Chapter 40 Microvascular Compression of the Vestibulocochlear Nerve

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1 Chapter 40 Dirk De Ridder and Aage R. Møller Keypoints 1. Microvascular contacts or compressions of the vestibulocochlear nerve can result in tinnitus. 2. For nonpulsatile tinnitus, the contact is most often at the central nervous system segment. 3. For pulsatile tinnitus and typewriter tinnitus, the contact is at the peripheral nervous system segment. The tinnitus is unilateral and characterized by intermittent paroxysms of tinnitus. (a) A typical development consists of progressively more frequent bouts of tinnitus, which last longer and longer. (b) If bilateral vascular compressions exist, the tinnitus alternates between the left and right side, and does not occur on each side simultaneously. 4. Associated symptoms are correlated with related contacts/compressions of nearby nerves and include overt or cryptogenic hemifacial spasms, geniculate neuralgia, optokinetically induced short bouts of disabling positional vertigo, and tinnitus frequencyspecific hearing loss. 5. Auditory brainstem responses (ABRs) correlate with disease progress and clinical symptoms and can be used diagnostically. (a) Tinnitus is causally related to a decrease in amplitude of peak II in the ipsilaterally elicited ABR. D. De Ridder (*) BRAI2N and Department of Neurosurgery, TRI Tinnitus Clinic Antwerp, University of Antwerp, Wilrijkstraat 10, 2650 Edegem, Belgium dirk.de.ridder@uza.be (b) Tinnitus frequency-specific hearing loss is causally related to prolongation of the ipsilateral interpeak latency (IPL) I III. (c) Prolongation of contralateral IPL III V occurs and is a sign of slowed signal transmission in the brainstem. 6. Magnetic resonance imaging sequences with constructive interference in steady state can visualize most vascular contacts/compressions of the auditory nerve. 7. Microvascular decompression should be performed before irreversible nerve damage is induced; clinically, the procedure should be performed before 4 5 years. Keywords Pulsatile Tinnitus Vascular conflict Microvascular compression MVC MVD Abbreviations AAO AAOO ABR CISS CVCS ENT HFS IPL MRI MVC MVD PNS TGN American Academy of Otolaryngology American Academy of Ophthalmology and Otolaryngology Auditory brainstem response Constructive interference in steady state Cochleovestibular compression syndrome Ear nose and throat Hemifacial spasm Interpeak latency Magnetic resonance imaging Microvascular compression Microvascular decompression Peripheral nervous system Trigeminal neuralgia A.R. Møller et al. (eds.), Textbook of Tinnitus, DOI / _40, Springer Science+Business Media, LLC

2 328 Introduction Definition of Microvascular Compression A blood vessel compressing a cranial nerve induces a nerve stimulation leading to a hyperactive cranial nerve syndrome [1, 2] with or without a loss of function. It is diagnosed almost solely based on the history taken, and a magnetic resonance imaging (MRI) is used for exclusion of other pathology and as a possible confirmation. Primary and Secondary Microvascular Compressions Microvascular compression (MVC) can occur as such or can be induced based on a general lack of space in the posterior fossa, such as seen in the Arnold Chiari malformation [3 5] or associated with space-occupying lesions. This can result in a direct compression [6, 7] or indirect compression [8], but can also occur contralaterally, possibly due to a decrease in intracranial space [9 13]. In Sindou s series [4] of 39 patients with Arnold Chiari malformation,1 nine suffered from trigeminal neuralgia. After decompressing the foramen magnum, five of these nine individuals got rid of their pain, whereas the remaining four persons required a second microvascular decompression (MVD) operation. In this series of trigeminal neuralgias treated by MVD, the nerve was compressed between the pons and petrous bone in 3.9% of persons studied, due to the small size of the posterior fossa [14]. Removal of a tentorial meningioma can improve sudden hearing loss related to an MVC of the vestibulocochlear nerve based on the same premises [8]. Signs and Symptoms of Microvascular Compression Examples of MVC syndromes are trigeminal neuralgia, glossopharyngeal neuralgia, hemifacial spasm [HFS], disabling positional vertigo, tinnitus, and otalgia.2 Arnold-Chiari malformation: displacement of the medulla and cerebellar tonsils and vermis through the foramen magnum into the upper spinal canal; often associated with other cerebral anomalies. 2 Otalgia: Earache. 