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1. Endoscopic anatomy of middle ear By- dr. vartika paliwal Jr-2

2. introduction The middle ear is recognized to be a very small space, almost inaccessible by microscope in some regions.  Despite the illumination and magnification offered by the operating microscope, it has proved to have distinct limitations. The straight-line view offered by the microscope produces blind spots during middle ear surgery. These limitations can be overcome with the complementary help of an endoscope, which allows “looking around corners.” Other than a wider and clearer view of middle ear anatomy, endoscopy allows better understanding of middle ear physiology and ventilation pathways that might cause pathology if impaired.

3. The middle ear can be conceptually divided into subspaces on the basis of their relationships with the mesotympanum: T he mesotympanum is the portion that an observer can visualize through the external ear canal by the use of an otoscope or a microscope. Posteriorly to it lies the retrotympanum , superiorly the epitympanum, anteriorly the protympanum , and inferiorly the hypotympanum ( ▶ Fig.)   Right ear. Drawing showing the hidden recess of the middle ear. ct , chorda; ma, malleus; in, incus; s, stapes; pr, promontory.

4. Retrotympanum The retrotympanum is a complex structure consisting of different spaces lying in the posterior aspect of the tympanic cavity ( ▶ Fig. ).   Left ear. The position of the retrotympanic complex with respect to the tympanic cavity (the red arrow shows where the retrotympanum is located). ma, malleus; pr, promontory; s, stapes; rw , round window; et, eustachian tube; ct , chorda; in, incus; plm , posterior ligament of the malleus; pos , posterior spine; dr , eardrum; sr , supratubal recess .

5. The retrotympanum is divided into the  superior retrotympanum  and the  inferior retrotympanum  ( ▶ Fig.) by a bony crest termed the  subiculum Right ear. The subdivisions of the medial compartments of the retrotympanum . Two different portions can be distinguished: an inferior retrotympanal space composed of the sinus subtympanicus lying between the finiculus inferiorly and the subiculum superiorly; and a superior retrotympanal space composed of the sinus tympani between the subiculum inferiorly and the ponticulus superiorly, with the posterior sinus separated from the sinus tympani by the ponticulus . pr, promontory; s, stapes; et, eustachian tube; f, finiculus ; ap, anterior pillar; pp, posterior pillar; jb , jugular bulb; ss, sinus subtympanicus ; sty , styloid complex; su , subiculum; p, ponticulus ; st , sinus tympani; ps , posterior sinus; pe, pyramidal eminence; fn , facial nerve; cp, cochleariform process.

6. We can identify four spaces in the superior retrotympanum : two spaces lying medial and anterior, and two spaces lying lateral and posterior to the third portion of the facial nerve and the pyramidal eminence ( ▶ Fig) The pyramidal eminence is the fulcrum of the retrotympanum . From this structure arise two bony structures: the chordal ridge extending outward and transversally toward the chordal eminence, and separating the facial recess superiorly and the lateral tympanic sinus inferiorly; and the ponticulus extending inward and transversally to the promontory region dividing the sinus tympani inferiorly and the posterior tympanic sinus superiorly . Right ear. Drawing representing the medial and lateral spaces of the retrotympanum . pr, promontory; s, stapes; su , subiculum; p, ponticulus ; st , sinus tympani; ps , posterior sinus; pe, pyramidal eminence; fn , facial nerve; fs, facial sinus; ls, lateral tympanic sinus; rw , round window; in, incus; ma, malleus; ct , chorda.

7. Sinus tympani The sinus tympani lies medial to the pyramidal eminence, the stapedius muscle, and the facial nerve and lies lateral to the posterior semicircular canal and vestibule. The superior limit of this space is represented by the ponticulus ; the inferior anatomical limit is represented by a prominent ridge extending from the styloid eminence to the posterior rim of the cochlear window niche: the subiculum The sinus has great variability in size and shape.

8. Left ear. Transcanal endoscopic view of the medial spaces of the retrotympanum by 0° endoscope. The subiculum is a ridge of bone arising from the styloid complex to the posterior pillar of the round window niche dividing the sinus tympani from the sinus subtympanicus . pr, promontory; s, stapes; f, finiculus ; jb , jugular bulb; ss, sinus subtympanicus ; sty , styloid complex; su , subiculum; p, ponticulus ; st , sinus tympani; ps , posterior sinus; pe, pyramidal eminence; fn , facial nerve; rw , round window; in, incus; ma, malleus.

