British Journal of Radiology 75 (2002),847-852 © 2002 The British Institute of Radiology
CT scanning of middle ear cholesteatoma: what does the surgeon want to know?
P D Yates, FRCS1,
L M Flood, FRCS1,
A Banerjee, FRCS1 and
K Clifford, FRCR2
1 Department of Otolaryngology, North Riding Infirmary, Newport Road, Middlesbrough TS1 5JE and 2 Department of Radiology, South Cleveland Hospital, Middlesbrough, UK
Correspondence: L M Flood, Consultant Otolaryngologist
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Abstract
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The history of surgery for middle ear cholesteatoma is of an evolution of techniques to meet the challenges of inaccessible disease and of post-operative cavity management. The concept has traditionally been of exploration guided by awareness and anticipation of all, possibly asymptomatic, complications. Modern imaging reliably demonstrates surgical anatomy, dictating the ideal approach, forewarns of complications and may reveal the extent of disease. An apparent resistance amongst otologists to universal CT scanning prior to mastoidectomy contrasts with the enthusiasm of skull base surgeons or rhinologists for appropriate imaging.
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Introduction
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A cholesteatoma is a collection of keratinizing squamous epithelium in the middle ear cleft associated with bone resorption, which may be congenital or acquired. It is accepted that most acquired cholesteatomas develop in a retraction pocket in the tympanic membrane, usually in the thinner pars flaccida, superiorly. Keratin squames normally migrate from the tympanic membrane along the ear canal to the external auditory meatus. The presence of a retraction pocket disturbs this clearance mechanism and leads to keratin accumulation in an expanding and destructive "pearl". Other mechanisms, such as implantation, migration through a perforation or even squamous metaplasia of middle ear mucosa, may be implicated.
The hazards of uncontrolled cholesteatoma demand a surgical approach in all but the worst anaesthetic candidates. The difficulty of eradicating this invasive disease in the complex anatomy of the temporal bone has inspired a variety of surgical approaches, each with its own proponents. The choice of technique has traditionally been determined by personal preference with pre-operative investigations limited to otoscopy and pure tone audiometry.
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Imaging protocol
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Imaging
In our practice [1], routine CT scanning of the middle ear has required coronal scans with the patient prone, without intravenous contrast and applying the following parameters: 512 matrix; 250 mm field of view, or zoom; 4 s scan time (the maximum available); 1.5 mm contiguous slices; 1 H filter (edge enhancement); 120 kV, 100 mA exposure; 1.5 mm table index (to give contiguous slices); fast scan mode; beam hardening correction switched on; approximately 25 slices performed per examination.
Radiation dosage
Calculated from a head phantom, using the above factors, the effective dose was found to be 2.319 mSv for 136 slices. Average dose per slice is therefore equal to 0.017 mSv. This is about the same dose as one chest X-ray. Therefore for 25 slices the total dose is 0.43 mSv.
The typical effective dose of a standard CT brain scan is, in comparison, over four times greater at 2 mSv. In practice, coronal scanning of the temporal bone produces minimal irradiation of the most sensitive target tissue, the lens of the eye, and the theoretical risk of cataract formation is far greater in CT scanning of the paranasal sinuses.
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Interpretation of scans (Figures 1–4 )
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The radiologist should provide the following clinically valuable information from the images obtained:- Degree of ventilation/opacification of middle ear cleft from the Eustachian tube to the mastoid tip.
- Erosion of ossicular chain.
- Access to the epitympanum as determined by the level of the dura laterally.
- Development/cellularity/sclerosis of the mastoid cortex.
- Dehiscence of the tegmen.
- Erosion of the labyrinth, especially the lateral semicircular canal.
- Status of the facial nerve
- Alterations in anatomy secondary to previous surgery.
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Figure 1. An ideal coronal CT scan with the external and internal auditory canal simultaneously displayed. The scutum is intact (arrow). The stapes suprastructure and incus body (arrowhead) are preserved but the long process of incus has been eroded. The tympanic membrane is retracted onto the stapes head but the middle ear is ventilated.
