ClinicalRadiology(1993) 47, 214 215
Case Report: Radiological Features of Epidermal Inclusion Cyst of the Tympanic Membrane G. A. VERNHAM, F. B R Y D E N * a n d S. MILLER~"
Departments of Otolaryngology and *Radiology, Victoria Infirmary, Glasgow and t Department of Radiology, Hairmyres Hospital, East Kilbride Computed tomography (CT) proved to be invaluable in diagnosing and ascertaining the extent of an epidermal inclusion cyst of the tympanic membrane secondary to a previous myringoplasty. V e r n h a m , G . A . , Bryden, F. & Miller, S. (1993). Clinical Radiology 47, 2 1 4 215. Case R e p o r t : R a d i o l o g i c a l F e a t u r e s o f E p i d e r m a l I n c l u s i o n Cyst o f the T y m p a n i c Membrane
E p i d e r m a l inclusion cysts m a y o c c u r in the t y m p a n i c m e m b r a n e a n d m i d d l e e a r as a c o m p l i c a t i o n o f m y r i n g o p l a s t y , as a result o f blast injury [2,3] o r o t h e r h e a d t r a u m a , a n d after o t h e r f o r m s o f surgery such as g r o m m e t insertion  a n d m a s t o i d surgery . Occasionally these cysts are. c o n g e n i t a l a n d a r e a n incidental finding on clinical e x a m i n a t i o n . C T has largely s u p e r c e d e d p l a i n r a d i o g r a p h s for i m a g i n g o f the p e t r o u s - t e m p o r a l b o n e . Inclusion cysts can be d e m o n s t r a t e d a n d the site a n d extent evaluated.
CASE REPORT A 49-year-old auxiliary nurse was seen 4 years after a right cortical mastoidectomy and myringoplasty to repair a sub-total perforation of the tympanic membrane. She complained of right otalgia for 3 weeks, which had not been relieved by treatment with oral amoxycillin and topical Otosporin ear drops. There was no history of otorrhoea, nor of any hearing impairment. Examination revealed a soft pearly white swelling apparently arising from the roof of the external auditory canal and almost filling the deep part of the canal. The previous myringoplasty involved an underlay technique in which the temporalis fascia graft is placed medial to the drum remnant. The ossicular chain was reported to be intact and mobile. A simple cortical mastoidectomy had also been performed as an exploratory procedure to exclude chronic infection in the mastoid, but no cholesteatoma was noted. Post-operative pure tone auditometry was normal. The diagnosis and extent of the lesion were unclear by clinical assessment alone, and entities such as inclusion cyst, cholesteatoma, keratosis obturans, osteoma, tympanosclerosis and squamous cell carcinoma were considered in the differential diagnosis. Direct coronal CT of the petrous temporal bone, performed with 1.5 mm contiguous slices (Fig. 1), revealed a large well-defined ellipsoidmass closely applied to the outer aspect of the tympanic membrane; there was no middle ear involvement or bony erosion. At subsequent surgical exploration, the mass was confirmed as a cyst arising in the superficial layer of the tympanic membrane, being so well encapsulated that it could be dissected away from the deeper layers. Histology: simple epidermal inclusion cyst. No recurrence has occurred 1 year later.
l a y e r o f the t y m p a n i c m e m b r a n e , o r within the t y m p a n i c cavity. T h e c o m p l i c a t i o n is m o s t p r e v a l e n t following a n o v e r l a y t y p e o f m y r i n g o p l a s t y , i.e. the graft is p l a c e d lateral to the d r u m r e m n a n t . F o r this reason, u n d e r l a y techniques are generally preferred, i.e. the graft is p l a c e d m e d i a l to the d r u m r e m n a n t . This case r e p o r t illustrates t h a t inclusion cysts m a y s o m e t i m e s occur after u n d e r l a y myringoplasty. D i r e c t c o r o n a l C T o f the a u d i t o r y a p p a r a t u s h a s r e p l a c e d c o n v e n t i o n a l t o m o g r a p h y as the r a d i o l o g i c a l i n v e s t i g a t i o n o f choice in suspected m i d d l e e a r disease. T h e i n t r o d u c t i o n o f scanners with the ability to p r o d u c e fine detail a n d thin slices p r o v i d e s a c c u r a t e i n f o r m a t i o n c o n c e r n i n g the extent o f a m a s s lesion a n d its r e l a t i o n s h i p to the t y m p a n i c m e m b r a n e , facial nerve a n d o t h e r vital structures. E p i d e r m a l inclusion cysts characteristically a r e welldefined a n d i m a g e as l o w - d e n s i t y lesions o n CT, a n d this a p p e a r a n c e facilitates the differential diagnosis. T h e dense a p p e a r a n c e o f an o s t e o m a  excludes this lesion.
DISCUSSION M o s t e p i d e r m a l inclusion cysts arise after surgical r e p a i r o f a p e r f o r a t e d t y m p a n i c m e m b r a n e by m y r i n g o plasty; Bennett r e p o r t e d a 10 p e r cent incidence in 201 ears subjected to v a r i o u s types o f surgery. T h e y m a y be sited b e n e a t h the skin o f the canal, in the superficial layer o f the t y m p a n i c m e m b r a n e (as in this case), in the deep Correspondence to: Mr G. A. Vernham, Department of Otolaryngology, Victoria Infirmary, Langside, Glasgow G42 9TY.
Fig. 1 - Direct coronal wide-window CT made at the level of the distal turn of the cochlea showing the malleus. There is a well-defined softtissue mass arising from the right tympanic membrane; there is no associated bone erosion.
TYMPANIC MEMBRANE INCLUSION CYST Malignant tumours are more likely to be ill-defined and irregular in outline, and often show associated local bony e r o s i o n .
Acknowledgement. The authors wish to thank David Crossan for his technical assistance with the illustration.
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