9.   Left ear. Transcanal endoscopic view of the retrotympanum and pyramidal eminence with a 0° endoscope. The ponticulus is a ridge of bone arising from the pyramidal eminence to the promontory region dividing the sinus tympani from the posterior sinus. pr, promontory; s, stapes; su , subiculum; p, ponticulus ; st , sinus tympani; ps , posterior sinus; pe, pyramidal eminence; fn , facial nerve; rw , round window; in, incus; ma, malleus; cp, cochleariform process.

10. Left ear. Endoscopic magnification of the medial boundary of the sinus tympani with a 45° endoscope. Classical shape: the sinus is located between the ponticulus and subiculum lying medial to the facial nerve and to the pyramidal process. pr, promontory; su , subiculum; p, ponticulus ; st , sinus tympani; ps , posterior sinus; pe, pyramidal eminence; fn , facial nerve; ss, sinus subtympanicus ; sty , styloid complex; pp, posterior pillar.

11. The morphology of the sinus tympani was classified on the basis of intraoperative findings and the anatomical variations of the ponticulus . The sinus tympani is classified into different types on the basis of shape as follow s- Classical shape :  the sinus is located between the ponticulus and subiculum lying medial to the facial nerve and to the pyramidal process Right ear. Sinus tympani with classical shape. pr, promontory; su , subiculum; p, ponticulus ; st , sinus tympani; ps , posterior sinus; pe, pyramidal eminence; fn , facial nerve; rw , round window; s, stapes .

12. Confluent shape : an incomplete ponticulus is present and the sinus tympani is confluent to the posterior sinus  Right ear. Sinus tympani with confluent shape. pr, promontory; su , subiculum; st , sinus tympani; pe, pyramidal eminence; fn , facial nerve; rw , round window; s, stapes.

13. Partitioned shape : a ridge of bone extending from the third portion of the facial nerve to the promontory area is present, separating the sinus tympani into two portions (superior and inferior)  Right ear. Sinus tympani with partitioned shape. pr, promontory; su , subiculum; sti , inferior portion of sinus tympani; sts , superior portion of sinus tympani; *, ridge of bone dividing the sinus tympani; pe, pyramidal eminence; fn , facial nerve; rw , round window; s, stapes; ps , posterior sinus; p, ponticulus .

14. Restricted shape : a high jugular bulb is present, reducing the inferior extension of the sinus tympani   Right ear. Sinus tympani with restricted shape. pr, promontory; st , sinus tympani; pe, pyramidal eminence; fn , facial nerve; rw , round window; s, stapes; ps , posterior sinus; jb , jugular bulb; p, ponticulus .

15. Several anatomical studies have focused on the depth of the sinus tympani. This is a very important detail because the deeper the sinus tympani the more difficult it is to achieve radical removal of cholesteatoma. For this reason it will be useful for the surgeon to study the extension of the sinus tympani before surgery. We have classified the depth of the sinus tympani into three types as follows- Type A : small sinus tympani. The medial limit of the third portion of the facial nerve corresponds to the depth of the sinus. In these cases, the sinus tympani is small and does not exhibit medial and posterior extension to the facial nerve. Type B : deep sinus tympani. The medial boundary of the sinus tympani lies medially with respect to the third portion of the facial nerve and does not exhibit posterior extension to the facial nerve. Type C : deep sinus tympani with posterior extension. The medial boundary of the sinus tympani lies medial and posterior to the third portion of the facial nerve. In these cases, the sinus tympani is very large and deep and all these patients have a well-developed mastoid.

16. Right ear. The depth classification of the sinus tympani. fn , facial nerve; rw , round window; pr, promontory; st , sinus tympani.

17. ponticulus T he ponticulus is a bony ridge extending from the pyramidal process to the promontory region, which separates the sinus tympani from the posterior tympanic sinus. The endoscopic approach allowed us to find three different variants of the ponticulus (Fig.  3 ). Classical morphology : in these patients the ponticulus is completely formed and it is like a ridge of bone extending from the pyramidal process to the promontory area; this structure represent the superior limit of ST dividing it from posterior sinus (Fig.  3 a). Incomplete ponticulus : in these cases, ST and posterior sinus are confluent (Fig.  3 b). Communicating ponticulus : in these subjects, the ponticulus is like a small bridge of bone and there’s a communication between ST and posterior sinus under it (Fig.  3 c).

