Although nerve conduction and electromyography (EMG) studies are used most often to evaluate peripheral nerve and muscle disorders, they also can be used to evaluate lesions of the cranial nerves. Outside of the brainstem, the cranial nerves, other than cranial nerves I (olfactory) and II (optic), are essentially the same as peripheral nerves, carrying motor, sensory, and autonomic fibers.
Mononeuropathies affecting cranial nerves VII (facial) and V (trigeminal) are the most common cranial nerve lesions evaluated in the EMG laboratory. The facial nerve can be directly stimulated and recorded using standard nerve conduction techniques. The blink reflex can be used to evaluate both the facial and trigeminal nerves. Facial and masticatory muscles, supplied by cranial nerves VII and V, respectively, can easily be examined with an EMG needle. As in other neuromuscular disorders, the electrophysiologic evaluation of facial and trigeminal neuropathies is used to confirm localization of the lesion, assess the underlying pathophysiology and severity of the lesion, and offer a prognosis. In fact, assessment of severity and prognosis are often the key issues addressed by the electromyographer in the most common cranial neuropathy of all, idiopathic facial palsy (i.e., Bell’s palsy).
Anatomy
Facial Nerve
The facial nerve, cranial nerve VII, is a complex nerve that carries several different fiber bundles, including the following:
- •
Motor fibers to all muscles of facial expression, as well as to the posterior belly of the digastric, stapedius, and stylohyoid muscles
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Parasympathetic motor fibers supplying the mucosa of the soft palate and the salivary and lacrimal glands
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Taste fibers to the anterior two thirds of the tongue
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Parasympathetic sensory fibers for visceral sensation from the salivary glands and the nasal and pharyngeal mucosa
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Somatic sensory fibers supplying a small part of the external auditory meatus and skin of the ear
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Proprioceptive sensory afferents from facial muscles
The facial nerve is formed by the conjoining of the facial motor root and the adjacent nervus intermedius . The facial motor root supplies the muscles of facial expression and arises from the facial motor nucleus located in the ventral lateral tegmentum of the lower pons. The nervus intermedius carries taste, sensory, and parasympathetic fibers and arises from the solitary nucleus/tract (medulla), trigeminal sensory nuclei (medulla–pons), and superior salivatory nucleus (pons), respectively.
The facial nerve, including the motor root and nervus intermedius, emerges from the brainstem at the cerebellopontine angle and enters the internal auditory meatus, next passing through the geniculate ganglion before traversing the facial canal. Within the bony facial canal, several branches arise from and leave the main facial nerve ( Figure 25–1 ). First, parasympathetic fibers are given off to the greater and lesser petrosal nerves , bound for the pterygopalatine and otic ganglia. A small motor branch arises next, to innervate the stapedius muscle in the inner ear. The chorda tympani then arises to carry taste fibers to the anterior two thirds of the tongue and parasympathetic fibers to the submandibular and sublingual salivary glands.
The facial nerve exits the skull at the stylomastoid foramen before coursing through the parotid gland. After the stylomastoid foramen, the nerve supplies the stylohyoid and the posterior belly of the digastric muscles, then gives off a cutaneous posterior auricular branch before dividing into its five major peripheral branches: temporal (a.k.a., frontal), zygomatic, buccal, mandibular, and cervical branches, which innervate the muscles of facial expression ( Figure 25–2 ).
Trigeminal Nerve
The trigeminal nerve, cranial nerve V, carries sensory fibers to the face and motor fibers to the muscles of mastication. It arises from several different nuclei in the brainstem, including one motor nucleus (mid-upper pons) and three separate sensory nuclei. The sensory nuclei include the main sensory nucleus (mid-upper pons), which mediates light touch; the nucleus of the spinal tract of V (pons to upper cervical cord), which mediates pain and temperature; and the mesencephalic nucleus of V (lower midbrain), which mediates proprioception from facial muscles. Exiting from the lateral mid-pons, the nerve is called trigeminal because it branches into three major peripheral nerves that arise from the trigeminal ganglion (also known as the semilunar or gasserian ganglion ), located just outside the brainstem on the petrous bone in the middle cranial fossa ( Figure 25–3 ). The cavity formed by the folds of dura that contain the trigeminal ganglion, surrounded by cerebrospinal fluid, is known as Meckel’s Cave. Whereas the trigeminal ganglion contains cell bodies of the sensory fibers from both the main sensory nucleus and the nucleus of the spinal tract of V, the cell bodies of proprioceptive sensory fibers from muscle spindles of trigeminal motor fibers are contained within the mesencephalic nucleus of V in the midbrain.
