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Electrophysiology of Cranial Nerve Testing: Trigeminal and Facial Nerves

Muzyka, Iryna M.; Estephan, Bachir

Journal of Clinical Neurophysiology: January 2018 - Volume 35 - Issue 1 - p 16–24
doi: 10.1097/WNP.0000000000000445
Invited Review

Summary: The clinical examination of the trigeminal and facial nerves provides significant diagnostic value, especially in the localization of lesions in disorders affecting the central and/or peripheral nervous system. The electrodiagnostic evaluation of these nerves and their pathways adds further accuracy and reliability to the diagnostic investigation and the localization process, especially when different testing methods are combined based on the clinical presentation and the electrophysiological findings. The diagnostic uniqueness of the trigeminal and facial nerves is their connectivity and their coparticipation in reflexes commonly used in clinical practice, namely the blink and corneal reflexes. The other reflexes used in the diagnostic process and lesion localization are very nerve specific and add more diagnostic yield to the workup of certain disorders of the nervous system. This article provides a review of commonly used electrodiagnostic studies and techniques in the evaluation and lesion localization of cranial nerves V and VII.

Department of Neurology, Mayo Clinic, Phoenix, Arizona, U.S.A.

Address correspondence and reprint requests to Iryna M. Muzyka, MD, Mayo Clinic, 5777 E Mayo Boulevard, Phoenix, AZ 85054, U.S.A.; e-mail: Muzyka.iryna@mayo.edu.

The authors have no funding or conflicts of interest to disclose.

The electrophysiological studies of the reflex activity mediated by the trigeminal and facial nerves allow the evaluation of their integrity and their central pathways. Their clinical applications include the evaluation of cranial neuropathies, polyradiculoneuropathies, peripheral neuropathies, brainstem lesions, and facial movement disorders. This review discusses the different established reflexes and techniques used to study these two nerves and their connections.

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THE TRIGEMINAL NERVE

Corneal Reflex

Neuroanatomy

The corneal reflex is designed to protect the eye and is believed to be purely nociceptive. The reflex afferents are A-delta fibers1,2 passing through long ciliary nerves and the ophthalmic division of the trigeminal sensory root to reach the pons. The central circuit is similar to that of R2 responses of the blink reflex (Fig. 1B), but it differs from R2 because it is purely nociceptive and thus is relayed through fewer and different interneurons that are more resistant to suprasegmental influences.1,3 The efferent limb comprises motor fibers of the bilateral facial motor nuclei in the facial nerve that terminate in the orbicularis oculi (OO) muscles.

FIG. 1

FIG. 1

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Technique of Testing and Application

The corneal reflex consists of bilateral involuntary eyelid closure (contraction of the OO) in response to stimulation of either cornea between spontaneous blinks. The stimulation can be mechanical or electrical. Responses are recorded simultaneously from bilateral inferior portions of the OO muscles with surface or needle electrodes placed in the same location as for the blink reflex.

Mechanical stimulation is evoked by application of a small 2-millimeter (mm) metal sphere to the cornea.1,4 When the sphere touches the cornea, contact is made between the cornea and the electrical trigger circuit that delivers the pulse.

Electrical stimulation of the cornea is done with a thin saline soaked cotton thread connected to the cathode of a constant current stimulator. The anode is placed on the earlobe or forearm. This stimulation produces controlled and reproducible responses with square pulses of 1 millisecond (ms) in duration and 0.1 to 3 milliamps (mA) in intensity yielding controlled and reproducible stimuli.

The reflex threshold in normal subjects rarely exceeds 0.5 mA. Mechanical and electrical stimuli elicit reflexes with similar latencies. Absolute latency values range from 36 to 64 ms with mechanical stimulation and 35 to 50 ms with electrical stimulation. The wide range of latencies narrows within age groups.

When measuring the latency, three pairs of latency times are assessed from stimulus artifact to onset of electromyographic (EMG) response (Fig. 1A).

