The anterior epitympanic recess (AER) is frequently involved in chronic infections and represents a common location of the more common pars flaccida growth [8, 25]. The complexity of the anatomical architecture of epi-tympanum necessitates accurate and deep knowledge of the surgeon about the spatial relationships present at this site, thus avoiding misguided procedure and inadvertent injury of vital structures [8, 26, 27]. HRCT has a crucial role in the preoperative detailed anatomical information [9, 28].
A thin bony plate, cog, protruding down from the tegmen tympani was found reaching antero-superior to the malleus head. The height of this bony plate was 1.5 to 3.0 mm, and when complete, it is seen to extend from the facial canal wall to the scutum . The cog is considered a dividing landmark between the anterior and the posterior epitympanic compartments, while the Tensor Tympani mucosal fold (TTF) is subdividing the anterior epitympanic compartment into anterior epitympanic recess (AER) and supra-tubal recess (STR) . Commonly, a vertical type of TTF (mucosal fold) is attached to the cog, and if this TTF is complete, the ventilation to anterior epitympanic recess can be blocked. In that scenario, removal of the cog and TTF can provide a sufficient ventilation route from the Eustachian tube to the epi-tympanum [10, 30]. Otherwise, in the abovementioned case, the only ventilation pathway will be the “isthmus” which is a narrow pathway between the epitympnum and mesotympanum. This “isthmus” or “isthmic ventilation” is provided through anterior and posterior “isthmi” in an isthmic membrane, the development of which is correlated to the development of the anterior epitympanic plate or so-called Sheehy’s cog.
This cog has different morphological variations, which are suggested to carry a good link to the embryological development of the isthmus or isthmic ventilation. A complete thin plate of “cog” is correlated to a more developed isthmic membrane and thus better isthmic ventilation. On the other hand, more incomplete shallow types of cog are associated with shorter isthmus and poor ventilation of epi-tympanum. This shows a good risk for the development of an attic cholesteatoma [31,32,33].
To best of our knowledge, few studies mentioned two different types of cog without correlation to chronic infection predisposition [10, 28]. Also, few studies investigated the radio-anatomical assessment of the KS and its predisposition to middle ear infection [13, 14] A single more recent prior study in oto-laryngology literature proposed a classification of morphological types of “cog” on an oligo-slice CT, which was followed in our study on multi-slice CT (MSCT) . The MSCT offers higher resolution and multi-planar reconstruction of multi-slice allowing reformations in orthogonal and double-oblique orientation. This is expected to provide higher visibility and better identification of types of anatomical variants under study.
In our study, retrospective CT imaging evaluation of diseased 86 temporal bones revealed 49 cholesteatomas and 37 non-cholesteatomatous CSOM. Three morphological variations of cog were noticed. It was a complete plate in 46.5%, pyramidal projection in 32.6%, while it was not visualized in 20.9% of temporal bones. Li et al.  had found a cog in all their studied patients with ME disease revealing complete and rudimentary types only. The “complete” type was most commonly seen (89%). Also, Li et al.  pointed to the correlation between types of TTF and abnormal anterior epitympanic recess aeration; however, they found no statistical difference between types of cog and types of TTF . On the other hand, El Anwar et al.  and Hong et al  made their studies on “non-pathologic” temporal bones. They reported that the cog was a constant boundary of the STR and that the “complete cog” was the commonest among the studied group, which was associated with single-cell STR; seen in (79.5%) of temporal bones [28, 31].
Lee et al.  assumed that the development of the chronic otitis media is relatively correlated to the type of the anterior attic bony plate. They found that in the simply diseased ears, the incidences were 40%, 44%, and 16% for the plate type, pyramid type, and anterior type, respectively, and in the cholesteatoma ear, 30%, 43%, and 27%, respectively, with a significant statistical difference. This study mad by Lee et al.  included normal and diseased ears. They classified cog into thin and long plate type (67%), thick pyramid type (28%), and more anteriorly located type (5%) in normal ears. Lee et al.  reported that 5% of completely healthy ears showed non-visualized cog owing to its more horizontal orientation making it hard to be visualized by HRCT. Lee et al.  explained the relation between cog morphology and ventilation that the plate type was associated with well-developed supra-tubal recess and better ventilation of the AER. The “pyramidal type” was associated with shallow or nearly absent supra-tubal recess. The “anterior type” was more frequent in cholesteatoma than in non-cholesteatoma with a significant statistical difference owing to association with less ventilation .
Our current study adopted the “cog” classification proposed by Lee et al. . In our study, the “pyramid type” was the commonest in patients diagnosed with cholesteatoma (in 45%) followed by anterior type (34.7%), while the plate type was the commonest in patients with non-cholesteatomatous CSOM (in 81.1%) with a significant statistical difference. This finding supports the etio-pathogenesis of cholesteatoma in a hypo-ventilated recess pre-disposed by incomplete types (II and III), in comparison to good ventilation facilitated by complete “cog” and isthmic membrane.
Recognition of another anatomical variant, “Köerner’s septum” before antro-mastoidectomy operations revealed to bear clinico-surgical relevance. During cholesteatoma surgery, a disregarded Köerner’s septum may be mistaken as the medial wall of the mastoid antrum resulting under-exposure of operative filed with high possibility for residual cholesteatoma [13, 14, 32]. Moreover, a prominent KS may be confused for the bony lamina covering of the sigmoid sinus with consequent inadvertent entry into antrum or facial nerve injury [13, 33, 34]. Some authors suggested that the middle portion of the Köerner’s septum is the cog itself, or an extension of it, and it was regarded as having a role in ventilation of the tympanic cavity [34, 35]. Multiple researches revealed that KS might be disrupted in more than one point along its course . In our study, 17.5% of temporal bones revealed non-visualized KS with 4 temporal bones showed coalescent mastoiditis. Köerner’s septum was recognized as a complete septum in 40.7% of our patients and as incomplete septum in 9.3% of cases .
In their study, Przewonzy et al.  reported Köerner’s septum in 6.5 to 47% of both healthy and diseased ears, while Wojciechowski et al.  reported the prevalence of Köerner’s septum as 6.58% of healthy ears, 30.4% in ears with retraction pockets, and 17.4% in simple chronic otitis media without retraction pocket [34, 35]. On the contrary, we found that the prevalence of the KS was 70.3% in patients with simple chronic otitis media and 34.7% in those with cholesteatoma, showing a significant statistical difference for being more prevalent in simple inflammation (p value = 0.002). A significant number of researches confirmed the impact of Köerner’s septum on the predisposition to chronic infection by facilitating blockage of isthmus during inflammation of the middle ear [36,37,38]. Thus, a prominent Köerner’s septum is associated with an increased risk of developing CSOM with or without cholesteatomas.
Preoperative HRCT therefore represents a mandatory guidance to the oto-surgeons. It submits information regarding the spread of disease together with detailed anatomic variations and possible complications to be avoided during surgery. The type of “cog” and Köerner’s septum may be written in the radiology report for CSOM cases [39,40,41]. The main limitation of this study was the absence of a healthy subject group to compare the incidence of cog and KS between them and diseased groups. It was difficult to differentiate rudimentary from eroded “cog” or “KS” as regard CT appearance. However, the incidence or complete erosions are a quite rare event.