1294

Before the precipitate launch of combination therapy (forelderly patients) at a cost of at least 10 million prescriptions(and who knows how many pounds) one must know that itsuccessfully reduces life-threatening complications. Suchevidence is awaited. Meanwhile, the priority should be thedevelopment of safer alternatives to existing anti-

inflammatory therapy.

IMAGING PATIENTS WITH SUSPECTEDACOUSTIC NEUROMA

THERE are approximately 45 new cases of acousticneuroma a year in Britain,1,2 but less than a tenth of patientswith insidious unilateral hearing loss have a neuroma; evenwhen patients with other signs of a cerebellopontine anglemass are included, only about 5 % of those referred forimaging are found to have eighth nerve tumours.3 Sincesurgery should be easier, safer, and carry a better functionalprognosis if neuromas are detected when small, a reliablescreening test is needed which, ideally, will also suffice forpreoperative assessment. Otologists and neurologists areguided by the precepts cheap before expensive, anodynebefore invasive. But this eminently reasonable approachbecomes invalid if the efficacy of the cheap or anodyne isunacceptably low or the yes/no answers provided are

inadequate for further management.Plain films and conventional tomography of the petrous

bones are wanting on both counts. Review of over 1400 casesup to 19803 revealed false-negative plain film results in athird of patients with acoustic neuroma (increased to morethan half if one large, atypical, accurately diagnosed series isexcluded), and in a quarter when tomography was used.Outside specialist centres, incorrect interpretations are notexceptional, and even in expert hands invasive confirmatorytests such as oil cisternography4 were done in almost fourtimes as many patients as finally proved to have neuromas.sThese figures may be acceptable in a community seeking tofoment imaging, but they are hardly appropriate today.Moreover, correlation between the extent of bonyabnormalities and tumour size is poor: plain films do notfurnish the data required for treatment planning. Likeconventional radionuclide studies these cheap, anodyne,and widely available techniques are unreliable for screening.Furthermore, when they are done in advance of moreinformative investigations, the costs and risks will beadditive. 7

In 1961, when no noninvasive confirmatory test wasavailable, 91 % of patients for whom views of the internalauditory meatuses were requested in major UK neurologicalcentres underwent no further imaging procedure; this

proportion had fallen, but only to 81 %, more than a decadeafter the introduction of X-ray computerised tomography(CT).8 Were those requesting plain films misled as to their

1 Behrend RC Epidemiology of brain tumours In Vinken PJ, Bruyn GW, edsHandbook of clinical neurology, vol 16 Tumours of the brain and skull part 1

Amsterdam North Holland, 1974 56-882 Anon Frequency of neurosurgical disorders in the UK Br J Neurosurg 1988, 2:

281-83

3. Hart RG, Davenport J Diagnosis of acoustic neuroma Neurosurgery 1981, 9: 450-634 Scanlan RL Positive contrast medium (iophendylate) in diagnosis of acoustic

neuroma. Arch Otolaryngol 1964, 80: 698-706.5 Valvassori GE The abnormal internal auditory canal the diagnosis of acoustic

neuroma Radiology 1969, 92: 449-596. Burrows EH Clinical reliability of posterior fossa scintigraphy Clin Radiol 1976, 27:

473-817. Schreiber MH Wilson’s law of diminishing returns AJR 1982; 138: 786-888 Moseley IF Longterm effects of the introduction of noninvasive investigations in

neuroradiology. Part 2 effects on management of individual patientsNeuroradiology 1988, 30: 193-200

value or, aware of their inefficacy, using radiography asoccupational therapy-"busy time".9CT has become the mainstay of imaging for acoustic

neuroma, and may not have increased the cost of case-

finding.10 With the simple machines of 15 years ago, mosttumours extending at least 1-5 cm into the posterior cranialfossa could be identified; review of almost 600 cases

published before 1980 suggested a false-negative rate below20%.3 With the introduction of CT air meatography in1979," all but the smallest intracanalicular tumours becameaccessible to the probing beam. Although intrathecal

injection of even a small amount of air robs the examinationof its largely anodyne nature, innumerable outpatients havebeen examined in this way. House and his colleagues12proposed combining the very high sensitivity but lowspecificity of auditory evoked responses with highly specificCT, resorting to meatography only should the latter show awide meatus but no tumour.The quality of the images of the petrous bones themselves

and the contents of the middle ear afforded before 1980 wassuch that plain films retained a complementary role inneuro-otology, sometimes indicating a different disease

process; but with modem high spatial resolution scanningand bone algorithm programmes this tenet no longer holdstrue. Moreover, CT and newer techniques such as magneticresonance imaging (MRI) provide all the information mostsurgeons demand for planning their approach, and share theimportant advantage of displaying the entire auditorypathway, thereby enabling a far wider range of alternativediagnoses.

It rapidly became evident that absence of the artifactswhich bedevil CT of the cerebellopontine angle gives MRI aclear advantage." The minimum size of demonstrableintracranial neuromas fell to a few millimetres,14 andmultiple imaging sequences, showing different tissue

characteristics, combined in a few cases with intravenousgadolinium compounds (better tolerated than CT contrastmedia), enable the examination to be conducted safely,rapidly, and with a very high degree of accuracy, even forintracanalicular lesions.15 Regrettably, MRI is costly andnot accessible to all patients with sensorineural hearing loss.Where the technique is unavailable, CT can usefully replaceit, albeit with more discomfort for some patients. Somecentres report 5% or less of positive results in those

undergoing meatography,16 and careful clinical and

audiological selection remains paramount if we are to avoidthe erstwhile commonly ill-conceived, wasteful approach toplain films, and make best use of expensive but reliabletechniques.l’ Utility of "cheaper", "simpler", or "less

invasive" tests, even for screening, is highly questionable.

9. Daves ML Radiologic overkill JAMA 1967; 200: 999-1000.10. House WF In discussion of Caparosa RJ Cost-benefit ratio in our search for

cerebellopontine angle tumors. Laryngoscope 1979; 89: 410-20.11. Sortland O. Computed tomography combined with gas cisternography in the

diagnosis of expanding lesions in the cerebellopontine angle Neuror adiology 1979;18: 19-22

12. Barrs DM, Brackmann DE, Olson JE, House WF Changing concepts of acousticneuroma diagnosis. Arch Otolaryngol 1985, 111: 17-21.

13. Young IR, Bydder GM, Hall AS, et al The role of NMR imaging in the diagnosis andtreatment of acoustic neuroma. AJNR 1983, 4: 223-24

14. Kingsley DPE, Brooks GB, Leung AWL, Johnson MA Acoustic neuromas

Evaluation by magnetic resonance imaging. AJNR 1985, 6: 1-515 Stack JP, Ramsden RT, Antoun NM, Lye RH, Isherwood I, Jenkins JPR Magnetic

resonance imaging of acoustic neuromas. the role of gadolinium-DPTA Br JRadiol 1988, 61: 800-05

16 Robertson HJ, Hatten HP, Keating JW False-positive CT gas cisternogram. AJNR1983, 4: 474-77

17 Larson EB Promoting effective use of newer imaging techniques AJR 1982, 138:788-89.