In contrast to the blink reflex, the corneal reflex does not evoke an early R1 response (Fig. 1A). When the cornea is stimulated (mechanically or electrically), there is direct (ipsilateral) and consensual (contralateral) response. With mechanical stimulation, the direct response latency should not exceed the consensual latency by more than 8 ms. The latencies of the direct responses evoked by stimulation of both corneas separately should never differ by more than 10 ms.1,4 With electrical stimulus, the difference between the direct and consensual responses should not exceed 5 ms, and the difference between direct responses should never exceed 8 ms (Table 1).

TABLE 1

TABLE 1

Lesions of the trigeminal nerve abolish ipsilateral responses to stimulation, whereas lesions of the facial nerve abolish the corneal reflex with bilateral eye stimulation.

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Blink Reflex

The blink reflex detects lesions involving the (ophthalmic) first division of the trigeminal nerve (V1), the facial nerve or their central pathways in the pons and medulla. Overend in 1896 observed that the reflex was the product of facial stimulation, for it was present in the blind. Kugelberg, in 1952 studied electromyographically the blink reflex evoked by electrical stimulation of the supraorbital nerve. The blink reflex is a surface EMG recording from the OO muscle of the reflex evoked by mechanical or electrical stimulation of the supraorbital nerve. Mechanically evoked blink reflexes show larger latency variation due to less well synchronized afferent volley.5

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Neuroanatomy

The afferent limb is mediated by the supraorbital branch of V1, and the efferent limb travels in the facial nerve. R1 is an oligosynaptic circuit response and is conducted via large myelinated fibers through the pons, to the principal sensory nucleus of the trigeminal nerve, the whole circuit lying within the pons, before reaching the ipsilateral facial nucleus (Fig. 1D). R2 is polysynaptic with afferent impulses being relayed to facial motor neurons through the dorsolateral pons and medulla before reaching the most caudal area of the spinal trigeminal nucleus.6 From there, impulses are conveyed through polysynaptic medullary pathways both ipsilaterally and contralaterally to the stimulated side of the face, before connecting to the facial nuclei. The crossing takes place in the caudal medulla.

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Technique of Testing

When an electrical stimulus is applied to the supraorbital nerve, surface electrodes deliver single stimuli of an intensity of 2 to 3 times the perception threshold. Surface recording electrodes are placed over bilateral inferior OO. The electrical stimulation of the supraorbital nerve elicits two responses: a first or early response (R1) that is relatively constant, well synchronized, unilateral, ipsilateral to the side of stimulation and accompanied by a short EMG response in the OO with latency of approximately 10 to 13 ms not visible clinically (considered a proprioceptive reflex). The second or late response (R2) is believed to be nociceptive, more variable and prolonged, bilateral and poorly synchronized with latency of approximately 30 ms6 (Fig. 1C). The amplitude measurements are usually not helpful in localization as differences as great as 40% can occur in normal subjects.7

R1 is a relatively stable response and is used for evaluation of the afferents from the supraorbital and pons regions, not influenced by supratentorial dysfunction (unless during the acute phase of an insult) and disorders of consciousness and cognition.8 ,page62 Absolute or side-to-side R1 latency prolongations are considered abnormal (Table 2).9

TABLE 2

TABLE 2

The R2 response correlates with closure of the eyelids and has the same latency as the corneal reflex. It is polysynaptic, more variable than R1, influenced by supratentorial processes, but is crucial in diagnosing medullary lesions. The R2 responses are attenuated during sleep and by sedating medications. With repeated stimulation, the R2 responses tend to habituate. The simultaneous recording of bilateral R2 responses is useful in differentiating an afferent (trigeminal) and efferent (facial) pattern of lesion.9

The blink reflex can also be elicited with stimulation of nontrigeminal inputs of peripheral nerves (somatosensory, acoustic, or photic).1 These reflexes involve central mechanisms, and responses have greater variability of latency, show more dispersion in time, vary with lengthy repetition of stimuli, and show other distinctive differences.10

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Applications

In lesions of the trigeminal nerve, ipsilateral R1 and bilateral R2 responses can be delayed or be absent depending on the severity of the lesion, but both R1 and bilateral R2 responses will be normal with stimulation of the unaffected side.11–13 This pattern is typically seen with proximal or distal trigeminal sensory neuropathy. Abnormalities in R1 only or both R1 and R2 may be seen in isolated trigeminal neuropathy. In postherpetic neuralgia, there are frequently significant abnormalities of the R1 blink reflex responses on the affected side compared with the normal side. However, in most patients with idiopathic trigeminal neuralgia, all blink reflexes are normal; an abnormality would suggest a structural lesion.11,13