The three major peripheral nerve divisions of the trigeminal nerve are the ophthalmic (V 1 ), maxillary (V 2 ), and mandibular (V 3 ) nerves. Each nerve exits the skull through a distinct opening: (1) the ophthalmic nerve through the superior orbital fissure, (2) the maxillary nerve through the foramen rotundum, and (3) the mandibular nerve through the foramen ovale. Each of the three major nerve branches contains sensory fibers, whereas motor fibers are carried solely in the mandibular nerve branches that supply innervation to the muscles of mastication (masseter, temporalis, medial, and lateral pterygoid muscles) and to the anterior belly of the digastric muscle, the mylohyoid, tensor veli palatini, and tensor tympani muscles. Branches of the trigeminal nerve supply light touch, pain, and temperature sensation to the skin of the face, the anterior half of the scalp, most of the oral and nasal mucosa, the anterior two thirds of the tongue, and the dura mater of the anterior and middle cranial fossae ( Figure 25–4 ).
Clinical
Facial Neuropathy
The most common cranial mononeuropathy is facial nerve palsy, which usually presents as idiopathic Bell’s palsy . Some cases are post-infectious, although a growing amount of evidence suggests that Bell’s palsy is a viral-induced cranial neuritis caused by herpes simplex virus-1 in many cases. In addition, the risk of Bell’s palsy is increased in patients with hypertension or diabetes and in pregnant women (the latter especially late in the pregnancy or in the early postpartum period).
Unilateral facial nerve dysfunction can also be seen in association with several disorders, most commonly in the setting of diabetes. In addition, facial palsy occurs with herpes zoster involving the geniculate ganglion (Ramsay Hunt syndrome), lymphoma, leprosy, cerebellopontine angle tumors such as acoustic neuroma, multiple sclerosis, stroke, and a host of other disorders ( Box 25–1 ). Bilateral facial weakness is less common; it may be seen in Guillain–Barré syndrome, Lyme disease, sarcoid, Melkersson–Rosenthal syndrome, tuberculous meningitis, and leptomeningeal lymphomatosis/carcinomatosis. Bifacial weakness also is noted in some neuromuscular junction disorders and in various muscular dystrophies.
Idiopathic Bell’s palsy
Associated with systemic disorders
Guillain–Barré syndrome *
* Often bilateral involvement.
Lyme disease *
Diabetes
Herpes zoster (Ramsay Hunt syndrome)
Vasculitis
Infiltrative lesions
Lymphoma
Leptomeningeal lymphomatosis/carcinomatosis *
Tuberculous meningitis *
Leprosy
Sarcoid *
Melkersson–Rosenthal syndrome *
Multiple sclerosis
Associated with tumors
Cerebellopontine angle tumor
Nasopharyngeal carcinoma
Associated with neuromuscular junction disorders
Muscular dystrophies
Stroke
The clinical presentation of facial nerve palsy depends on the location, pathophysiology, and severity of the lesion. A central lesion (proximal to the facial nerve nuclei) causes contralateral weakness primarily of the lower facial musculature, with relative sparing of the orbicularis oculi and frontalis muscles, which are bilaterally innervated. Furthermore, with central lesions there may be facial movement during laughing or crying because the pathways that mediate responses to emotional stimuli are different from those that mediate voluntary facial movement. Peripheral lesions (at or distal to the facial nerve nuclei) cause ipsilateral facial paralysis that affects both the upper and lower facial musculature, resulting in an inability to wrinkle the forehead, close the eye, or smile. In addition, there may be dysfunction and absent taste sensation over the anterior two thirds of the tongue, depending on which branches are involved as the nerve courses through the facial canal.
