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The investigation of acoustic neuroma
Clinical Radiology (1991) 44, 232-235
The Investigation of Acoustic Neuroma A. G. C L I F T O N , P. D. P H E L P S and G. A. S. L L O Y D
Department of Radiology, The Royal National Throat, Nose and Ear Hospital, London Gadolinium-enhanced magnetic resonance (GdMR) is the long awaited, easy and definitive investigation for imaging or excluding small acoustic neuromas. However, on the grounds of cost alone it is unrealistic to submit every patient with mild unilateral sensorineural deafness to GdMR. A screening regime is described which combines simple but effective imaging studies of the internal auditory meatus with clinical testing of vestibular function and objective audiometry. The 'two out of three positive' approach based on these tests has proved to be a reliable screening regime for selecting patients for GdMR. Clifton, A.G., Phelps, P.D. & Lloyd, G.A.S. (1991). Clinical Radiology 44, 232 235. The Investigation of Acoustic Neuroma
Conventional zonography
The early diagnosis of an acoustic neuroma remains a priority, since operative risk is directly proportional to its size. Various tests are now available for early diagnosis, some of which can be used for screening large numbers of patients, while other more sophisticated and expensive techniques provide a complete diagnosis. The modern investigation of acoustic neuroma is therefore a two-stage process: a screening method to identify patients requiring further study, followed by the definitive examination.
The bony outline of the internal auditory canal is readily imaged by transorbital zonography or thick section t o m o g r a p h y of the petrous bones (Fig. 1). Only three or four sections are usually required. In some positive cases abnormal enlargement may be obvious upon simple inspection, but minor enlargements require comparison of the two sides by measurement. The vertical diameter of the canal is readily revealed by zonography and the two sides can usually be measured on the same radiograph. Figure 2 is a histogram of these measurements in 100 normal patients. It shows a symmetrical, unimodal distribution with a standard deviation of 0.5 mm. Eight other reports substantiate this figure (Kobayashi and Zusho, 1987). Ninety-five per cent of normal subjects can be expected to lie within two standard deviations from the
METHODS Screening Method Many patients present with otological symptoms consistent with an early acoustic neuroma, but only one in approximately 100 patients will have a tumour (Pfleiderer et al., 1988). It is clear that a low-cost yet highly sensitive screening method is required to select those patients who require the definitive investigation. N o single technique has yet been devised for this task, but a combination of three tests has been suggested by Terkildsen and Thomsen (1983), who described a screening protocol comprising caloric testing, evoked response audiometry and conventional tomograms of the internal auditory meatuses. They found that no additional acoustic neuroma was identified by subsequent definitive investigation when two of these three tests were positive. In a retrospective study Pfleiderer et al. (1988) found, in 64 patients with proven acoustic neuromas, that this protocol correctly predicted the presence of a tumour in 97%. Smith et al. (1990) modified this protocol and devised a 'weighted' system of scoring designed to reduce the number of false positive investigations. However, they employed computed tomographic (CT) air meatography as the definitive test for acoustic neuroma instead of gadolinium-enhanced magnetic resonance ( G d M R ) imaging and were, therefore, anxious to avoid subjecting normal patients to a technique with a significant morbidity. When G d M R is used as the definitive examination, emphasis should be placed on reducing false negative examinations rather than reducing the number of negative G d M R examinations.
,
Correspondence to: Dr A. G. Clifton, Department of Neuroradiology, Atkinson Morley's Hospital, Copse Hill, Wimbledon, London SW20.
Fig. 2 - Histogram showingthe distribution of the comparative internal auditory meatus measurements (measured from the radiograph in millimetres) in 100 normal subjects. S.D. = 0.5.
Fig. 1 -Zonogram of the internal auditory canals (arrows) showing widening of the right-sided canal caused by an acoustic neuroma.