1 D. De Ridder and A.R. Møller Other clinical syndromes such as spasmodic torticollis [15], cyclic oculomotor spasm with paresis [16], superior oblique myokymia [17, 18], and abducens spasm [19] may also be initiated by vascular compressions of the respective cranial nerves (nerves intermedius, spinal accessory nerve, and oculomotor and trochlear nerves). The incidence of the different MVC syndromes seems to be related to the length of the central nervous system (CNS) segment [20]. MVC of cranial nerves usually occurs unilaterally and, thus, induces unilateral symptoms [21 25] characterized by paroxysmal and intermittent spells of hyperactivity. The paroxysms typically become more frequent over time, the intermittent symptom-free periods become shorter and terminate in a constant dysfunction [26 28]. The symptoms of MVC can often be evoked by specific triggers [21, 26 28]. MVC syndromes are most common in late middle age (mean age 50 years) [21 25]. MVC of the vestibulocochlear nerve can cause any of the following paroxysmal symptoms depending on the place of compression: vertigo disabling positional vertigo [29], tinnitus [30, 31], hearing loss [32], or ear pressure (Dirk De Ridder unpublished observation). MVC rarely presents bilaterally (1 12%) [23, 25, 33, 34]; if it does, the pain or spasm alternates sides and never occurs on both at the same time. There usually is a delay between the onset of symptoms from one side and the development of symptoms of the other side [24, 35, 36], with only 2 3% of the bilateral cases starting simultaneously. Bilateral MVC has a higher incidence in familial cases [33, 35]. MVC syndromes that affect more than one cranial nerve occur rarely (incidence 2.8%) [24]. The combination may occur unilaterally (1.5%) or bilaterally (1.3%). The mean age is higher than for unilateral symptoms, 63.2 vs years, which is similar to bilateral MVCs (61.4 years) [24]. If one blood vessel contacts two or more cranial nerves, symptoms do not develop at the same moment in time [37]. The bestknown double compression syndrome is called the tic convulsif, consisting of a combination of HFS and trigeminal neuralgia [38], which can occur even bilaterally [37, 39]. These data suggest that if bilateral tinnitus is due to MVC it is expected that the left- and the right-sided component should start at different moments in time and with a different pitch. Theoretically, true bilateral tinnitus (i.e., with same pitch) could occur if the compression is at the level of the cochlear nucleus.

3 40 Cochleovestibular Compression Syndrome A recent meta-analysis has confirmed that blood vessels in contact with the vestibulocochlear nerve can result in otological symptoms, including hearing loss and tinnitus [40, 41]. Whereas initially it was proposed that only vascular compression of the root entry zone of a cranial nerve could cause symptoms [42], it was later suggested that any vascular contact along the CNS segment (between the internal acoustic meatus and the brainstem) could result in tinnitus [20]. Vascular loops inside the internal acoustic meatus along the morphologically more resistant peripheral nervous system (PNS) segment, however, were described to produce either typewriter tinnitus [43] or pulsatile tinnitus [41, 44]. Typewriter tinnitus consists of paroxysms of tinnitus perceived as Morse code, machine gun-like staccato, or typewriter sound and has been shown to be responsive to treatment with carbamazepine [43], thus analogous to trigeminal neuralgia. 329 disease, the spells are shorter lasting and have no aura and no postictal period. In a chronic stage, persistent instability is noted [25, 46]. It is of interest, however, that in Ryu s study, 73% of the patients with a MVC were diagnosed as having Ménière s disease [28]. The main electrophysiological difference between Ménière s disease and cochleovestibular compression syndrome (CVCS) is that in Ménière s disease there are no abnormalities in peak II and interpeak latency (IPL) I III of the auditory brainstem response (ABR) [47]. Two more nerves are in close relationship with the cochleovestibular nerve: the intermediate and the facial nerve. Vascular contact with the nervous intermedius is associated with geniculate neuralgia [22]. At the acute stage, intermittent paroxysmal bouts of otalgia occur; at a later stage, a deep, dull hemifacial pain develops [22]. Vascular contact with the root exit zone of the facial nerve can result in HFS [48, 49] and concomitant contact with the cochleovestibular nerve. The same vessel can cause auditory signs including low frequency tinnitus and hearing loss [31, 50]. Diagnostic Criteria of Cochleovestibular Compression Syndrome Characteristic Features of Tinnitus as a MVC Syndrome Based on the analogy with other vascular compression syndromes, tinnitus caused by MVC would be expected to be unilateral and have short-lasting paroxysms with the tinnitus-free intervals becoming progressively shorter ending in constant tinnitus. This kind of tinnitus would be expected to occur in middle-aged individuals and would not be anticipated to be associated with a flat hearing loss, as the typical MVC disorders (HFS and TGN) are not associated with complete weakness or complete loss of sensation in the entire distribution of the cranial nerve. Persistent compression can result in changes in the characteristics of pain and sensory impairment [26]. In a similar fashion, long-standing HFS could result in facial palsy or Bells s palsy [45]. Similarly, chronic vestibular nerve compression can lead to hypofunctioning of the labyrinth, clinically