In patients with idiopathic Bell’s palsy, complete facial paralysis involving the upper and lower face generally occurs within 24 hours and inevitably is accompanied by pain behind the ipsilateral ear. The etiology is thought to be inflammation of the facial nerve, which causes swelling and compression of the nerve in the facial canal.
In most patients, the prognosis is excellent, with full recovery of function over several weeks to months. However, in more severe cases, usually those associated with significant axonal loss, some permanent facial weakness remains, or aberrant reinnervation may occur as the nerve regenerates. Aberrant reinnervation can take one of two forms: (1) an axon that previously innervated a particular muscle grows down a different fascicle and innervates a different muscle than the original one, or (2) a single axon branches into two or more axons that go to two or more different muscles. Either type of aberrant reinnervation can result in synkinesis of facial movements. For example, closing the eye (orbicularis oculi) may be accompanied by movement of the lips (orbicularis oris). Clinically, these reinnervation abnormalities may vary from being very subtle to very severe. In the most extreme case, synkinesis may lead to massive contractions on one side of the face. As most people blink spontaneously every few seconds, synkinesis involving the orbicularis oculi and other facial muscles can clinically appear very similar to hemifacial spasm (see below), although the etiology is quite different.
Aberrant reinnervation may also occur between the motor axons of the facial nerve and the parasympathetic axons (i.e., nerve fibers derived from the facial motor root and nerve fibers derived from the nervus intermedius). Thus, parasympathetic axons may innervate motor endplates, and, conversely, motor axons may innervate the parasympathetic endplates. This may result in lacrimation, salivation, and/or hemifacial sweating when the facial muscles are activated. One can imagine the embarrassing situation wherein tears rather than saliva are produced while eating.
Hemifacial Spasm
Hemifacial spasm is a chronic and often progressive disorder usually associated with chronic compression of or injury to the facial nerve. The disorder is characterized by involuntary contractions that affect one or multiple muscles on one side of the face. The spasms typically occur initially around the eye, and later spread to involve other ipsilateral facial muscles. The contractions are often irregular and persist during sleep. Although several unusual causes of chronic irritation have been reported in the literature, the most common etiology is an aberrant blood vessel lying in contact with the facial nerve near its exit zone from the brainstem. The spasms are thought to be generated by damage to some axons of the facial nerve with ephaptic transmission to other nearby axons. Surgical decompression of the blood vessel away from the facial nerve often results in complete recovery.
As noted above, massive reinnervation and subsequent synkinesis of the facial muscles may occur following an idiopathic facial palsy, leading to a pattern nearly identical clinically to hemifacial spasm. However, the underlying pathophysiology of hemifacial spasm (damage to the facial nerve with ephaptic transmission) differs from that of post-paralytic facial syndrome (massive synkinesis that occurs with spontaneous blinking, due to aberrant reinnervation of muscles following idiopathic facial palsy).
Trigeminal Neuropathy
Trigeminal neuropathy is less common than facial palsy. It generally occurs as a purely sensory neuropathy in association with connective tissue disorders, most notably Sjögren syndrome or systemic lupus erythematosus. In addition, trigeminal neuropathy can be seen in association with toxic neuropathies, sometimes in isolation. Rarely, patients with local or metastatic cancer present with isolated involvement of the mentalis branch of V 3 (so-called “numb chin syndrome”). Isolated motor involvement of the trigeminal nerve is seen occasionally, usually in association with mass lesions or after surgery.
Patients with purely sensory dysfunction of cranial nerve V present with numbness over the ipsilateral face. The distribution of numbness depends on the extent of nerve involvement and on which branches of the trigeminal nerve are involved. Involvement of the motor branch causes difficulty chewing and deviation of the jaw to the contralateral side when opening the mouth.
