The Epitympanic Compartments (Anterior and Posterior Epitympanum) and the Concept of Upper and Lower Units

The epitympanum is divided into 2 compartments: a large posterior compartment and a smaller anterior compartment. The demarcation between the anterior and posterior epitympanum depends on the anatomic variations of important structures such as the cog and the tensor fold. In most people, the demarcation between the anterior and posterior epitympanum is represented by the transverse ridge or cog. The cog is a bony septum that detaches from the tegmen tympani cranially, leading vertically toward the cochleariform process in front of the malleus head.

Much of the posterior epitympanic volume is occupied by the body and short process of the incus together with the head of the malleus. The lateral portion of the posterior epitympanum is narrow and is divided by the lateral incudomalleolar fold in 2 further portions, the superior and inferior lateral attic, positioned separately one above the other.

The incudomalleolar fold originates at the posterior extremity of the short process of incus and the lateral portion of the posterior incudal fold, continuing anteriorly between the body of the incus, the head of the malleus and the lateral aspect of the attic. At this level, the fold bends inferiorly, joining the posterior malleolar ligament fold and the lateral malleolar ligament fold, with which it forms the medial and superior aspect of the Prussak space ( Fig. 1 ).

The malleolar ligament folds and membranous folds representing the complete epitympanic diaphragm, and the 2 major middle ear ventilation pathways of the epitympanic compartments ( blue arrow ) and Prussak space ( orange arrow ). ( A ) Anterior view. ( B ) Posterior view. ( C ) Axial view according to Palva. ( D ) Lateral view of Prussak space. AES, anterior epitympanic space; amlf, anterior malleal ligamental fold; as, anterior spine; bin, body of the incus; cp, cochleariform process; ct, corda tympani; et, eustachian tube; fn, facial nerve; hma, head of the malleus; imlf, lateral incudomalleal fold; in, incus; ma, malleus; mlf, lateral malleal ligamental fold; PES, posterior epitympanic space; pil, lateral and medial posterior incudal ligaments; plm, posterior malleal ligamental fold; prs, Prussak space; ps, posterior spine; s, stapes; sr, supratubal recess; tf, tensor fold. The orange arrow indicates the major ventilation pathway through the posterior pocket.

The inferior lateral attic is bounded superiorly by the lateral incudo-malleolar fold. This anatomic area is therefore in a lower position than the epitympanic diaphragm in communication with the underlying mesotympanum. Ventilation of the inferior lateral attic is provided by the mesotympanic region. In a more cranial position than the inferior lateral attic lies the superior lateral attic, whose floor or inferior limit is represented by the incudomalleolar fold. Together with the medial attic, this anatomic area is called the superior attic or upper unit.

The 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 the tegmen tympani, the lower limit is the second (intratympanic) portion of the facial nerve, and laterally it is bounded by the bony lateral wall of the atticus. The superior attic is therefore ventilated through the isthmus ( Fig. 2 ).

Right ear. The posterior isthmus after posterior atticotomy maintaining the integrity of the ossicular chain (the red arrow represents the main ventilation route from the ET to the antrum through the isthmus). aes, anterior epitympanic compartment; cp, cochleariform process; fn, facial nerve; in, incus; lsc, lateral semicircular canal; ma, malleus; pe, pyramidal eminence; pes, posterior epitympanic compartment; s, stapes; tf, tensor fold; ttc, tensor tympani canal.

The lower unit is formed by the reduced space represented by the Prussak space, which is separated in its anatomy and ventilation from the upper unit by its vault, represented by the lateral malleolar ligament fold. This inferior epitympanic portion is ventilated in most cases from the posterior pocket through mesotympanum (see Fig. 1 panel D).

The 2 ventilatory trajectories of the epitympanic units are therefore separated from each other; this pathophysiologic concept is important in transcanalar endoscopic surgery, because surgical treatment is based on the restoration of ventilation and on the unification of the upper unit with the lower unit, through the creation of a large tympanic isthmus and an accessory route through the tensor fold. The surgical solution must ensure the ventilation of all parts of the epitympanum.

The anterior epitympanum is delimited anteriorly by the root of the zygomatic arch (a thick bony plate that separates it from pericarotic cells), superiorly by the tegmen tympani (which separates it from the meninges), laterally by the tympanic bone and chorda tympani, and medially by a bony wall that separates it from the geniculate fossa, which contains the homonymous ganglion. Its inferior limit is represented by the tensor fold, which, if complete, separates it from the underlying sovratubaric recess. The tensor fold presents a variable anatomy: according to Palva and colleagues, the tensor fold is incomplete in only 25% of cases, allowing an alternative ventilation route directed from the sovratubaric recess toward the attic ( Fig. 7 ). It extends laterally from the semicanal of the tensor tympani muscle to the lateral aspect of the protympanum, posteriorly adhering to the cochleariform process and to the tensor tympani tendon, and extends anteriorly to the root of the zygomatic bone to provide the epitympanic floor.

If it inserts on the transverse crest, its direction is almost vertical, whereas, if inserts on the tubaric tegmen, its direction is horizontal.

In most cases the curvature is about 45° and its most frequent insertion lies at the central portion of the anterior sovratubaric-epitympanic tegmen. According to our observations on patients affected by attic cholesteatomas, a complete tensor fold has been observed in almost all the patients studied, and the direction of the fold was in most cases horizontal. In general, the width of the underlying sovratubaric recess varies depending on its angle ( Fig. 3 ).