expressed as gait instability [25]. Both parts of the vestibulocochlear nerve might be compressed at the same time, and symptoms from the vestibular nerve would be expected in individuals with tinnitus from MVC. A similar evolution is noted, with progressively more vertiginous spells and shorter symptom-free periods [25, 28]. In contrast to Ménière s Selection Criteria 1. Intermittent paroxysmal spells of tinnitus lasting only seconds (a) Hearing loss at the tinnitus frequency 2. Associated ipsilateral symptoms from adjacent cranial nerves (a) Cryptogenic or overt HFSs (b) Bouts of otalgia or feeling pressure in the ear (c) Vertiginous spells: short lasting, optokinetically induced 3. Positive MRI for vascular compression 4. Positive brainstem auditory evoked potential using Møller s criteria Classification of Cochleovestibular Compression Syndrome The characteristics of CVCS can be classified into four different groups based on the American Academy of

4 330 Ophthalmology and Otolaryngology s (AAOO) (later renamed the American Academy of Otolaryngology [AAO]) criteria of Ménière s disease [51, 52], relating to the certainty of the diagnosis of CVCS as the cause of tinnitus [53]: Possible CVCS: initially intermittent unilateral tinnitus spells without associated symptoms. Probable CVCS: possible CVCS with associated symptoms (vertigo spells; ipsilateral cryptogenic or overt HFS; ipsilateral pressure feeling in the ear, ipsilateral ear pain, or deep, dull hemifacial pain; ipsilateral frequency-specific hearing loss). Definite CVCS: probable CVCS with abnormal ABR and/or abnormal MRI. Certain CVCS: definite CVCS is surgically proven. Pathophysiology of the CVCS A cranial nerve has two parts, a CNS segment and a PNS segment separated by a transition zone, known as the root entry or root exit zone (for sensory and motor nerves, respectively) or Obersteiner Redlich zone. The length of the CNS segment is different in every cranial nerve, with sensory fibers, in general, having a longer CNS segment than motor fibers [54]. For the VIIIth cranial nerve, the CNS segment encompasses the entire cisternal trajectory of the cochleovestibular nerve with the root entry zone located at the entrance of the internal auditory canal, thus the root entry zone is located at the internal auditory meatus. Functional Anatomy The cochlear nerve contains approximately 30,000 axons [55], 90% of which are myelinated (type I) and 10% of which are unmyelinated (type II) [56]. (For details, see Chaps. 8 and 36.) Myelinated nerve fibers represent the afferent neurons from the inner hair cells and the efferent neurons to the outer hair cells. Unmyelinated nerve fibers, on the contrary, represent the efferent neurons to the inner hair cells and the afferent neurons from the outer hair cells [56]. D. De Ridder and A.R. Møller The average axon diameter of the PNS segment is fairly constant at ±3 mm [56] or mm [57], suggesting conduction velocities of approximately 12 m/s [58] (11.6 ± 1.6 m/s). Whether differences exist in fiber spectrum, especially with regards to fiber diameter between apical and basal fibers in humans, is still debated, so it is not known whether a direct correlation exists between axonal diameters and tonotopy in humans; however, it has been suggested [57]. The auditory system is tonotopically organized. This means neurons sensitive to specific acoustic frequencies are topographically arranged in an orderly manner [59 62]. As the cochlea is tonotopically organized (Von Bekesy s place theory of pitch perception) as well as the cochlear nuclei, the inferior colliculus, and the auditory cortex the cochlear nerve has to be tonotopically organized too [31], as shown in animal studies [63]. The cochlear nerve (as other cranial nerves) rotates as it travels through the auditory canal and cisternal segment of the subarachnoidal space toward the cochlear nucleus [64]. The tonotopy follows this rotation as well. This tonotopy has been demonstrated in humans as well as in studies of MVDs of the vestibulocochlear nerve [31]. It has also been demonstrated by means of an MRI technique using 3D reconstructions of high-resolution (0.6 mm slice thickness), heavily T2-weighted images (constructive interference in steady state, CISS) [65] also known as virtual endoscopy [66]. Pathophysiological Model of CVCS Several hypotheses have addressed the pathology of MVC in general. Some of them concern the cranial nerve and some concern the respective nucleus. HFS has been studied extensively, and evidence for hyperactivity in the facial motonucleus has been presented [48]. There is no evidence supporting the old hypothesis that blood vessels elongate and their brain sags with age [2, 67 70]. It is not known whether the formation of vascular loops in the posterior fossa that can come close to cranial nerve increases with age [69]. MVC has been claimed to cause focal demyelination (see Chap. 84), but little evidence of demyelination or other morphological changes in cranial nerves in individuals with symptoms of cranial nerve vascular compression has been published.