RigM
1.5
I
Left
0"5
0
0.5
I
I .5
233
INVESTIGATION OF ACOUSTIC NEUROMA
(a)
(a)
(b)
(b)
(e)
Fig. 3 Right acoustic neuroma shown by MRI. (a) Axial Tl-weighted sequence (TE26, TR700). The tumour returns a low signal. (b) Coronal T2-weighted sequence (TE80, TR1500). The tumour returns a high signal.
Fig. 5 - Small right acoustic neuroma. (a) Unenhanced CT bone window view (W'W4000, WL600) imaging an abnormally wide internal auditory canal. (b) Enhanced CT slice of the level of the internal auditory canals. A small enhancing mass is just visible emerging from the right meatus. (e) Gadolinium enhanced M R I T 1-weighted sequence o f the same patient reveals the full size of the tumour.
Fig. 4 Left acoustic neuroma shown by coronal Tl-weighted gadolinium enhanced MRI. The tumour enhances vividly, contrasting with the low signal returned by the normal right seventh and eighth nerves (arrow).
mean. When measured on the radiographs with a magnification factor of 30%, a discrepancy in the size of the internal auditory meatuses of over 1 m m can be expected in only 5% of normal subjects. As stated above, the expected incidence of acoustic neuroma in the population
under examination is 1%. Therefore, an acoustic neuro m a is a strong possibility if the size discrepancy of the internal auditory canals exceeds 1 m m on the suspected side; and this is a clear indication for definitive examination by G d M R . In his studies of tomographic measurement of the internal auditory meatus, Valvassori (1969) considered the internal auditory meatus to be abnormal if there was widening of 2 m m or more of any portion of the meatus in comparison with the contralateral side, stating that when present this is almost certain evidence of an acoustic tumour. He also considered that widening by 1-2 m m suggested a tumour. Since the purpose of using bone studies as a screening procedure is to select patients for G d M R - n o t to make the d i a g n o s i s - widening of more than 1 m m should be a positive indication for further examination and will ensure that 90% of acoustic neuromas will be selected for G d M R . It is the function of the two clinical tests, evoked response audiometry and caloric responses,.to select the remaining 10%.
234
CLINICAL RADIOLOGY
with Meniere's disease. Also, the procedure is unsuitable in patients with a hearing loss above 75dB, and thus inapplicable in many patients. If patients with poor hearing are first excluded, claims of high success rate, e.g. 98% (Johnsen and Selters, 1987) are unrealistic. Diagnostic information may therefore be recorded in only 54% of acoustic neuromas (Pfleiderer et al., 1988). Caloric responses
(a)
The caloric test is based upon nystagmus caused by convection currents in the endolymph induced by thermal changes in the external auditory canal. The technique was first described by Fitzgerald and Hallpike (1942), and positive results in acoustic neuromas have been reported to be as high as 100% (Dix and Hallpike, 1958). However, the high positive results in some 0f the early series merely reflected the tendency of mosr'tumours to present late; in small tumours, positive results may be as low as 50% (De La Cruz, 1981). Linthicum and Churchill (1968) recorded reduced vestibular responses in 50% of small acoustic neuromas, 78% of medium-sized and 93% of large tumours. It is apparent that small tumours frequently yield a symmetrical caloric response, and Flood et al. (1984) have suggested that a normal response may occur if the tumour is small and arises from the inferior vestibular nerve, which is not evaluated during the routine caloric technique. Of the 58 patients tested by Pfleiderer et al. (1988) 54 were positive and four negative, i.e. an abnormal caloric response in 93% of patients with proven acoustic neuromas. Definitive Investigation