In facial nerve lesions stimulation of the involved side can produce prolonged or absent R1 and ipsilateral R2 depending on the severity of the lesion, with normal contralateral R2 responses. Stimulation of the uninvolved side will produce normal R1 and ipsilateral R2 with prolonged or absent contralateral R2 responses. This pattern is typically observed in idiopathic or secondary facial neuropathies.

The R1 component of the blink reflex is mediated at the level of the lateral midpons and facial nucleus. This probably courses ventrolaterally to the medial longitudinal fasciculus, approaching the intrapontine segments of the abducens, facial, and vestibular nerves.14,15 Provided that trigeminal nerve functions are intact (i.e., there is a normal corneal reflex, normal trigeminal sensory function, and no masseter paresis), masseter reflex abnormalities (see below) indicate ipsilateral brainstem lesions between the level of the trigeminal motor and oculomotor nuclei, whereas blink reflex R1 abnormalities indicate lesions between the lateral midpons and medial caudal pons.15,16

Because of the differences in the circuitry of the R1 and R2 responses, an interesting possibility emerges from the study of the onset latency of the responses: the latency of the R1 depends more on the trigeminal and facial conduction times than on the intraaxial synaptic connectivity, with only one or two interneurons in the circuit.1 The reverse occurs with regard to the R2, for which the latency is more dependent on interneuronal synapsis than on peripheral nerve conduction time. Therefore, delays are observed predominantly in the R1 response in lesions involving the peripheral nerve and in the R2 response in lesions involving the trigeminal complex in the brainstem, predominantly medullary (Kimura, 1975, Kimura, 1982, Kimura and Lyon, 1972, and Kimura et al., 1970). However, delays in the R1 response can also be detected in clinical manifest or silent pontine lesions in multiple sclerosis (Kimura, 1975).

Blink reflex findings involving both R1 and R2 can be helpful in detecting subclinical sensory ganglionopathy of various etiologies and may be useful in providing evidence of patchy sensory involvement.13 While subclinical trigeminal abnormalities can be seen in patients with sensory ganglionopathy, sometimes involving R1 and/or R2 responses, overall the most frequent finding is an abnormal R2 response.17 Taimour et al. reported abnormal blink reflex responses in paraneoplastic sensory ganglionopathy cases. This was believed to possibly reflect the patchy nature of the disease.17

In patients with chronic inflammatory demyelinating polyneuropathy, there is no apparent correlation between clinical disability or disease duration and the R1 response. The blink reflex is useful for functional evaluation of trigeminal and facial nerves in chronic inflammatory demyelinating polyneuropathy because of this lack of correlation between clinical features and electrophysiological findings.18 Delayed responses to supraorbital nerve stimulation are found in hereditary motor and sensory neuropathy type 1 (Kimura 1971). Delay is most significant in the distal segment of the facial nerve, although no facial weakness may be present. By contrast, delayed latencies in acute inflammatory demyelinating polyradiculoneuropathy are often associated with facial weakness. Axonal length-dependent peripheral neuropathies rarely affect the blink reflex.

The blink reflex can also be obtained by stimulation of infraorbital and mental nerves. The responses can be of help for the assessment of the site of the lesion in the trigeminal spinal nuclei in brainstem vascular lesions (Valls-Solé et al., 1996), and in patients with suspected lesions of the maxillary or mandibular divisions of the trigeminal nerve.