5 40 Focal demyelination, if it exists because of MVC, could cause ectopic excitation [68, 71 73] (see Chap. 84). Such ectopic excitation might cause dysfunction of the cochlear nerve, most likely leading to a reorganization of the auditory nuclei in the auditory brainstem through activation of neural plasticity. Subsequently, the entire auditory tract, including the auditory cortex, can become hyperactive, resulting in gamma band activity, which may cause tinnitus [74, 75]. Microvascular Compressions Can Result in Tinnitus due to Abnormal Signal Transmission Animal (cat) studies have described the tonotopic organization of the auditory nerve. The tonotopic organization of the human auditory nerve [31] has been related to the site of vascular contact and the frequencyspecific dysfunction of the cochlear nerve revealed as the frequency-specific hearing loss and a frequencyspecific tinnitus [31, 41]. Nonfrequency-specific click evoked auditory brainstem potentials are used routinely in an attempt to discover early demyelination. If the close contact with a blood vessel causes demyelination, frequency-specific ABR would be expected to be able to detect such focal demyelination. (For details about the anatomy of the auditory nerve, see Chap. 36 and [76].) The neural generators of the auditory evoked responses (ABRs) in humans have been determined [59, 76]. The generators of the ABR in humans are not the same as the generators of the ABR in animals, including those in monkeys [76]. Peak I in humans is generated in the distal part of the cochlear nerve; peak II is generated in its CNS segment; peak III in the cochlear nuclei; peak IV in the superior olivary complex; peak V in the lateral lemniscus; and peak VI in the inferior colliculus (see Table 40.1) [76]. The IPL I III would therefore be expected to be increased. If the vascular compression occurs at the CNS segment [20], peak II would be expected to be affected. Evoked potentials, in general, are the result of synchronized firing pattern as a reaction to a sensory stimulus [76]. The more synchronized the nerves fire, the higher the summated amplitude will be. If MVC of the cochlear nerve creates functional impairment of 331 some fibers, the temporal coherence of firing will decrease, resulting in a decrease of the amplitude of peak II. This hypothesis is supported by clinical findings that show a peak II decrease in individuals with tinnitus ipsilateral to MVCs with recurrence of peak II when surgical decompression is successful [53]. This suggests that the tinnitus is causally related to dysfunctional signal transmission at the site of compression in the initial stage of compression. 1. Chronic MVC results in frequency-specific hearing loss at tinnitus frequency. In the first 2 years, no significant changes in ABR are noted in patients presenting with tinnitus and MVC [53]. Once peak II decreases are noted, IPL I III prolongs [53].The fact that the IPL I III prolongation is related to the duration of the tinnitus furthermore suggests that this is a dynamically progressive pathology [53] and that the effect of vascular contact with blood vessels creates changes over time, both electrophysiologically [53] and clinically [53]. The IPL I III prolongation seems to be significantly related statistically to the degree of tinnitus after normalization for age [53]. Postoperatively, a shortening of the IPL I III is not related to a clinical improvement in tinnitus but to an improvement in tinnitus frequencyspecific hearing loss [53]. Schwaber and Hall [46] analyzed auditory brainstem evoked potentials in cochleovestibular compressions: IPL I III interval difference ³0.2 ms occurs in 66% of patients with a diagnosis of an MVC syndrome. Wave II amplitude <33% (in comparison with the contralateral) occurs in 57%. Contralateral IPL III V interval difference ³0.2 ms occurs in 30%; the ipsilateral IPL I III absolute interval ³2.3 ms occurs in 24%. Contralateral IPL III V absolute interval ³2.2 ms occurs in 2% of patients diagnosed with an MVC syndrome. This is associated with hearing loss for high frequencies in 65% of patients, a mid-frequency hearing loss in 27% of patients, and a low frequency loss in 8% of patients. A flat hearing loss was not seen in patients diagnosed with a MVC in Schwaber s series [46]. While the ABR changes (increased IPL I III) indicate that the conduction velocity in the auditory nerve has decreased, intracranial recordings from patients undergoing MVD operations for severe tinnitus [77] did not find any significantly increased latencies when compared with individuals with some hearing loss who did not have tinnitus, confirming that IPL I III is related

