HRCT imaging of acquired cholesteatoma: a pictorial review

Cholesteatoma is a condition prevalent around the world, predominantly in developing countries. Acquired cholesteatoma is a common complication of unsafe/atticoantral chronic otitis media. Literally, it means the presence of “skin in the wrong place.” The cholesteatomatous sac is lined by stratified squamous epithelium constituting the matrix, which secretes the acellular keratin debris within [1]. An outer perimatrix contains mesenchymal cells which produce proteolytic bone destroying enzymes [2, 3].

Cholesteatoma can be classified as congenital or acquired. Another system of categorization could be according to the location of the cholesteatoma, which could be in the external auditory canal or middle ear cavity and other pneumatised parts of the temporal bone. HRCT temporal bone is currently advocated in all cases suspected of cholesteatoma clinically prior to surgery for assessment of the extent of disease, complications, and presence of variant anatomy.

Various theories exist regarding the formation of cholesteatoma; however, none of them explain it completely.

Clinically, the lesion presents with foul-smelling otorrhea, earache, and hearing loss. On otoscopic examination, it classically appears as a cluster of pearly white structures in the attic region. It is also suspected beneath polyps protruding from the pars flaccida or when there is a marginal TM perforation or granulation tissue [8].

Erosion of facial nerve canal can lead to features of lower motor neuron palsy manifesting as an inability to close ipsilateral eye, drooping of the corner of the mouth, and drooling of saliva. A large proportion of patients with facial canal erosion may not have features of facial nerve paresis. A labyrinthine fistula will lead to vertigo and nystagmus; however, these symptoms may be present in the absence of a fistula, often due to the diffusion of toxic substances into the intact labyrinthine. Intracranial complications can be life-threatening, patients presenting with fever, headache, and nuchal rigidity due to meningitis. The most common etiology of the cerebellar abscess is otogenic and patients present with fever and ataxia and other balance abnormalities.

Non-contrast axial scans parallel to the axis of and through the petrous temporal bone are acquired on a multidetector CT scanner. Axial scanning is done with helical technique (120 kVp, high-resolution matrix (512 × 512), section thickness 0.6–1 mm, FOV 15–20 cm) in bone algorithm. Head is kept in neutral position and true axial images parallel to Reid’s line (joining inferior margin of the orbit to the center of the orifice of EAC) are acquired. Coronal multiplanar reconstruction is made perpendicular to the axial scans.

Cholesteatoma can be congenital or acquired. Congenital cholesteatoma of the temporal bone is formed due to lack of involution of distinct squamous cell rest, which should have involuted to become normal endothelium [1]. Middle ear cavity including mastoid, petrous, and squamous portion of the temporal bone and TM are frequent sites of congenital cholesteatoma. Another location is intradural (cisternal) usually involving the cerebellopontine angle (CPA) and the patient presents with varying degrees of hearing loss [9, 10]. A solitary lytic calvarial lesion may be cholesteatoma in the diploic space. They are usually incidental findings on imaging and history of ear discharge or previous otologic procedure is lacking. Usually, the TM is intact and retraction pockets are not seen in such cases. Presence of cholesteatoma in the anterosuperior part of the middle ear cavity just above the opening of the eustachian tube should raise the suspicion of congenital etiology [2].

EAC cholesteatoma is uncommon and usually occurs in an age group older than middle ear cholesteatoma. It appears as a soft tissue mass in the canal particularly along its floor with associated bony destruction and periostitis. It occurs due to the invasion of the squamous epithelium lining the EAC into the underlying bone. However, it is difficult to distinguish between cholesteatoma, carcinoma, and malignant otitis externa using HRCT alone [2, 11].

Among the middle ear cholesteatoma, the most common type is pars flaccida variety and is seen characteristically in the Prussack’s space, a niche in the epitympanum lateral to the ossicles. These cause medial displacement of the malleus and erosion of the bony scutum, extending into the aditus and antrum posteriorly (Figs. 1 and 2). Widening of the aditus and formation of a common cavity are a characteristic feature [12, 13] (Fig. 3).

Pars tensa variety is less common and usually arise within retraction pockets. Pars tensa cholesteatoma displaces the ossicles laterally in contrast to pars flaccida variety [9] (Fig. 4). Careful inspection of sinus tympani and facial recess is necessary as these areas are not visible on otoscopic examination (Fig. 5). Another important location in the middle ear is anterior epitympanic recess also called supratubal recess, which is the medial most extension of the epitympanum. A bony bar called cog which is attached to the tegmen anterior to the malleus forms the boundary between anterior epitympanic recess and epitympanic cavity. Medially, this recess is in close relation to the proximal part of tympanic segment of the facial nerve and geniculate ganglion, and dehiscence of the bony partition can make the facial nerve prone to injury [14, 15] (Fig. 6)

The hallmark of cholesteatoma is bony destruction. Presence of soft tissue density in the middle ear cavity coexistent with ossicular and mastoid bony erosion is highly specific for cholesteatoma. In attic cholesteatomas, the ingrowth of epithelium from the pars flaccida into the epitympanum is evident by the erosion of the scutum.