(b) Fig. 6 CT air meatography. (a) Normal right air meatogram. Air is seen in the internal auditory meatus, and the seventh and eighth nerves can be identified (arrow). (b) Small acoustic n e u r o m a (arrow) is shown emerging from the internal auditory canal. Air does not outline the
canal. E v o k e d response a u d i o m e t r y
Two variations of this technique are used for detecting acoustic neuromas: brainstem evoked response audiometry (BSER) and electrocochleography. The latter technique being invasive, albeit with an extremely low morbidity, it cannot be considered suitable. By contrast, BSER is a non-invasive and highly sensitive test which can be carried out as an out-patient procedure. A normal reading virtually excludes an acoustic neuroma, and false negative results can be expected in only about 3% of patients (Pfleiderer et al., 1988). Unfortunately, a high rate of false positives is encountered, especially in patients
In those patients screened out by the protocol, a specific, more sensitive test is required for imaging the tumour. G d M R has now replaced CT as the definitive method. It offers the most certain demonstration of an acoustic neuroma pre-operatively. MRI provides better soft tissue contrast and discrimination compared with CT, and is free of artefacts. Even without prior injection o f a paramagnetic substance, large acoustic neuromas are visible in all pulse sequences (Fig. 3). Those tumours with small extracanalicular components are best seen in the T 1- and proton density-weighted sequences, while wholly intracanalicular lesions may be identified only in the Tl-weighted images. According to Valvassori et al. (1988) small tumours that thicken the nerve are easily recognizable in narrow internal auditory canals, but they may be missed in large canals because of partial volume averaging. Unenhanced MRI has been reported as less sensitive than CT air meatography (Haughton et al., 1986). ~However, the use of intravenous gadolinium, which acts indirectly by shortening tissue relaxation times and particularly in pathological tissues, produces an increased signal in Tl-weighted sequences. This affects especially the inversion recovery sequences (Curati et al., 1986; Stack et al., 1988a). Several authors (Stack et al., 1988b; Watabe and Azuma, 1989) have described the G d M R appearances of acoustic neuromas, some of which had been inadequately demonstrated by CT or unenhanced MRI (Fig. 4). It seems likely that all neuromas enhance with gadolinium and, therefore, will be demonstrated irrespective of their size; however, this claim has yet to be proved by controlled trials.
INVESTIGATIONOF ACOUSTICNEUROMA The present d r a w b a c k s of M R I are lack of availability and high cost, i n c l u d i n g the cost o f the g a d o l i n i u m (phelps, 1989). It is possible to e x a m i n e four patients in 1 h, using the following protocol: g a d o l i n i u m is injected intravenously a n d T l - w e i g h t e d 5 m m sequences are obtained t h r o u g h the petrous b o n e s in axial a n d c o r o n a l planes. E n h a n c e m e n t is best d e m o n s t r a t e d with the T1weighted spin echo sequences which are quicker to o b t a i n than u n e n h a n c e d T2-weighted sequences (Stack et al., 1988b; Phelps, 1989). Since G d M R is relatively n o n - i n v a s i v e a n d involves no exposure to ionizing radiation, these t u m o u r s can be readily followed up. A recent study (Valvassori a n d G u z m a n , 1989) f o u n d that the rate of growth of acoustic neuromas, a l t h o u g h unpredictable, is usually m i n i m a l or only moderate. T u m o u r growth, w h e n it occurs, can be observed in early follow-up studies (1 year or less); a n d the rate o f n o t u r n o u t growth exceeds the p r o b a b i l i t y of rapid t u m o u r growth. Valvassori a n d G u z m a n (1989) suggest that if the p a t i e n t is over 50 years old; if