6 D. De Ridder and A.R. Møller 332 Table 40.1 Summary of relative time duration and possible mechanism related to electrophysiological changes and clinical symptoms for microvascular compression of the VIIIth cranial nerve for tinnitus Time (years) Mechanism ABR Clinical 0 2 Vascular compression No ABR changes Intermittent tinnitus? >2 Disrupted signal transmission Peak II decrease ipsilateral Tinnitus >4 Demyelination? IPL I III prolongation ipsilateral Hearing loss at tinnitus frequency >4 Compensation in brainstem IPL III V contralateral? to hearing loss and not tinnitus, per se. When compensated for hearing loss, individuals with tinnitus do not have significant changes in auditory evoked potentials from the peripheral part (IPL I III) of the ascending pathways but a slight change in the potentials recorded from the inferior colliculus. Signals transmitted via the compressed nerve fibers arrive at the cochlear nuclei in delay (IPL I III prolongs) in comparison with the contralateral input. Because auditory input arrives bilaterally, this slowing down of nerve conduction in the auditory nerve of the affected ear (ipsilateral IPL I III) will be counterbalanced by slowing down the auditory signals coming from the contralateral ear (De Ridder, submitted). As this slowing down can only occur in the brainstem, this will result in an increase in IPL III V in the contralateral side. As such, a pathophysiological explanation can be proposed for Møller s criteria of MVC syndromes of the cochleovestibular nerve. Criteria of microvascular compression of the VIIIth nerve [29]: Ipsilateral IPL I III ³2.3 ms Contralateral IPL III V ³2.2 ms IPL I III difference ³0.2 ms IPL III V difference ³0.2 ms IPL I III difference ³0.16 ms if low or absent peak II IPL III V difference ³0.16 ms if low or absent peak II Peak II amplitude <33% 2. Chronic tinnitus might be due to tinnitus frequencyspecific hearing loss. Whereas initially tinnitus is causally related to abnormal signal transmission in the peripheral part of the cochlear nerve at the site of the compression, electrophysiologically demonstrated by peak II decrease ipsilateral to the tinnitus side, chronic tinnitus might be the result of deafferentation due to hearing loss caused by slowing down of signal transmission in the peripheral part of the cochlear nerve, electrophysiologically related to IPL I III prolongation. It is known that the most common cause for tinnitus is auditory deprivation, inducing the development of an auditory phantom percept [78]. Therefore, it is likely that when the compression has resulted in a hearing loss this will result in tinnitus, specifically at the frequency of hearing loss [31, 53, 79 81]. It has also been shown that the neural network in the brain that generates tinnitus changes with time [82], with a marked change before and after 4 years of tinnitus duration. This could explain why tinnitus that has lasted a long time is more difficult to treat by surgical decompression than acute tinnitus [28, 30, 31, 83, 84]. MVD is less successful in treatment of tinnitus that has lasted for longer than 3 5 years than tinnitus that has lasted a shorter period [31], coinciding temporally with the tinnitus-related brain network changes. Conclusion It is evident from several studies that MVD operations are more successful in treating tinnitus that has not lasted too long (less than 3 5 years). Studies have shown cure rates of 30% of patients and 30% improved. Worsening of tinnitus caused by MVD operations and other complications are rare but can be severe and life threatening. After a MVD operation, the hearing threshold of the frequency of the tinnitus may improve if IPL I III normalizes and peak II reoccurs.

7 40 The following pathophysiological mechanism can be suggested for tinnitus: when a blood vessel comes into contact with the auditory part of the VIIIth nerve and starts interfering with normal signal transmission, initially no electrophysiological changes can be retrieved. After 2 years, when enough fibers are involved a decrease in peak II on the ABR develops. When the close contact with a blood vessel continues, IPL I III may increase, associated with hearing loss at the tinnitus frequency. This signal transmission slowing at the side of the compression is compensated by a contralateral slowing in the brainstem (contralateral IPL III V prolongs). When hearing loss develops, tinnitus might relate more to the deafferentation, which induces network changes in the brain based on neural plasticity, and tinnitus at that stage has become a phantom percept. 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