability underlies these synkinetic discharges.

4.3. Hemifacial Spasm

This unilateral paroxysmal contraction of facial muscles may follow Bell's palsy or occur with a mass compressing the facial nerve, but, in most instances, hemifacial spasm arises without a clear cause. Surgical exploration often reveals an arterial loop impinging the nerve in such "idiopathic" cases. EMG activity during spasm shows bursts of MUP firing at rates of 80 to 150 Hz. Brief bursts often repeat at rates of 5 to 20 per second but prolonged tonic discharges also occur. During voluntary blinking, synkinetic discharges appear in orbicularis oris or other facial muscles normally not involved in blinking. The synkinesis of idiopathic hemifacial spasm may vary from moment to moment, unlike the constant synkinesis observed after Bell's palsy.

Nerve conduction and reflex studies reveal abnormal responses in cases of hemifacial spasm. Blink reflexes demonstrate synkinetic R1 and R2 responses in muscles such as the orbicularis oris (that is, those not normally activated by supraorbital nerve stimulation), that are more variable than those observed after Bell's palsy. Motor conduction studies with selective stimulation of individual facial nerve branches will evoke CMAPs from muscles supplied by other divisions. This may reflect either ephaptic transmission or hyperexcitability of facial neurons. Hyperexcitability may also manifest as after-discharges following CMAP.

4.4. Trigeminal Neuralgia/Neuropathy

Neuralgia is the most common affliction of the trigeminal nerve. It manifests as paroxysmal pain within one or more branches, affecting (in order of decreasing frequency) the maxillary, mandibular, and ophthalmic divisions. Although a sensation of mild residual numbness can persist in the affected area, the neurological exam shows no deficit. Trigeminal neuralgia complicates many cases of multiple sclerosis; however, it most often occurs in isolation. Surgical exploration of such idiopathic cases frequently reveals a vascular loop contacting proximal portions of the nerve.

Electrophysiological evaluation is normal in cases of trigeminal neuralgia, except for rare prolongation of the R1 component of the blink reflex. Other abnormalities in reflex studies or EMG of trigeminal muscles argue for the alternative diagnosis of trigeminal neuropathy. Trigeminal neuropathy with evidence of denervation on EMG often reflects a neoplastic or traumatic etiology.

Pure sensory trigeminal neuropathy occurs in patients with connective tissue disorders (sensory neuronopthy). These cases show abnormalities in R1 and R2 components of the blink reflex as well as abnormal masseter silent periods. The sensory trigeminal neuropathy complicating Sjogren's syndrome produces blink reflex and silent period abnormalities, but spares proprioceptive neurons within the mesencephalic nucleus and consequently the mas-seter (jaw jerk) reflex remains normal, because these neurons are located within the central nervous system.

4.5. Spinal Accessory Neuropathy

The most common causes of spinal accessory neuropathies are surgical trauma and tumors, but occasionally they occur without clear etiology. Many of these spontaneous cases appear akin to brachial neuritis, likely involving disimmune mechanisms. Motor nerve conduction studies and EMG confirm the diagnosis and localization, with assessment of the sternoclei-domastoid important in determining the proximal extent of pathology. On the whole, electro-physiology is less helpful in prognosis than it is for cases of facial neuropathy. The amplitude of trapezius CMAPs on early conduction studies does not predict outcome well. Cases with very low initial CMAP amplitudes and marked denervation on EMG can recover surprisingly well. In part, this may reflect compensatory reinnervation directly from cervical roots.

EMG of cranial muscles plays an important role in the diagnosis of ALS. Detection of den-ervation outside the cervical and lumbar regions becomes particularly significant in patients with extensive spondylosis or other spinal pathology that could confound denervation identified in appendicular muscles. EMG abnormalities can be observed well before clinical evidence of weakness in facial or bulbar muscles. Fibrillations and positive waves provide the most definitive evidence of denervation but may be difficult to recognize. Reported studies of cranial muscles in ALS have found fibrillations (in order of decreasing frequency) in the tongue, facial nerve muscles, and trigeminal nerve muscles. Evidence of reinnervation (increased MUP duration and amplitude) is less definitive but more common. Quantitative analysis of MUP parameters has revealed abnormalities in up to 60% of cranial nerve muscles in ALS patients without bulbar signs or symptoms.

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