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Figure 2. (a) The scutum is eroded (white arrow) with an attic opacity eroding the malleus head. Medially the cochlea is intact with the labyrinthine and horizontal portions of the facial nerve displayed (black arrow). (b) More posteriorly the middle ear connects with a well preserved, opaque and very extensive cellular mastoid; potentially a huge cavity with a modified radical mastoidectomy.
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Figure 3. (a) Laterally there is a very low lying dura (open arrow), which would restrict surgical access to the opaque attic. Erosion of the scutum is again demonstrated. (b) Posteriorly the arch of the lateral semicircular canal is eroded causing an asymptomatic fistula (arrow). (c) Ultimately the cholesteatoma extends into a small, dense, sclerotic mastoid cavity. The canal of the descending portion of the facial nerve is intact (black arrow).
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Figure 4. A woman with history of right total hearing loss after radical mastoidectomy who developed a mild hearing loss in her left, only hearing, ear. She also had a temporary left facial weakness, initially attributed to a Bell's palsy. CT scans confirmed the otoscopically apparent cholesteatoma, with (a) erosion of the scutum and malleus head (arrow). (b) Despite lack of vertigo there is a fistula of the lateral semicircular canal (arrow) and attenuated tegmen. Exposure of the fistula at surgery risked a profound bilateral sensorineural hearing loss in the only functioning ear, so the keratin matrix was preserved. (c) CT of the contralateral ear demonstrates the open mastoid cavity but also the unsuspected fistula of this lateral canal that had resulted in destruction of cochlear function at operation. This defect compares with (d) the intact dome of the semi-circular canal in another, similar, mastoid cavity.
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Discussion
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CT imaging of the paranasal sinuses has been accepted as an essential investigation in the pre-operative planning of endoscopic sinus surgery. It has not, however, gained wide acceptance as an essential aid to planning surgery for cholesteatoma. Most otologists reserve scans for selected cases:- Complications of chronic suppurative otitis media (CSOM).
- Suspected congenital abnormalities.
- Loss of landmarks owing to previous surgery.
Routine CT scanning prior to all surgery of cholesteatoma can only be justified if it can be shown that clinical management is influenced.
Cooperation between radiologist and an otologist sufficiently flexible to tailor surgical management according to radiological findings is the ideal. Advantages of scanning will then include:
- A visual aid to pre-operative counselling of the patient.
- Avoidance of unnecessary surgery owing to its high degree of sensitivity/specificity for middle ear disease.
- A prediction of the anatomy, ease of surgical access and extent of disease, all of which guide surgical approach.
- Anticipation of complications of chronic suppurative otitis media.
The lack of effective, non-surgical management for cholesteatoma, and the potential complications if untreated, require surgical eradication. The earliest technique, a radical mastoidectomy, converted the external canal and middle ear into a large, empty cavity devoid of ossicles or tympanic membrane. Poor epithelialization generally caused a chronic discharging cavity. The later modified radical mastoidectomy aimed to preserve the tympanic membrane, thereby sealing off the Eustachian tube from the cavity. The open mastoid bowl is still associated with discharge and a need to regularly attend an out-patient clinic for suction clearance.
Subsequent development of the intact canal wall technique, the combined approach tympanoplasty, allowed access through an intact bony canal and separately through a transmastoid approach to the facial recess and middle ear (Figure 5). Unfortunately the prospect of preserving relatively normal anatomy, avoiding cavity problems and producing better hearing results, did not entirely stand the test of time. Residual cholesteatoma, i.e. failure to eradicate the disease, occurs in 13–36% of cases, and recurrent cholesteatoma in 5–13% of cases, following this procedure [2]. The high incidence of residual or recurrent cholesteatoma in relatively inaccessible sites, such as the stapes or sinus tympani makes a re-exploration, or "second look tympanotomy", mandatory after 1 year.
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Figure 5. Axial scan through a healthy middle ear cleft and descending facial nerve. The arrow demonstrates the transmastoid, intact canal wall, approach to the facial recess. Medially, the descending facial nerve (arrowhead) is marked by the pyramidal eminence, the origin of the Stapedius tendon. Further medially is a second cleft, the relatively inaccessible sinus tympani.