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Jaw Jerk (Masseter Reflex)

Clinically, this reflex is usually confined to distinguishing between normal and brisk reactions because during clinical examination alone, it is difficult to detect unilateral interruption of the reflex. In addition, even in healthy subjects, reflex movement of the mandible is often impossible to separate from the slight excursion of the jaw with mechanical percussion.1

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Neuroanatomy

The jaw Jerk (Mandibular or Masseter reflex) evaluates the third division of the trigeminal nerve (V3). It is a monosynaptic tendon reflex that is one of the first reflexes to develop in utero and can be recorded in preterm infants.1 Tapping the chin causes contraction of the masseters—jaw-closing muscles. Afferent neurons are located centrally unlike all other stretch reflexes (in the dorsal root ganglia), in the mesencephalic nucleus in the midbrain (Fig. 2B). The efferent motor neurons are located in the pontine trigeminal motor nucleus and activate only the ipsilateral masseter muscle. Afferent impulses from masseter muscle spindles travel through the motor root of V3 to the mesencephalic nucleus and activate the motor nucleus monosynaptically. The efferent limb of the reflex arc causes the ipsilateral masseter muscle to contract. Whether the afferent fibers travel in the trigeminal motor root36,47 or the trigeminal sensory root11,19,20 remains controversial. In addition, the masseter reflex also differs from the extremity deep tendon reflexes because it is potentiated rather than inhibited by vibration.12

FIG. 2

FIG. 2

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Technique of Testing

When the jaw-jerk reflex is assessed electrophysiologically, the reflex hammer is equipped with a microswitch that triggers the sweep of the oscilloscope when striking the examiner's finger resting over the subject's chin (15,21 Stevens JC, Smith BE, 1996, Daube, Clinical Neurophysiology, p.321–325). Electromyographic responses are recorded simultaneously and bilaterally with either surface electrodes placed on each masseter muscle belly two-third the distance between the zygoma and the lower edge of the mandible, or small diameter concentric needle electrodes inserted into each masseter (in elderly or obese individuals). The reference electrode is placed just below the mandibular angle or over the zygoma.

It is often absent in elderly people and therefore has no definite clinical significance if absent bilaterally, a situation which may occur in healthy subjects. The normal range of latencies is 5 to 10 ms (Table 3).22 The amplitude is widely variable, and measurement is not considered clinically useful (Kimura, 2001). Since reflex latencies may vary with successive trials, comparison of simultaneously recorded contralateral latencies responses is more meaningful than analyzing absolute values.14 A difference of more than 0.8 ms or a consistent unilateral absence of the reflex is taken to be abnormal (Fig. 1A).

TABLE 3

TABLE 3

The masseter reflex is not technically as precise as the blink reflex. Technical problems can include standardizing the mechanical tap and changes in the tone of the masseter muscle (both inter- and intra-individual variability); therefore, four consecutive responses are used to show consistency. If the jaw is not sufficiently relaxed, background muscle activity may obscure the response. This reflex is strongly influenced by dental occlusion and can be asymmetrical or even absent in some patients with temporomandibular disorders.13,23

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Applications

This reflex assesses function of the mandibular division of the trigeminal nerve (V3). The most common abnormality is the absence of the reflex rather than prolongation of the latency. A unilateral delay or absent response suggests a lesion either in the trigeminal nerve or the brainstem. Using the jaw reflex in conjunction with needle examination of the masseter muscle may document a peripheral lesion if there is evidence of denervation.11,24,25 Because the afferent nerve cell body is at the mesencephalic nucleus, patients with primarily sensory symptoms and hyporeflexia who have a normal masseter response would more likely be diagnosed with a ganglionopathy rather than a sensory neuropathy.26 This is also why the response is typically normal in patients with Friedrich ataxia.27

The observation of an absent reflex response in the face of a normal masseter EMG study is observed in ipsilateral midbrain lesions. However, with pontine lesions, both EMG and jaw jerk may be abnormal, due to efferent cell bodies' involvement.11,24,25 Polysynaptic brainstem reflexes such as the blink reflex R2 response show increased latencies and decreased amplitudes with supratentorial lesions, particularly those of the lower postcentral area.28 The masseter reflex is monosynaptic and therefore is not similarly influenced, making it a reliable measure of direct involvement of the reflex arch.28 Masseter reflex latency and amplitude are not observed to be shifted beyond the normal range by suprasegmental and cerebellar influences.28

Using both the masseter and blink reflexes in patients with internuclear ophthalmoplegia due to multiple sclerosis or lacunar brainstem infarctions, lesions can be localized either to the midbrain or pons. If the abnormality is limited to the masseter reflex, this suggests a midbrain lesion, but an abnormal blink reflex R1 latency indicates involvement of the rostral pons.