18. Different morphologies of ponticulus .  a  classical morphology,  b  incomplete ponticulus ,  c  communicating ponticulus .  st  Sinus tympani,  p   ponticulus ,  rw  round window,  pr  promontory,  ps  posterior sinus,  s  stapes,  pe  pyramidal eminence,  fn  facial nerve

19.   Left ear. Ponticulus with bridge morphology, endoscopic view with 45° endoscope. White arrow shows the communicating space between the sinus tympani and the posterior sinus. pr, promontory; su , subiculum; st , sinus tympani; ps , posterior sinus; pe, pyramidal eminence; p, ponticulus ; fn , facial nerve; s, stapes.

20. subiculum The subiculum is a bony ridge extending from the posterior lip of the round window niche outward to the styloid eminence region, which separates the sinus tympani from the sinus subtympanicus . The subiculum may be present—in which case the sinus tympani is separated by the inferior retrotympanum —or may be absent, and in this case the sinus tympani is confluent with the inferior retrotympanum . The bridge subiculum is a rare conformation; when present, under this bridge of bone there is a communication between the inferior retrotympanum and the sinus tympani 

21. The subiculum is a ridge of bone arising from the styloid complex to the posterior pillar to the round window niche, dividing the sinus tympani from inferior retrotympanum . Right ear. In this case the sinus tympani is confluent with the inferior retrotympanum (sinus subtympanicus ) and the subiculum is absent. Left ear. Endoscopic views of the retrotympanum in patients with bridge subiculum (0° endoscope).

22. Pyramidal eminence and subpyramidal spaces In our experience performing endoscopic middle-ear surgery, close and variable relationships have been noted between ST, posterior tympanic sinus (PTS) and the pyramidal eminence (PE). Pneumatization of the retrotympanum may extend to a variable degree into a recess under the PE. We described this space in an unpublished work and we called this anatomical finding the ‘ subpyramidal space’ (SS). This space is limited laterally by the medial aspect of the pyramidal process, medially by the lateral wall of the tympanum, inferiorly by the ponticulus , and posteriorly and superiorly by the Fallopian canal, and it could be in direct anatomical continuity with the ST or with the PTS, depending on the position of the ponticulus . Features of this space (particularly its depth) vary significantly and we found that it could range from a total absence due to a complete development of the medial aspect of the pyramidal process, up to a clear representation of the SS with a significant depth. The more SS is deep, the more a surgical approach is at high risk of leaving residual cholesteatoma. A good knowledge of these anatomical spaces may help in reducing the risk of residual cholesteatoma during middle ear surgery.

23. The subpyramidal space.  a – b   retrotympanic space with endoscope 45° angle view in a subject with a confluent shape of the ST;  c  magnification of posterior sinus and subpyramidal space ( arrow );  d  exploration of subpyramidal space laying under the pyramidal eminence.  st  Sinus tympani,  pr  promontory,  ps  posterior sinus,  s  stapes,  pe  pyramidal eminence,  fn  facial nerve,  lc  lateral semicircular canal,  an  antrum,  ss   subpyramidal space,  PES  posterior epitympanic space,  cp  cochleariform process,  ma  malleus

24. Inferior retrotympanum Some authors have studied the anatomy of the inferior retrotympanum , but the region has been largely neglected in the literature. This is probably because of the poor accessibility of that space during conventional microscopic procedures. In fact, Proctor in his studies had already identified almost all of the structures in that region, based on several temporal bone dissections. He identified a fairly constant structure, a ridge of bone connecting the basal helix of the cochlea to the jugular wall of the tympanum, in relation to the anterior pillar of the round window niche: the sustentaculum promontorii . He called it the sustentaculum (from the Latin  sustentaculum , support) because he thought that it sustained the inferior tympanic artery, enveloping it during the development of the middle ear. The present authors have confirmed the presence of that structure in relation to the anterior pillar of the round niche, identifying two variants: a ridge shape and a bridge shape. Thus rename the sustentaculum promontorii the  finiculus  .

25. In a more cranial position lies the superior lateral attic whose inferior limit is represented by the incudo -malleolar fold. This anatomical area together with the medial attic is the so-called superior attic or upper unit. Superior attic is in communication with the mesotympanum through the underlying tympanic isthmus and posteriorly it is opened to the aditus ad antrum. Its upper limit is tegmen tympani, while the lower one is given by the second portion (intratympanic) of the facial nerve and laterally it is bounded by the lateral bony wall of the Atticus. The whole superior attic is ventilated through the isthmus. Epitympanic compartments receive their aeration via the large tympanic isthmus between the medial part of the posterior incudal ligament and the tensor tendon.

