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Imaging of the Postoperative Middle Ear, Mastoid, and Internal Auditory Canal
Introduction
Interpretation of CT or MR imaging examinations performed on patients with a history of middle ear (ME), mastoid, or neurotologic surgery can be challenging. This is greatly simplified by knowing the surgical procedures used and the expected postoperative appearance. Knowledge of the normal postoperative appearance and the current clinical situation is the key to recognizing recurrent disease.
The goal of surgery in the ME, mastoid, and posterior fossa is the elimination of disease and, if possible, the preservation or restoration of normal function. The approaches and procedures discussed may be performed singularly or in combination.
Transcanal Approach
Surgery is performed through the external auditory canal (EAC) with the aid of a microscope and ear speculum. Visualization of the tympanic membrane (TM) is limited anteriorly and posteriorly without drilling of bone. Drilling may occur anywhere in the bony canal depending on the need for exposure or for removal of disease.
Drilling is the preferred approach for EAC stenosis, exostosis, EAC osteomas, myringitis, repair of central perforations, placement of pressure equalization tubes, and so forth. There may be little evidence of postoperative change in the EAC. Typical CT findings include thinning, irregularity or flattening of bone, soft tissue thickening, and bone wall defects. A larger appearing bony EAC may be evident from drilling of bone posteriorly lateral to or at the level of the isthmus ( Fig. 10.1 ). Advantages include limited surgical dissection, so postoperative pain is diminished. As a result, patients normally return to their presurgical functional status within 1–2 days.
Retroauricular and Endaural Approaches
For the endaural approach, an incision is made between the tragus and helix. For the retroauricular approach, an incision is made posterior to the ear with the ear reflected anteriorly. Both provide excellent exposure to the EAC and facilitate tympanoplasty when the transcanal approach cannot give the necessary visualization. These are the primary approaches for many otologic mastoid procedures. The approach chosen depends on the surgical goals and training of the surgeon; the retroauricular approach is more commonly used in the United States. Both approaches require the use of a dressing that can be removed the next day. The postoperative recovery period is 4–5 days longer than that of the transcanal approach.
Meatoplasty and Canaloplasty
Meatoplasty involves surgery of the external auditory meatus, whereas canaloplasty is performed more medially primarily in the osseous portion of the EAC. Both involve removal of disease while restoring patency of the meatus/canal.
Myringoplasty
Myringoplasty, also known as “type 1 tympanoplasty,” refers to surgery performed on the eardrum. The aim of myringoplasty is to restore the normal functions of the TM. The procedure may be simple, such as patching small perforations, or more complex, including lasering of chronic myringitis, removal of myringosclerotic plaques, or, in the case of larger perforations, reconstructing the entire TM with a variety of grafts (temporal fascia, tragus perichondrium, etc.). The TM postoperatively can be quite variable in appearance on CT, appearing so thin as to be barely perceptible or thickened focally/diffusely. Blunting of the anterior angle of the EAC is common; this “acute” angle is formed by the TM and the adjacent anterior wall of the EAC.
Tympanoplasty
Although tympanoplasty and myringoplasty seem to be synonymous, the term “tympanoplasty” may include surgery involving the TM, ossicles, and associated ME disease.
A tympanoplasty classification developed by Wullstein in 1956 included five types.
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Type 1—Tympanoplasty with intact ossicular chain.
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Type 2—Malleus partially eroded.
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Type 3—Malleus and incus eroded, TM/graft extends to the intact stapes.
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Type 4—Stapes superstructure eroded but footplate is mobile.
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Type 5—Reconstruction with a fixed stapes footplate, a graft extending to a horizontal semicircular canal fenestration.
With the advancement of surgical techniques/instruments and development of ossicular prosthesis, the classification has been modified over the years. For example, the Type 5 tympanoplasty is now of historical interest only ( Fig. 10.2 ).
Although Wullstein’s classification describes the anatomic situation, there have been attempts to modify it with regards to prognosis and functional results. In 1971, Austin included the status of the ossicular chain as a prognostic factor for hearing results. In 1973, Belluci proposed a dual classification, also taking into consideration the presence or absence of ME disease; the spectrum extended from Group 1, a disease-free dry ear, through Group 4, a draining ear with nasopharyngeal malformation (cleft palate, choanal atresia). In 1994, Kartush described the Middle Ear Risk Index, which combines the Belluci and Austin schemes with additional prognostic factors, including the status of the TM, presence/absence of cholesteatoma, ME effusion/granulation tissue, history of prior surgery, and smoking status.
Ossiculoplasty
Ossiculoplasty reconstructs a sound-conducting mechanism between the TM/graft and oval window. The surgery performed is dependent on the ossicular deficit. Variations include the use of a prosthesis, autologous ossicle (most commonly the incus), bone cement, cartilage, etc.
In general terms, a partial ossicular replacement prosthesis extends from the TM, malleus, or incus to an intact stapes with mobile footplate. A total ossicular replacement prosthesis (TORP) extends to the footplate, a term used even if other ossicles and the stapes superstructure are present. A stapes prosthesis is a form of TORP but is often referred to as a “stapes prosthesis.” Because of the wide variety of ossicular and prosthesis combinations, it is best to describe the actual anatomic situation ( Fig. 10.2 ).
The prosthesis and materials used are numerous and are in general made from calcified particles or hydroxyapatite, metal (e.g., titanium), or plastic (e.g., plastipore). Stacked cartilage and bone cement may also be used with or without a prosthesis. Specific prostheses are not discussed here because they are numerous and in constant evolution. It is helpful to become familiar with the techniques and prosthesis used locally by your surgeons.
There are two important points regarding ossiculoplasty. First, look for continuity of the ossicular chain-prosthesis from the TM/graft to the stapes or footplate ( Fig. 10.3 ). Off-axis reformatted images are often valuable for this purpose. The ossicular reconstructions used are quite variable and not always a “straight line” or centered on the footplate ( Fig. 10.2C ). The prosthesis may extend just deep to the footplate but should not extend into the medial half of the vestibule ( Fig. 10.3C ). It is important to remember that the apparent penetration below the footplate estimated by CT may be more or less than the actual penetration anatomically, particularly in the case of a metallic prosthesis with streak artifact. Second, audiometry trumps imaging. That is to say, if there is significant conductive hearing loss (CHL), even with a “satisfactory” imaging appearance, the patient will need revision surgery.
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