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Masseter Inhibitory Reflex (MIR)

Stimulation of the masseter muscle stretch receptors induces not only the jaw jerk but also inhibitory effects that can be easily observed as a silent period when the stretch is applied during masseter contraction. The silent period in response to chin taps begins at approximately 10 to 12 ms and lasts for 20 to 40 ms.29 This was believed to be due to unloading of spindles or activation of Golgi tendon organs28 while electrical stimuli evoke a double phase of silence.

Electrical stimulation anywhere within the mouth or on the skin innervated by the maxillary and mandibular trigeminal divisions may evoke a reflex inhibition in the jaw-closing muscles. This phenomenon can be produced by electrical or mechanical stimulation of the infraorbital or mentalis nerves during voluntary contraction of the masseter muscle, which evokes a reflex inhibition and an EMG silent period of the jaw-closing muscles (masseter, temporalis) and transitory relative or absolute decrease in EMG activity evoked in the midst of an otherwise sustained contraction.30 This reflex is believed to have a primary role of protecting against powerful jaw closure during biting and chewing.

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Neuroanatomy

After stimulation of the mental or infraorbital nerve, impulses reach the pons through the sensory mandibular or maxillary root of the trigeminal nerve, respectively15 (Fig. 3B). The SP1 response is probably mediated by inhibitory interneuron located close to the ipsilateral trigeminal motor nucleus. The inhibitory interneuron projects onto jaw-closing motor neurons bilaterally. The whole circuit lies in the midpons.31 The afferents for SP2 descend in the spinal trigeminal tract and connect through a polysynaptic chain of excitatory interneurons, probably located in the lateral reticular formation, at the level of the pontomedullary junction.

FIG. 3

FIG. 3

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Technique of Testing

The recording electrode is placed on the masseter muscle bilaterally (same location as for the jaw jerk). Subjects are instructed to clench the teeth as hard as possible for approximately 2 to 3 seconds. The reflex can be measured properly only if the patient is able to clench the teeth and produce a full EMG interference pattern. Single electric shocks are delivered to the mental or infraorbital nerves through surface electrodes placed over the appropriate foramina. A stimulus intensity of approximately 2 to 3 times the reflex threshold (usually 20–50 mA) yields the best results. It is always necessary to perform several trials. Some authors measure the latency at the last EMG peak crossing of the isoelectric line, whereas others record the beginning of the electrical silence. Each of these methods is clinically satisfactory, provided the normal value determination is maintained and intraindividual measurement differences (between right and left sided stimuli) are avoided.1

With an electrical stimulus, two electrical silent periods occur interrupting the voluntary EMG activity in the ipsilateral and contralateral masseter muscles15,30–32 (Fig. 3A). The first (SP1) corresponds to the SP after a mechanical tap and the second one (SP2) begins 30 to 60 ms after the stimulus (Smith BE, 2009, Daube, Clinical Neurophysiology, p.538–539). Prolongation of either absolute latencies, ipsilateral-contralateral latency difference, or side-to-side latency difference when recorded from one muscle, is considered abnormal (Table 4). Perhaps because electrical stimuli yield mixed nociceptive and nonnociceptive inputs, whether the SP1, SP2, or both components are nociceptive reflexes remains controversial.

TABLE 4

TABLE 4

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Applications

The silent period may be absent in patients with trigeminal sensory neuropathies that result in significant impairment of reflex mechanism involved in chewing.33 It is also sometimes absent in tetanus and may be unilaterally absent in patients with hemimasticatory spasm.34 The latency of SP1 is often delayed in patients with demyelinating neuropathies.35 MIR is especially useful to confirm conduction delay secondary to demyelination in severe neuropathies with absent sensory and muscle action potentials as well as blink reflex, as it can still be measured. MIR remains normal in axonopathies.