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Relevance of anatomy
The intact canal wall and "open" canal wall down techniques both have their proponents. Some otologists argue that a single stage canal wall down procedure (modified radical mastoidectomy) provides maximum long-term patient benefit. Others argue that even in difficult cases where more than two stages are required to eradicate disease, combined approach tympanoplasty is preferable to modified radical mastoidectomy. Ideally the otologist will employ both techniques, the choice being influenced by radiological anatomy. The closed, intact canal wall approach requires wide access, ideally through a large cellular mastoid, whilst the canal wall down approach is preferable in the small sclerotic system (Figure 6), which is commonly associated with cholesteatomas in adults. Open exploration will then produce only a small cavity with better prospects for epithelialization. The modified radical cavity in a large cellular system holds poorer prospects for healing. Equally, the intact canal wall technique in tiny mastoid cavity is technically difficult because of poor access. Similarly, foreknowledge of the extent of the disease and mastoid system may determine whether drilling starts at the outer attic wall, proceeding posteriorly, in a small system or starts with a direct approach to the antrum through a well pneumatized mastoid. Low lying dura lateral to the attic is also a potential hazard (Figure 3a
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Figure 6. Coronal scan centred on the stylomastoid foramina (arrows). The patient's healthy ventilated, extensive, cellular, left mastoid system contrasts with the sclerotic bone and the tiny antral cavity, produced by cholesteatoma, on their right.
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Prediction of complications
Studies of sensitivity/specificity of CT in detecting asymptomatic complications prior to surgical exploration may reflect earlier techniques, e.g. axial imaging, but false negatives are inevitable. The wise surgeon will always approach danger areas, such as the lateral semicircular canal, the facial nerve and stapes footplate, with caution. However, one can only detect bone erosion and abnormal exposure by dissection of cholesteatoma from these structures, with obvious hazards. If forewarned, the operator may decide to leave cholesteatoma matrix as a protective cover, which becomes harmless once exteriorized.
Interpretation of CT scans obviously requires an otologist prepared to benefit from the knowledge of an expert radiologist. With time, surgeon and radiologist gain increased experience of correlating CT with eventual surgical findings. Coronal scans are also relatively easily understood by the patient. A few minutes discussion of the images, demonstrating the course of the facial nerve, the relationship of the inner ear and the damage to the ossicles, can be of great help in pre-operative counselling. Such scans can illustrate both the need, but also the hazards, of surgery (Figure 7). Imaging can also enhance the trainee's knowledge of surgical anatomy. Although a relatively minor consideration as yet, any pre-operative documentation of disease can be of medicolegal value.
Clinical examination and otoscopy are used to diagnose cholesteatoma. CT can determine its extent by revealing the combination of a soft tissue mass and bone erosion with 80% specificity [3, 4]. Unfortunately, cholesteatoma sac, associated granulation tissue, mucosal oedema and effusion may be indistinguishable on CT [5, 6]. Although cholesteatoma is said to show a lower attenuation than granulation tissue, the difference is subtle and enhanced MRI may be more reliable in differentiation [7].
Complications of cholesteatoma are related to bone erosion, limiting the value of MRI. MRI can confirm suspicion of defects in the tegmen tympani or dural plate on CT, and is of especial value in diagnosing cerebral herniation [8, 9]. It can exclude dual pathology in patients presenting with facial nerve palsy or sudden sensorineural hearing loss, associated with cholesteatoma. MRI also reliably detects disease extension to the petrous apex, lateral sinus thrombosis or intracranial sepsis [10].
Few authors recommend scanning as routine prior to all mastoid surgery [11], but with improving resolution, and therefore sensitivity, this may evolve. In planning revision surgery, especially after intact canal wall procedures, residual diseased air cells in the sinodural angle, tegmen, mastoid tip and petrous apex, together with recurrent cholesteatoma, can be demonstrated [12].
Few intratemporal complications of CSOM require such immediate surgery as to prevent radiology. The direst emergency, intracranial sepsis, will certainly require neuroradiology but sophisticated imaging of the temporal bone may well have to be sacrificed.
In our practice, CT evaluation has become the norm prior to the majority of mastoid surgery, and is the subject of a prospective study of its clinical relevance.
Received for publication June 20, 2001.
Revision received November 15, 2001.
Accepted for publication February 14, 2002.
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References
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