26. Left ear. Transcanal endoscopic view of the inferior retrotympanum and hypotympanum. pr, promontory; s, stapes; in, incus; ma, malleus; te , tegmen of round window niche; su , subiculum; fu, fustis ; co, sinus concameratus ; sp , sustentaculum promontorii ; et, eustachian tube; ct , chorda.

27.   Left ear. Endoscopic magnification of the inferior retrotympanum (0° endoscope) and round window niche. pr, promontory; te , tegmen of round window niche; su , subiculum; fu, fustis ; co, sinus concameratus ; sp , sustentaculum promontorii ; ma, malleus; ap, anterior pillar; pp, posterior pillar; rw , round window.

28. Tympanic isthmus.  a  magnification of the isthmus with a 0° endoscope;  b  magnification of the isthmus with a 45° angle endoscope;  c  scheme of the isthmus.  ma  Malleus,  in  incus,  s  stapes,  cp  cochleariform process,  tt  tensor tendon of the malleus,  PES  posterior epitympanic space,  ct  corda tympani,  tf  tensor fold,  fn  facial nerve,  pe  pyramidal eminence,  lc  lateral semicircular canal

29. epitympanum The epitympanic space is a pneumatized portion of the temporal bone superior to the mesotympanum. Epitympanic diaphragm” consists of three malleal ligamental folds (anterior, lateral, and posterior), the posterior incudal ligamental fold and two purely membranous folds (the tensor fold and the lateral incudomalleal fold) together with the malleus and incus. From this anatomical point of view it is possible to classify the epitympanum in two different compartments: a larger and posterior one (posterior epitympanic space—PES) and a smaller and anterior compartment (anterior epitympanic space—AES) The lateral portion of posterior epitympanum is narrower and it is divided in two further portions by the lateral incudo-malleal fold. They are separated and arranged one above the other: the superior and inferior lateral attic (Fig.  6 a–c). Inferior lateral attic is bounded superiorly by the lateral incudo-malleal fold and it is located between the more declive portion of short process and body of the incus medially and the medial aspect of the scutum laterally.  Mesotympanic region guarantees ventilation of the inferior lateral attic.

30. Epitympanic spaces and their ventilation routes.  a  posterior view;  b  medial to laterally view;  c:  lateral view.  Long curved arrow  ventilation route of the epitympanic-mastoid compartments,  short arrow  ventilation route of the Prussack space,  ma  Malleus,  in  incus,  s  stapes,  cp  cochleariform process,  AES  anterior epitympanic space,  PES  posterior epitympanic space,  pos  posterior spine,  et  Eustachian tube,  imlf  lateral incudomalleal fold,  mlf  lateral malleal fold,  tf  tensor fold,  plm  posterior malleal ligamental folds,  alm  anterior malleal ligamental folds

31. protympanum The protympanic space is a pneumatic portion of the middle ear that lies anteriorly to the mesotympanum and inferiorly to the AES (Anterior Epitympanic Space). The cochleariform process and the tensor fold with the tensor tympani canal represent the upper limit of protympanic space, while posteriorly its limit is generally considered the promontorium .  Tympanic portion of Eustachian tube starts from the protympanum . Above and medially to the Eustachian tube opening runs the internal carotid artery. Bone over this structure could be thick or pneumatized with some cells in there ( protympanic cells). Another important reason to explore this recess is that when we find protympanic cells in a patient with cholesteatoma disease involving the protympanic space, we have to pay more attention because these cells might hide the presence of cholesteatoma persistence.

32. Supratubal recess view with a 45° endoscope in the protympanic space.  a  right ear with wide perforation of the drum;  b  magnification of the protympanum with a good view of the anatomic relationship between Eustachian tube and the supratubal recess.  ma  Malleus,  in  incus,  s  stapes,  et  Eustachian tube,  sr   supratubal recess,  ct  corda tympani,  pos  posterior spine,  rw  round window,  pr  promontory,  plm  posterior malleal ligamental folds

33. CONCLUSION W ith endoscopic middle ear surgery we think that “in vivo” endoscopy of middle ear is the best way to explore the tympanic cavity and to understand the importance of ventilation routes, which we believe are the most important pathogenetic causes in chronic middle ear disease. Endoscopy allowed us to understand that ventilation in middle ear doesn’t mean just Eustachian tube function, but also isthmus blockage, complete or incomplete tensor fold and mastoid pneumatization. The goal of surgery in the chronic pathology of the middle ear should be restoration of normal ventilation of the attical -mastoid area. These were the main reasons why we did a review of middle ear anatomy, especially focused on how it appears to the surgeon while performing an endoscopic procedure .

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