the t u m o u r is small (1 cm or less); a n d particularly if useful hearing is retained, or the t u m o u r is situated in the only hearing ear, then the patient should n o t be operated u p o n but rather followed u p by M R I . The first e x a m i n a t i o n should be carried out at 8 m o n t h s , a n d if n o change occurs, a n o t h e r M R I e x a m i n a t i o n should be p e r f o r m e d at 18 m o n t h s . CT should be used only if G d M R is n o t available. The examination should be m a d e with b o n e as well as b r a i n settings a n d the b r a i n slices should include contrast enhancement. E n h a n c e d C T d e m o n s t r a t e s masses in the cerebellopontine angle if sufficiently large. Extracanalicular acoustic n e u r o m a s as small as 1 cm or less in diameter can be detected (Valvassori, 1984). Some a u t h o r s claim that C T is diagnostically unreliable if the t u m o u r protrudes less t h a n 1-1.5 cm (Schubiger et al., 1978; Bentson et al., 1980). False negative or equivocal results are encountered with small t u m o u r s , particularly the intracanalicular variety, due to b e a m h a r d e n i n g or petrous b o n e artefacts (Fig. 5). CT air m e a t o g r a p h y m a y be necessary to delineate small t u m o u r s (Fig. 6). This technique, a l t h o u g h simple to p e r f o r m (Phelps a n d Lloyd, 1982), is n o t w i t h o u t morbidity ( G r e e n b e r g e r et al., 1987; Phelps, 1988). Greenberger a n d his colleagues f o u n d that 30 out of the 84 patients in their series developed headaches sufficiently severe to p r e v e n t t h e m from working. F o r this procedure patients require 24 h hospital a d m i s s i o n a n d the examination n o w survives only as part of the pre-operative neuro-otological assessment of patients with intractable vertigo. G d M R is more cost-effective t h a n C T air meatography. G a d o l i n i u m - e n h a n c e d M R is n o w the definitive test for the diagnosis of acoustic n e u r o m a a n d has been recommended as a screening test (Editorial, 1988), b u t it seems undesirable as well as u n e c o n o m i c to o b t a i n these scans in every p a t i e n t with unilateral hearing loss since only 1% of such patients referred from E N T clinics for radiological e x a m i n a t i o n will, in fact, have t u m o u r s . The use o f the 'two out of three positive' a p p r o a c h followed by G d M R as advocated by Terkildsen a n d T h o m s e n (1983) is a highly specific m e t h o d of screening ,and d i a g n o s i n g patients with suspected acoustic n e u r o m a s .
235
REFERENCES
Bentson, JR, Mancuso, AA, Winter, J & Hanafee, W (1980). Combined gas cisternographyand edge-enhancedcomputed tomography of the internal auditory canal. Radiology, 136, 777 779. Curati, WL, Graif, M, Kingsley, DPE, Neindorf, HP & Young, IR (1986). Acoustic neuromas: Gd-DTPA enhancement in MR imaging. Radiology, 158, 447 451. De La Cruz, A (1981). Acousticneuromas: evaluation and management. Revue de Laryngologie Otologie Rhinologie (Bordeaux), 102, 35 43. Dix, MR & Hallpike, CS (1958). The otoneurological diagnosis of tumours of the VIIlth nerve. Proceedings of the Royal Society of Medicine, 51, 889-896. Editorial (1988). Imaging patients with acoustic neuroma. Lancet, 3, 1294. Fitzgerald, G & Hallpike, CS (I942). Studies in human vestibular function. Brain, 65, 115 180. Flood, LM, Brammer, RE, Graham, MD & Kemink, JL (1984). Pitfalls in the diagnosis of acoustic neuromas. Clinical Otolaryngology, 9, 165-170. Greenberger, R, Khangure, MS & Chakera, TMH (1987). The morbidity of air meaatography: a follow up of 84 patients. Clinical Radiology, 38, 535 536. Haughton, VM, Rimm, AA, Sobocinski, KA, Papke, RA, Daniels, DL, Williams, AL et al. (1986). A blinded clinical comparison of MR imaging and CT in neuroradiology. Radiology, 160, 751 755. Johnsen, EW & Selters, WA (1987). In Handbook of Neurological Diagnosis'. Eds. House, JW and O'Conner, AF. p. 76. Marcel Dekker, New York. Kobayashi, H & Zusho, H (1987). Measurement of internal auditory meatus by polytomography. I Normal subjects. British Journal of Radiology, 60, 209-214. Linthicum, EH & Churchill, D (1968). Vestibular test results in acoustic tumour cases. Archives of Otolaryngology, 88, 604 607. Pfleiderer, AG, Evans, KL, Grace, ARH & Lloyd, GAS (1988). A screening protocol used for the detection of acoustic neuromas: a clinical evaluation. Clinical Otolaryngology, 13, 145 151. Phelps, PD & Lloyd, GAS (t 982). High resolution air CT meatography: the demonstration of normal and abnormal structures in the cerebellopontine cistern and the internal auditory meatus. British Journal of Radiology, 55, 19-22. Phelps, PD (1988). The morbidity of CT air meatography. Clinical Radiology, 39, 216-217. Phelps, PD (1989). GdMRI Good value at 150 DM a vial? Clinical Otolaryngology, 14, 1 2. Schubiger, O, Valvanis, A & Menges, H (1978). Computed tomography for small acoustic neuromas. Neuroradiology, 15, 287 290. Smith, SM, Turnbull, LW, Sellar, RJ, Murray, JAM & Best, JK (1990). A modified screeningprotocol for the diagnosis of acoustic neuromas. Clinical Otolaryngology, 15, 167-171. Stack, JP, Antoun, NM, Jenkins, JPR, Metcalfe, R & Isherwood, I (1988a). Gadolinium-DTPA as a contrast agent in magnetic reso nance imaging of the brain. Neuroradiology, 30, 145 154. Stack, JP, Ramsden, NM, Antoun, NM, Lye, RH, Isherwood, I & Jenkins, JPR (1988b). MRI of acoustic neuromas: the role of GdDTPA. British Journal of Radiology, 61,800 805. Terkildsen, K & Thomsen, J (1983). Diagnostic screening for acoustic neuromas. Clinical Otolaryngology, 8, 295-296. Valvassori, GE (1969). The abnormal internal auditory canal: the diagnosis of acoustic neuroma. Radiology, 92, 449-459. Valvassori, GE (1984). Radiologic evaluation of eight nerve tumours. American Journal of Otolaryngology, 5, 270-280. Vatvassori, GE, Morales, GF, Palacios, E & Dobben, GE (1988). MR of the normal and abnormal internal auditory canal. American Journal of Neuroradiology, 9, 115 119. Valvassori, GE & Guzman, M (1989). Growth rate of acoustic neuromas. American Journal of Otology, 10, 174-176. Watabe, T & Azuma, T (1989). TI and T2 measurements of meningiomas and neuromas before and after Gd-DTPA. American Journal of Neuroradiology, 10, 463-470.
The Investigation of Acoustic Neuroma A. G. C L I F T O N , P. D. P H E L P S and G. A. S. L L O Y D
Department of Radiology, The Royal National Throat, Nose and Ear Hospital, London Gadolinium-enhanced magnetic resonance (GdMR) is the long awaited, easy and definitive investigation for imaging or excluding small acoustic neuromas. However, on the grounds of cost alone it is unrealistic to submit every patient with mild unilateral sensorineural deafness to GdMR. A screening regime is described which combines simple but effective imaging studies of the internal auditory meatus with clinical testing of vestibular function and objective audiometry. The 'two out of three positive' approach based on these tests has proved to be a reliable screening regime for selecting patients for GdMR. Clifton, A.G., Phelps, P.D. & Lloyd, G.A.S. (1991). Clinical Radiology 44, 232 235. The Investigation of Acoustic Neuroma
Conventional zonography
The early diagnosis of an acoustic neuroma remains a priority, since operative risk is directly proportional to its size. Various tests are now available for early diagnosis, some of which can be used for screening large numbers of patients, while other more sophisticated and expensive techniques provide a complete diagnosis. The modern investigation of acoustic neuroma is therefore a two-stage process: a screening method to identify patients requiring further study, followed by the definitive examination.