An afferent abnormality (absent or delayed direct and crossed responses to unilateral stimulation) indicates a lesion along the afferent path (intra- or extra-axial before the site of crossing of the impulse volley).31 SP1 is more susceptible than SP2 to extra-axial lesions, such as trigeminal neuropathy.2

Brainstem lesions may show an afferent delay or block to unilateral stimulation and abnormal crossed responses to contralateral stimulation, or abnormal crossed responses to stimulation of either side. The lesion involves the dorsal pontine tegmentum at the level of the midpons if both SP1 and SP2 are affected, and the lower pons or the pontomedullary junction if SP2 alone is affected.31 A unilateral efferent abnormality (absent or delayed responses confined to the muscle on one side, irrespective of the side of stimulation) is extremely rare except in cases of unilateral masticatory spasm.36

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Trigeminal Somatosensory-Evoked Potentials

Different approaches have been used to study the trigeminal system by the evoked potential techniques. As these techniques are not frequently used, clinically their practical application and utility remain poorly defined.

The main problems associated with somatosensory-evoked potentials (SEPs) from the trigeminal territory are stimulus artifact (with electrical stimulation), unwanted stimulus spread to facial muscles innervated by the facial nerve, and direct or reflex muscle activities (with mechanical stimulation), which contaminate scalp recordings.37–39 In addition to the limitations related to puff stimulation that appeared to activate receptors with differing thresholds.40 The suggested solution was the use of invasive techniques37,39,41 such as the use of electrical pulses to the tooth pulp or gums and needle electrode stimulation at the mental foramen. Despite the last method reliability, its invasiveness limited its use.34

Another consideration was the use of short versus long latency potentials.42 Very short latency-evoked potentials were recorded by Leandri et al. who described an invasive technique to stimulate the mental nerve without simultaneous activation of the surrounding muscles, with a technique similar to that used to stimulate the infraorbital and supraorbital nerves.

Inferior alveolar nerve (IAN) SEPs have been used to evaluate the sensory function of the mandibular branch through the mental nerve, and a separate technique was reported with stimulation of the mental nerve at the mandibular foramen, with recording early evoked potentials over the scalp.38 With IAN SEPs, it became possible to evaluate nerve function and its central connections noninvasively. The use of near nerve electrode over the mentalis nerve causes localized stimulation without triggering movement of the jaw and muscle artifact. Hand stimulation of the IAN endings with surface electrodes has been successfully demonstrated42–45 and normative data have been published.38,40,46–48 Arcuri et al.38 2006 described a reliable technique of a regular pattern of peak events that were stable and not contaminated by muscle artifact.

In a study by Rossini at el. 2016, IAN SEPs were recorded in a noninvasive technique and represented a noninvasive objective method to evaluate sensory nerve function in the maxillofacial region with very similar results. Latencies obtained by this method were more stable then amplitudes, and mean latency values were similar to those reported by Arcuri et al. 2006.

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Laser-Evoked Potentials and Contact Heat-Evoked Potentials

Owing to the growing interest in the evaluation of neuropathic pain and somatosensory pain pathways, assessment of nociceptive cerebral–evoked potentials using laser-evoked potentials (LEPs) has been extensively studied and proven to be a very reliable method. It is considered the gold standard objective test that is highly specific for small fiber/spinothalamic tract function evaluation. Because of the short conduction distance and high receptor density, the trigeminal LEPs are of higher amplitude and recorded easier than with limb stimulation.2,49,50

Due to the radiant heat delivered through laser beams and the risk of eye damage and skin burns, another method of stimulating the skin with contact heat conveyed through thermofoil thermodes with brain electrical source analysis was developed and became of practical application in the recent years. Both methods LEPs and contact heat-evoked potentials (CHEPs) stimulate the same type of nociceptors in cutaneous A-delta and C fibers. The main difference in stimulation for LEPs versus CHEPs during the thermode skin application of stimulus is a slope of the temperature rise which is 70 C/s (centigrade/second) for CHEPs that is considerably less than that for LEPs, which is about 1,000 C/s.51–53

A large multicenter international study by Granowsky et al.54 provided previously lacking data set of normative values that facilitated the clinical use of CHEPs. The data are valid only for the equipment, set up, and stimulation parameters used in the study. Normative values at different body parts (face, upper and lower limbs, cervical and lumbar spine) were analyzed for both males and females, compared between both sides of the body, and correlated their amplitudes and latencies with age and sex. Contact heat-evoked potentials latencies and amplitudes were similar on both sides of the body, hence were both reliable and sufficient for normative data with unilateral assessment. Prolonged latencies and reduced amplitudes were associated with aging and correlated with age-related changes in thermal pain perception. Females responded with higher amplitudes and shorter latencies compared with males, but both genders reported similar pain scores.