Ossicular erosion is more frequently encountered in pars tensa cholesteatoma than pars flaccida location of soft tissue. Ossicles may not be visualized, appear rarefied, or show small erosions. Lenticular process of the incus is most prone to erosion, due to poor ligamentous support and precarious arterial supply by end arteries. Disruption of the “ice cream cone” configuration of ossicles in epitympanum is seen frequently in attic cholesteatoma where the “ice cream scoop” is the head of the malleus and the body of the incus represents “the cone” (Figs. 7 and 8). Careful inspection of the footplate of stapes at oval window niche is also particularly important. Although the degree of ossicular erosion does not alter the need for surgery, it can help prognosticate the patient regarding recovery of hearing loss after surgery and aids in planning the reconstructive procedure. For instance, patients showing erosion of stapes superstructure experience a much higher degree of hearing loss compared to patients with intactness of the same. Correlation for ossicular erosion between CT and surgical findings is higher for the malleus, body, and short process of incus when compared to the long process of incus and stapes.

However, in the absence of bony erosions, diagnosis of cholesteatomas is challenging on CT. Presence of non-dependant soft tissue goes in favor of cholesteatoma. Also, mass effect on the ossicles is frequently seen in cholesteatomas, even in the absence of frank destruction [2].

Erosion of the facial nerve canal is an important complication. Tympanic/horizontal segment is the most frequently affected. A limitation exists in evaluating the facial canal as it can be so thin even in the non-pathological ear that it becomes difficult to assess the presence of its erosion in the affected ear. This explains for the poor radiological correlation with the surgical findings [16, 17]. In addition, visualizing the tympanic portion of the facial canal is found to be especially difficult when there is an adjacent pathologic soft tissue in the mesotympanum [8]. Routinely, axial and coronal reconstructions are seen to evaluate the facial canal (Fig. 9a, b). Double oblique sagittal images, along the plane of tympanic segment, aid in better visualization of this segment (Fig. 10).

Labyrinthine fistula is a dreaded complication of cholesteatoma. The most commonly affected is the lateral semi-circular canal due to its close proximity to the middle ear cavity (Fig. 11). Extensive disease can affect cochlea and other semi-circular canals also. Fistula formation is seen as direct contact between the soft tissue and membranous labyrinth. Hence, detailed inspection of the labyrinth is necessary in axial and coronal planes. Even then, very small fistulae can be missed. The presence of cholesteatoma in close proximity to the bony labyrinth also needs to be reported even in the presence of intact intervening bone, due to the danger to the underlying labyrinth during surgery [18].

Erosion of lateral mastoid cortex is also commonly seen (Fig. 9b).

Extensive bony destruction of the mastoid and ossicles can occur which may give the appearance similar to postsurgical cases termed as automastoidectomy. Spontaneous evacuation of cholesteatoma may be seen with this, termed as mural cholesteatoma leaving behind no residual mass [2] (Fig. 12).

Other sites of erosion include tegmen tympani (Fig. 13) and sinodural plate (Fig. 7). Erosion of the sinodural plate is known to be a common implicating cause of transverse sinus thrombosis, meningitis, cerebritis, and abscess. In suspected intracranial complications, MRI is the investigation of choice.

The differential diagnosis of soft tissue in the middle ear with bony erosions includes cholesteatoma, facial nerve schwannoma/hemangioma, paraganglioma, rhabdomyosarcoma, LCH (Langerhans cell histiocytosis), squamous cell carcinoma, metastasis, and GCT (giant cell tumor) [1]. The presence of retraction of tympanic membrane points towards chronic suppurative otitis media as the etiology [18].

HRCT is unreliable in differentiating residual or recurrent cholesteatoma from other entities which usually occur without bone destruction, like granulation tissue, cholesterol granuloma, post-inflammatory ossicular fixation, mucosal edema, fibrosis, scar tissue, and fluid. They are also frequently encountered in the post-operative period. MR imaging can usually aid in making the correct diagnosis as cholesteatomas are hypointense on T1WI, hyperintense on T2WI, do not show any enhancement, and demonstrate diffusion restriction on DWI. Cholesterol granulomas on the other hand show hyperintense signal on T1WI due to the presence of intrinsic blood products [2]. Post-inflammatory ossicular fixation is a complication of non-cholesteatomatous chronic otitis media and appears as soft tissue density encasing the ossicles particularly in the oval window niche with or without calcific foci. Absence of ear discharge and the presence of calcification or fibro-osseous tissue are pointers towards non-cholesteatomatous etiology [18].

HRCT temporal bone has certain limitations. It is difficult to differentiate cholesteatoma from granulation tissue, pus, or fluid. Early disease with the absence of osseous erosion is challenging to pick. The integrity of the facial canal cannot be accurately commented upon, especially in the presence of adjacent soft tissue in the middle ear. Due to the acquisition of the non-contrast scan, intracranial complications are frequently overlooked.

HRCT temporal bone offers an insight into the extent of cholesteatoma and the various complications before the surgeon embarks onto the definitive line of management. It also helps detects recurrent/residual middle ear disease to avoid unnecessary second-look surgery.