The bony outline of the internal auditory canal is readily imaged by transorbital zonography or thick section t o m o g r a p h y of the petrous bones (Fig. 1). Only three or four sections are usually required. In some positive cases abnormal enlargement may be obvious upon simple inspection, but minor enlargements require comparison of the two sides by measurement. The vertical diameter of the canal is readily revealed by zonography and the two sides can usually be measured on the same radiograph. Figure 2 is a histogram of these measurements in 100 normal patients. It shows a symmetrical, unimodal distribution with a standard deviation of 0.5 mm. Eight other reports substantiate this figure (Kobayashi and Zusho, 1987). Ninety-five per cent of normal subjects can be expected to lie within two standard deviations from the
METHODS Screening Method Many patients present with otological symptoms consistent with an early acoustic neuroma, but only one in approximately 100 patients will have a tumour (Pfleiderer et al., 1988). It is clear that a low-cost yet highly sensitive screening method is required to select those patients who require the definitive investigation. N o single technique has yet been devised for this task, but a combination of three tests has been suggested by Terkildsen and Thomsen (1983), who described a screening protocol comprising caloric testing, evoked response audiometry and conventional tomograms of the internal auditory meatuses. They found that no additional acoustic neuroma was identified by subsequent definitive investigation when two of these three tests were positive. In a retrospective study Pfleiderer et al. (1988) found, in 64 patients with proven acoustic neuromas, that this protocol correctly predicted the presence of a tumour in 97%. Smith et al. (1990) modified this protocol and devised a 'weighted' system of scoring designed to reduce the number of false positive investigations. However, they employed computed tomographic (CT) air meatography as the definitive test for acoustic neuroma instead of gadolinium-enhanced magnetic resonance ( G d M R ) imaging and were, therefore, anxious to avoid subjecting normal patients to a technique with a significant morbidity. When G d M R is used as the definitive examination, emphasis should be placed on reducing false negative examinations rather than reducing the number of negative G d M R examinations.
,
Correspondence to: Dr A. G. Clifton, Department of Neuroradiology, Atkinson Morley's Hospital, Copse Hill, Wimbledon, London SW20.
Fig. 2 - Histogram showingthe distribution of the comparative internal auditory meatus measurements (measured from the radiograph in millimetres) in 100 normal subjects. S.D. = 0.5.
Fig. 1 -Zonogram of the internal auditory canals (arrows) showing widening of the right-sided canal caused by an acoustic neuroma.
RigM
1.5
I
Left
0"5
0
0.5
I
I .5
233
INVESTIGATION OF ACOUSTIC NEUROMA
(a)
(a)
(b)
(b)
(e)
Fig. 3 Right acoustic neuroma shown by MRI. (a) Axial Tl-weighted sequence (TE26, TR700). The tumour returns a low signal. (b) Coronal T2-weighted sequence (TE80, TR1500). The tumour returns a high signal.
Fig. 5 - Small right acoustic neuroma. (a) Unenhanced CT bone window view (W'W4000, WL600) imaging an abnormally wide internal auditory canal. (b) Enhanced CT slice of the level of the internal auditory canals. A small enhancing mass is just visible emerging from the right meatus. (e) Gadolinium enhanced M R I T 1-weighted sequence o f the same patient reveals the full size of the tumour.
Fig. 4 Left acoustic neuroma shown by coronal Tl-weighted gadolinium enhanced MRI. The tumour enhances vividly, contrasting with the low signal returned by the normal right seventh and eighth nerves (arrow).
mean. When measured on the radiographs with a magnification factor of 30%, a discrepancy in the size of the internal auditory meatuses of over 1 m m can be expected in only 5% of normal subjects. As stated above, the expected incidence of acoustic neuroma in the population
under examination is 1%. Therefore, an acoustic neuro m a is a strong possibility if the size discrepancy of the internal auditory canals exceeds 1 m m on the suspected side; and this is a clear indication for definitive examination by G d M R . In his studies of tomographic measurement of the internal auditory meatus, Valvassori (1969) considered the internal auditory meatus to be abnormal if there was widening of 2 m m or more of any portion of the meatus in comparison with the contralateral side, stating that when present this is almost certain evidence of an acoustic tumour. He also considered that widening by 1-2 m m suggested a tumour. Since the purpose of using bone studies as a screening procedure is to select patients for G d M R - n o t to make the d i a g n o s i s - widening of more than 1 m m should be a positive indication for further examination and will ensure that 90% of acoustic neuromas will be selected for G d M R . It is the function of the two clinical tests, evoked response audiometry and caloric responses,.to select the remaining 10%.
