994

Air CT Cisternography of Anterior Inferior Cerebellar Artery Loop Simulating an ~ntracanalicular Acoustic Neuroma Makhan S. Khangure, ·2 and Saeid Mojtahedi ,· 3

Acoustic neuromas larger than 1 cm are usually apparent on routine computed tomography (CT) after contrast en­hancement, but smaller lesions escape detection [1, 2]. Metrizamide CT cisternography can demonstrate small le­sions, but it is not accurate for the detection of intracanalic­ular lesions [1, 3]. Pantopaque meatocisternography can detect intracanalicular lesions but has a high percentage of inconclusive results [4]. Air CT cisternography is considered to be the most sensitive procedure for the demonstration of very small and totally intracanalicular acoustic neuromas [5, 6]. A case is presented where partial filling of the internal auditory canal, simulating an intracanalicular tumor, was due to a loop of the anterior inferior cerebellar artery (AICA).

Case Report

A 51-year-old man had progressive right hearing loss over 5 years. He also complained of occasional loss of balance and tinni­tus, especially when exposed to loud noise. There was no history

A B

Received March 19, 1982; accepted after revision May 27, 1982.

of ear infection or labyrinthitis. Audiovestibular examination was consistent with a right retrocochlear lesion and the left ear was normal.

Air CT was performed in each canal in the appropriate lateral decubitus position. After the introduction of 7 mm 3 of air via a lumbar puncture, 2 mm sections were obtained with a Picker Synerview 600.

The left canal showed normal filling with air (fig . 1 A), but , on the right side (fig . 1 B), there was soft tissue occupying the medial half of the canal, with air filling the lateral segment. This was interpreted as representing an intracanalicular acoustic neuroma.

At translabyrinthine exposure the AICA was seen to form a loop within the canal and was placed between the superior vestibular nerve and the facial nerve. No tumor was noted.

Discussion

Air CT cisternography is considered a safe and simple procedure with a high degree of accuracy in detecting acoustic neuromas [5, 6]. Nonfilling of the canal during

Fig . 1 .-Air CT cisternograms. A , Right lateral decubitus position. Air out­lines left cerebellopontine angle cistern and fills internal auditory canal. Appear­ance of neurovascu lar bundle suggests that AICA enters internal auditory canal. B, Left lateral decubitus position . Air fills ri ght cerebellopontine angle c istern and lateral half of internal aud itory canal. Soft-tissue density over medial half of canal is thought to be intracanalicular tumor , although it may also represent partial volume effect of bone. At surgery the loop of AICA was seen to enter the canal.

'Department of Radiology, Universi ty of Iowa Hosp itals and Clinics, Iowa City. IA 52242 . ' Present address: Department of Diagnostic Radiology, Royal Perth Hospital, Perth, Western Aust ralia 6001 . 3Present address: Departmenl of Rad iology, University of Chicago , Box 429, 950 E. 59th St. , Chicago , IL 6063 7. Address reprint requests to S. Mojtahedi.

AJNR 4:994-995, July / AU9ust 1983 0195-6108 / 83 / 0404-0994 $00.00 © American Roentgen Ray Society

AJNR :4 , Jul. ! Aug . 1983 AICA LOOP SIMULATING ACOUSTIC NEUROMA 995

Pantopaque meatocisternography may be due to a narrow canal, arachnoid webs , an abnormal arterial loop, or tumor [4, 7].

In a dissection of cadaver temporal bones, Mazzoni [8] found the AICA loop close to the internal auditory canal in 80% of specimens. The apex of the convexity of the vascular loop extended into the canal in 40% of specimens and approached the porus acousticus in 27% of specimens.

The loop of the AICA is well visualized on high-resolution air CT examinations as it lies in the cerebellopontine angle or porus acousticus [9]. To our knowledge the AICA loop has not been demonstrated on air CT when it lies well within the canal.

Nonfilling of the internal auditory canal due to arachnoid adhesions simulating an acoustic neuroma has been re­ported [10]. Partial filling of the internal auditory canal with air would be expected with some small intracanalicular neuromas, but in such a situation the possibility of a vascular loop within the canal simulating a tumor must be considered . Air CT scanning after a bolus of intravenous contrast mate­rial may differentiate the vascular structure from an intra­canalicular tumor.

REFERENCES

1. Davis KR , Parker SW, New PFJ , et al. Computed tomography of the acoustic neuroma. Radiology 1977; 124 : 81-86

2. Dubois PJ , Drayer SP, Bank WO , Deeb ZL, Rosenbaum AE . An evaluation of current diagnostic radiolog ic modalities in the investigation of acoustic neuril emmomas. Radiology 1978;1 26 : 173-179

3. Rosenbaum AE , Drayer BP , Dubois PJ , Bl ack FO . Visualization of small ex tracanalicular neurilemmomas by metri zamide c is­ternographi c enhancement. Arch Oto/aryngo/1978 ; 104 : 239-

243 4 . Fisch VP, Neozoleki J , Wellaver J. Diagnostic value of meato­

cisternography. Arch Oto/aryngol 1975; 10 1 : 339- 343 5. Sortland O. Computed tomography combined with gas c istern ­

ography for the diagnosis of expanding lesions in th e cerebel­lopontine angle. Neuroradiology 1979;18 : 19 - 22

6. Kricheff II , Pinto RS , Bergeron RT , Cohen N. Ai r-CT c istern - • ography and canalography for small acoustic neuromas. AJNR

1980;1 : 57-63 7. Brookler KH, Hoffman RA. Acoustic neuroma or vasc ular loop?

Am J Oto/1979 ;1 :32-36 8 . Mazzoni A. Internal auditory canal arterial relationships at the

porus acou sticus . Ann Otol Rhinol Laryngol 1969;78: 7 9 7 -

815 9. Phelps PD, Lloyd GS. High resolution air CT meatog raphy: the

demonstration of normal and abnormal structures in the cere­bellopontine cistern and internal auditory meatus. Br J Radiol

1982;55 : 19-22 10. Downey EF Jr, Buck DR , Ray JW. Arachnoiditis simulating

acoustic neuroma on air-CT c isternography. AJNR

1981;2:470-471