Trigeminal Neuralgia
Trigeminal neuralgia, also known as tic douloureux , is a condition characterized by episodes of severe pain in the distribution of one or more branches of the trigeminal nerve. It occurs most frequently in the maxillary division. Inconsequential stimuli, such as light touch over the cheek, eating, or brushing the teeth can trigger excruciating pain. There is no associated sensory or motor dysfunction in the fifth nerve distribution and standard nerve conduction and EMG evaluations will be normal. Blink reflex studies usually are normal, although rarely the R1 component may be abnormal on the affected side (found in <5% of patients).
Electrophysiologic Evaluation
Facial Neuropathy
The facial nerve can be evaluated using a combination of direct facial nerve stimulation, the blink reflex, and needle EMG examination ( Box 25–2 ). The electrophysiologic evaluation is directed toward answering four major questions:
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Is the lesion central or peripheral?
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If the lesion is peripheral, what is the extent of involvement? That is, are all branches of the nerve involved, or is the lesion selective?
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What is the underlying pathophysiology: demyelinating, axonal loss, or elements of both?
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What is the prognosis for recovery?
Facial nerve:
- 1
Facial nerve studies:
- a
Stimulation of the whole facial nerve, stimulating below and anterior to the mastoid or directly anterior to the tragus, recording a facial muscle (typically the nasalis or orbicularis oculi); bilateral studies; OR
- b
Stimulation of facial branches:
- i
Frontal branch. Stimulating three to four fingerbreadths lateral to the eye, recording the frontalis muscle; bilateral studies.
- ii
Zygomatic branch. Stimulating over the zygomatic bone just anterior to the ear, recording the nasalis muscle; bilateral studies.
- iii
Mandibular branch. Stimulating over the angle of the jaw, recording the mentalis muscle; bilateral studies.
- i
- a
- 2
Blink reflex studies, stimulating the supraorbital nerve, recording orbicularis oculi muscles; bilateral studies
- 3
Needle electromyographic examination, examining muscles from the major branches, including frontalis (temporal branch), orbicularis oculi (zygomatic branch), orbicularis oris (buccal branch), and mentalis (mandibular branch)
Trigeminal nerve:
- 1
Blink reflex studies, stimulating the supraorbital nerve, recording orbicularis oculi muscles; bilateral studies
- 2
Needle electromyographic examination, examining the masseter and temporalis muscles
Nerve Conduction Studies
Using a handheld stimulator, the facial nerve can be stimulated either below the ear anterior to the mastoid or directly anterior to the tragus ( Figure 25–5 ). The patient should be in a relaxed state, lying supine on the examining table. The stimulating cathode should be placed anterior and superior to the anode, although it may be necessary to rotate the anode to reduce the stimulus artifact or to avoid direct stimulation of the masseter. The active recording electrode consists of a a standard disk electrode placed over the nasalis muscle, with the reference electrode placed over the contralateral nasalis. The inferior orbicularis oculi also is commonly used for recording, with the active electrode placed on the skin just lateral and inferior to the pupil at mid-position, with the reference electrode placed on the skin over the lateral canthus of the eye. The nasalis muscle also is commonly used for recording, with the reference electrode placed on the contralateral nasalis muscle. Any of the following muscles can be used for recording: the frontalis, nasalis, orbicularis oculi or oris, mentalis, or platysma, although a needle recording electrode may be necessary for some muscles. The reference electrode can be placed over the same muscle on the contralateral side of the face. A surface ground electrode is placed over the forehead or chin.
Selective branches of the facial nerve can be stimulated more distally, including the temporal branch recording the frontalis muscle, the zygomatic branch recording the nasalis, the buccal branch recording the orbicularis oris, the mandibular branch recording the mentalis, or the cervical branch recording the platysma. Avoiding direct stimulation of the masseter muscle is important and can be accomplished by watching for contraction of the masseter during nerve stimulation.