234
CLINICAL RADIOLOGY
with Meniere's disease. Also, the procedure is unsuitable in patients with a hearing loss above 75dB, and thus inapplicable in many patients. If patients with poor hearing are first excluded, claims of high success rate, e.g. 98% (Johnsen and Selters, 1987) are unrealistic. Diagnostic information may therefore be recorded in only 54% of acoustic neuromas (Pfleiderer et al., 1988). Caloric responses
(a)
The caloric test is based upon nystagmus caused by convection currents in the endolymph induced by thermal changes in the external auditory canal. The technique was first described by Fitzgerald and Hallpike (1942), and positive results in acoustic neuromas have been reported to be as high as 100% (Dix and Hallpike, 1958). However, the high positive results in some 0f the early series merely reflected the tendency of mosr'tumours to present late; in small tumours, positive results may be as low as 50% (De La Cruz, 1981). Linthicum and Churchill (1968) recorded reduced vestibular responses in 50% of small acoustic neuromas, 78% of medium-sized and 93% of large tumours. It is apparent that small tumours frequently yield a symmetrical caloric response, and Flood et al. (1984) have suggested that a normal response may occur if the tumour is small and arises from the inferior vestibular nerve, which is not evaluated during the routine caloric technique. Of the 58 patients tested by Pfleiderer et al. (1988) 54 were positive and four negative, i.e. an abnormal caloric response in 93% of patients with proven acoustic neuromas. Definitive Investigation
(b) Fig. 6 CT air meatography. (a) Normal right air meatogram. Air is seen in the internal auditory meatus, and the seventh and eighth nerves can be identified (arrow). (b) Small acoustic n e u r o m a (arrow) is shown emerging from the internal auditory canal. Air does not outline the
canal. E v o k e d response a u d i o m e t r y
Two variations of this technique are used for detecting acoustic neuromas: brainstem evoked response audiometry (BSER) and electrocochleography. The latter technique being invasive, albeit with an extremely low morbidity, it cannot be considered suitable. By contrast, BSER is a non-invasive and highly sensitive test which can be carried out as an out-patient procedure. A normal reading virtually excludes an acoustic neuroma, and false negative results can be expected in only about 3% of patients (Pfleiderer et al., 1988). Unfortunately, a high rate of false positives is encountered, especially in patients
In those patients screened out by the protocol, a specific, more sensitive test is required for imaging the tumour. G d M R has now replaced CT as the definitive method. It offers the most certain demonstration of an acoustic neuroma pre-operatively. MRI provides better soft tissue contrast and discrimination compared with CT, and is free of artefacts. Even without prior injection o f a paramagnetic substance, large acoustic neuromas are visible in all pulse sequences (Fig. 3). Those tumours with small extracanalicular components are best seen in the T 1- and proton density-weighted sequences, while wholly intracanalicular lesions may be identified only in the Tl-weighted images. According to Valvassori et al. (1988) small tumours that thicken the nerve are easily recognizable in narrow internal auditory canals, but they may be missed in large canals because of partial volume averaging. Unenhanced MRI has been reported as less sensitive than CT air meatography (Haughton et al., 1986). ~However, the use of intravenous gadolinium, which acts indirectly by shortening tissue relaxation times and particularly in pathological tissues, produces an increased signal in Tl-weighted sequences. This affects especially the inversion recovery sequences (Curati et al., 1986; Stack et al., 1988a). Several authors (Stack et al., 1988b; Watabe and Azuma, 1989) have described the G d M R appearances of acoustic neuromas, some of which had been inadequately demonstrated by CT or unenhanced MRI (Fig. 4). It seems likely that all neuromas enhance with gadolinium and, therefore, will be demonstrated irrespective of their size; however, this claim has yet to be proved by controlled trials.