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THE FACIAL NERVE

Blink Reflex

As blink reflex is helpful in the localization of focal demyelinating peripheral facial nerve palsies. For example, if on direct stimulation of the facial nerve, the latency of the response is normal, whereas with supraorbital nerve stimulation the latency is prolonged, this finding indicates that the lesion is proximally located somewhere between the facial nerve nucleus and the stylomastoid foramen, whereas if direct stimulation is abnormal, it would be more distal. An abnormality in the blink reflex response may therefore be seen in patients with Bell palsy, traumatic lesions of the facial nerve, and acoustic neuromas. Abnormality of the blink reflex in these scenarios can be useful in prognosis.55,56 When the blink reflex is absent on the involved side, the prognosis is poor in a majority of the cases. When the reflex is normal or only R1 is delayed, the prognosis is excellent.55 Delayed responses or the reappearance of the previously absent responses suggest a conduction defect without substantial axonal loss, from which the patient will likely recover completely or nearly completely.1

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Lateral Spread and Facial Synkinesis

With simultaneous recordings from the OO (orbicularis oris) or mentalis muscles, facial synkinesis can be measured objectively (Fig. 4A). In normal individuals, after supraorbital nerve stimulation at the supraorbital notch, only the OO muscle contracts, and an abnormal synkinetic response is not obtained from other facial muscles. A manifestation of hemifacial spasm or of aberrant regeneration after a facial nerve lesion, synkinetic R1 and R2 responses may be obtained from other facial muscles (orbicularis oris, mentalis) (Fig. 4B). However, in hemifacial spasm, synkinesis (when present) may be related to ephaptic transmission at the site of injury because of focal demyelination and is believed to be a peripheral process, not occurring at the level of the nucleus.57,58 Vascular compression of the nerve has been demonstrated in 65% of patients with hemifacial spasm.34 The phenomenon has also been described in intraaxial brainstem lesions, tumors of the posterior fossa, basilar meningitis, and arteriovenous malformations. With hemifacial spasm, routine blink reflexes are usually normal, although there may be an increase in R1 amplitude and a late activity, suggesting hyperexcitability of the reflex. In addition, there may be lateral spread (due to ephaptic transmission) that can be demonstrated with stimulation of either mandibular or zygomatic branches of the facial nerve while recording from the OOs or mentalis muscle, respectively. Microvascular decompression of the facial nerve generally results in loss lateral spread and relief of hemifacial spasm.34,57,58

FIG. 4

FIG. 4

However, synkinesis involving R1 and R2 blink reflex responses does not occur in other facial movement disorders such as essential blepharospasm, facial myokymia, habit spasm, oro-facial dystonia, and focal cortical seizures. Therefore, in atypical facial movement disorders, the presence of facial synkinesis would substantiate the diagnosis of hemifacial spasm.

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SUMMARY

The described electophysiological studies of the trigeminal and facial nerves and their application in the right clinical contexts, provide useful diagnostic tools to localize the lesion to the peripheral or central nervous systems and sometimes assist in prognostication. Moreover, the addition of these tests to the routine diagnostic armamentarium (nerve conduction studies, EMG, SEP, MRI, etc.) in combination with the neurological examination and disease phenotype, increases the yield of identifying and/or classifying the peripheral nerve process, and ultimately guiding further disease-targeted evaluation and therapy.

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Keywords:

Trigeminal nerve; Facial nerve; Corneal reflex; Blink reflex; Jaw-jerk reflex; Masseter reflex; Masseter inhibitory reflex; Trigeminal somatosensory-evoked potentials; Laser-evoked potentials; Contact heat-evoked potentials

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