As in any other motor conduction study, the amplitude of the distal compound muscle action potential (CMAP) is proportional to the number of intact motor axons, whereas the distal latency reflects conduction time along the fastest-conducting fibers of the distal segment of the facial nerve. The degree of axonal loss has direct implications for the prognosis and the time required for recovery. In general, amplitudes 50–75% lower than the contralateral side are associated with a poorer prognosis, a prolonged recovery time, and aberrant reinnervation. Note that it is important to check the facial CMAP at least 6 days after facial weakness develops so that enough time will have passed for wallerian degeneration to have occurred for motor fibers. If a patient with a facial palsy is evaluated before the symptoms are 3 days old, wallerian degeneration will not have begun. If evaluated between 3 and 5 days, wallerian degeneration may not be complete, and thus the degree of axonal loss and the prognosis will not be assessed accurately.
Blink Reflex Studies
Direct facial nerve stimulation evaluates only the distal segments of the nerve. The blink reflex study measures the entire reflex arc between the trigeminal and facial nerves, including proximal segments of the facial nerve (see Chapter 5 ). Therefore, to evaluate the proximal facial nerve segments, the blink reflex study is used in combination with direct facial nerve stimulation. Lesions of the facial nerve result in abnormalities of the ipsilateral R1 and R2 components of the blink reflex, whereas the contralateral R2 response remains normal. When the contralateral normal side is stimulated, the opposite pattern is seen: normal ipsilateral R1 and R2 responses and an abnormal contralateral R2 response.
The blink reflex can also be used in chronic facial palsies to look for electrophysiologic evidence of aberrant reinnervation. One can perform a blink reflex study stimulating the supraorbital nerve, co-recording the orbicularis oculi and mentalis muscles. If ipsilateral R1 and R2 responses are present in both muscles, aberrant reinnervation likely is present because the mentalis muscle does not usually participate in the blink reflex.
Electromyographic Approach
A small, fine, concentric needle should always be used to study the facial muscles, with muscles from the major branches being sampled. The easiest muscles to sample include the frontalis (temporal branch), orbicularis oculi (zygomatic branch), orbicularis oris (buccal branch), and mentalis (mandibular branch). Muscles innervated by cranial nerve V (masseter, temporalis) should also be sampled to look for evidence of more widespread cranial nerve dysfunction. The motor unit action potentials (MUAPs) in facial muscles tend to be smaller and shorter than those in limb muscles. In addition, the onset firing frequency is higher than in most limb muscles (8–10 Hz as opposed to 4–5 Hz). Accordingly, one should become well practiced in the needle examination of facial muscles so as not to mistake the normally small MUAPs for myopathic motor unit potentials.
On needle EMG, myokymic discharges may be seen in the facial muscles of patients with multiple sclerosis, brainstem tumors (especially pontine gliomas), or Guillain–Barré syndrome, or in patients who have received prior irradiation to the face and neck area.
Needle EMG can also be used to look for evidence of synkinesis that results from aberrant reinnervation. Small concentric needle electrodes can be placed simultaneously in muscles innervated by different facial nerve branches, with the electromyographer looking for co-contraction. For example, if MUAPs fire in the mentalis when the patient is asked to close the eyes, synkinesis is likely present. One must always take care not to confuse simultaneous co-contraction of muscles under voluntary control with involuntary co-contraction of muscles, which indicates synkinesis.
Hemifacial Spasm
Nerve Conduction Studies and Blink Reflex
Direct facial nerve conduction studies are usually normal in hemifacial spasm. However, the blink reflex and other specialized nerve conduction studies looking for lateral spread (ephaptic transmission) may be useful in demonstrating abnormalities. Selective facial nerve conduction studies can be done by stimulating an individual facial nerve branch and co-recording muscles innervated by different branches. For example, the zygomatic branch can be stimulated in a patient with hemifacial spasm, with the orbicularis oculi (zygomatic branch) and mentalis (mandibular branch) simultaneously recorded. One looks for a delayed lateral spread response (presumably ephaptic) in the mentalis that occurs just after the response at the orbicularis oculi ( Figure 25–6 ). As in other nerve conduction studies, when a facial nerve branch is stimulated, the depolarization travels both orthodromically and antidromically. In hemifacial spasm, the antidromic volley presumably travels to the area of nerve injury and spreads ephaptically to adjacent fiber branches, resulting in a delayed response in muscles innervated by adjacent facial nerve branches. After successful decompression of the facial nerve, this lateral spread response disappears.