INVESTIGATIONOF ACOUSTICNEUROMA The present d r a w b a c k s of M R I are lack of availability and high cost, i n c l u d i n g the cost o f the g a d o l i n i u m (phelps, 1989). It is possible to e x a m i n e four patients in 1 h, using the following protocol: g a d o l i n i u m is injected intravenously a n d T l - w e i g h t e d 5 m m sequences are obtained t h r o u g h the petrous b o n e s in axial a n d c o r o n a l planes. E n h a n c e m e n t is best d e m o n s t r a t e d with the T1weighted spin echo sequences which are quicker to o b t a i n than u n e n h a n c e d T2-weighted sequences (Stack et al., 1988b; Phelps, 1989). Since G d M R is relatively n o n - i n v a s i v e a n d involves no exposure to ionizing radiation, these t u m o u r s can be readily followed up. A recent study (Valvassori a n d G u z m a n , 1989) f o u n d that the rate of growth of acoustic neuromas, a l t h o u g h unpredictable, is usually m i n i m a l or only moderate. T u m o u r growth, w h e n it occurs, can be observed in early follow-up studies (1 year or less); a n d the rate o f n o t u r n o u t growth exceeds the p r o b a b i l i t y of rapid t u m o u r growth. Valvassori a n d G u z m a n (1989) suggest that if the p a t i e n t is over 50 years old; if the t u m o u r is small (1 cm or less); a n d particularly if useful hearing is retained, or the t u m o u r is situated in the only hearing ear, then the patient should n o t be operated u p o n but rather followed u p by M R I . The first e x a m i n a t i o n should be carried out at 8 m o n t h s , a n d if n o change occurs, a n o t h e r M R I e x a m i n a t i o n should be p e r f o r m e d at 18 m o n t h s . CT should be used only if G d M R is n o t available. The examination should be m a d e with b o n e as well as b r a i n settings a n d the b r a i n slices should include contrast enhancement. E n h a n c e d C T d e m o n s t r a t e s masses in the cerebellopontine angle if sufficiently large. Extracanalicular acoustic n e u r o m a s as small as 1 cm or less in diameter can be detected (Valvassori, 1984). Some a u t h o r s claim that C T is diagnostically unreliable if the t u m o u r protrudes less t h a n 1-1.5 cm (Schubiger et al., 1978; Bentson et al., 1980). False negative or equivocal results are encountered with small t u m o u r s , particularly the intracanalicular variety, due to b e a m h a r d e n i n g or petrous b o n e artefacts (Fig. 5). CT air m e a t o g r a p h y m a y be necessary to delineate small t u m o u r s (Fig. 6). This technique, a l t h o u g h simple to p e r f o r m (Phelps a n d Lloyd, 1982), is n o t w i t h o u t morbidity ( G r e e n b e r g e r et al., 1987; Phelps, 1988). Greenberger a n d his colleagues f o u n d that 30 out of the 84 patients in their series developed headaches sufficiently severe to p r e v e n t t h e m from working. F o r this procedure patients require 24 h hospital a d m i s s i o n a n d the examination n o w survives only as part of the pre-operative neuro-otological assessment of patients with intractable vertigo. G d M R is more cost-effective t h a n C T air meatography. G a d o l i n i u m - e n h a n c e d M R is n o w the definitive test for the diagnosis of acoustic n e u r o m a a n d has been recommended as a screening test (Editorial, 1988), b u t it seems undesirable as well as u n e c o n o m i c to o b t a i n these scans in every p a t i e n t with unilateral hearing loss since only 1% of such patients referred from E N T clinics for radiological e x a m i n a t i o n will, in fact, have t u m o u r s . The use o f the 'two out of three positive' a p p r o a c h followed by G d M R as advocated by Terkildsen a n d T h o m s e n (1983) is a highly specific m e t h o d of screening ,and d i a g n o s i n g patients with suspected acoustic n e u r o m a s .
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