Right ear. The variations in size of the supratubal recess. The dimensions of this recess depend on the inclination of the tensor fold ( right ). The more vertical the tensor fold, the wider the supratubal recess. When the tensor fold is a horizontal, the supratubal recess is not present ( left ). et, eustachian tube; in, incus; is, isthmus; ma, malleus; s, stapes; sr, supratubal recess; tf, tensor fold.

During surgery, in sectorial ventilatory disorders caused by isthmus block, it is essential to create an alternative direct ventilatory route between the protympanum and anterior attic from a section of the central portion of the tensor fold.

The anterior epitympanum can be formed from a single large air cell, or by several small air cells, and this makes the anterior epitympanum a variable anatomic space in an anterior-posterior direction. In a recent study conducted at our clinic, subjects affected by cholesteatoma limited to the attic showed a reduced volume of the bony boundaries of the anterior epitympanum. The small anterior epitympanic cavities might be proof of selective attic dysventilation.

Epitympanic Diaphragm and Epitympanic Ventilation Patterns

The concept of epitympanic diaphragm was raised for the first time by Lemoine in 1950 ; the investigators described that the diaphragm was made up of various structures and membranous ligaments that, together with the malleus and the incus, form the floor of the epitympanic compartment.

The investigators also described the Prussak area as a structure located inferiorly to this diaphragm, therefore dividing it from the epitympanic compartments by the lateral malleolar ligament fold, considered the Prussak space roof.

In addition to the folds described by Lemoine, Palva added 2 more duplicated folds: the tensor fold and the lateral incudomalleolar fold. The complete diaphragm therefore comprises the 3 malleolar ligament folds (anterior, lateral, and posterior), the posterior incudal fold, and the 2 duplicated membranous folds (tensor fold and the lateral incudomalleolar fold) associated with the incus and the malleus.

Palva classified Proctor’s anterior tympanic isthmus as a single entity that is always present, and defined it simply as the tympanic isthmus, whereas the ventilation route posterior to the incus, an irregular feature named by Proctor the posterior isthmus, provides inconsistent ventilation from the fossa incudis.

The tympanic isthmus described by Palva represents a wide ventilation route for the epitympanum, excluding the Prussak space.

This structure extends anteriorly from the tensor fold to the pyramidal process (inferiorly and posteriorly) and to the medial portion of the posterior incus ligament (superiorly and posteriorly). Its medial limit is the attic bony wall and the lateral limit is the body of the incus, the incus short process, and the head of the malleus.

The space bounded by these structures is called the middle attic, which becomes the mesotympanum inferior to the body incus.

The anterior portion of the tympanic isthmus, superior to the level of the tensor tympani tendon, represents a wide communication with the anterior epitympanum. In healthy ears, the tympanic isthmus is an open structure with no fold.

Although Palva noticed a wide opening just behind the incus short process in 25% of Proctor posterior tympanic isthmus, in most cases this potential posterior ventilation route was blocked by the posterior incus ligament fold.

Given this, all the compartments leading to the epitympanic diaphragm receive air through the only ventilation way that is always present, the tympanic isthmus route, located between the medial aspect of the posterior incus ligament and the tensor fold.

However, Palva noticed that membranous folds that formed the epitympanic diaphragm could have structural defects resulting in incomplete folds.

In this way, additional ventilation patterns arise for the structures above the epitympanic diaphragm. Most of the defects were of the tensor fold (29% of cases), followed by the lateral incudomalleolar fold in its anterior portion (15% of cases).

The Prussak Space

The medial and inferior aspects of the Prussak space are formed respectively by the neck and the short process of the malleus.

The superior limit is the fold of the lateral malleolar ligament, which also represents the floor of the lateral malleolar space; this ligament inserts laterally on the medial wall of the scutum. The lateral malleolar ligament fold is integral in most case, because according to Palva only 19% of subjects showed a defect in the anterior portion of the fold, whereas, in some rare cases (7% of the subjects examined by Palva), the defect was in the posterior portion of the ligament, the latter involving the lateral malleolar ligament fold, creating a communication between the upper epitympanic portion (upper unit) and the lower (lower unit), resulting in a communication between the Prussak space and the lateral malleolar space and creating a change in the classic epitympanic diaphragm.

The anterior aspect of the Prussak area is bounded by a thin, membranous fold among the tympanic membrane and the anterior malleolar ligament fold, which inserts laterally on the tympanic membrane and medially on the neck and long process of the malleus.

In some cases, this fold is absent, causing a further anterior ventilation trajectory to the Prussak space.

The lateral aspect is represented by the Sharpnell membrane.

The posterior wall is represented by a large posterior pocket (the posterior pocket of von Tröltsch), which is the main route of ventilation. This posterior pocket is bounded laterally by the pars tensa and pars flaccida of the tympanic membrane, and medially by the posterior malleolar ligament fold, which originates from the posterior portion of the malleus neck and the upper third of the malleus handle and inserts posteriorly in the posterior tympanic spine ( Fig. 1 ; Panel D). This posterior pocket develops in a posterior-inferior direction and opens at the most cranial portion of mesotympanum, so, in most people, ventilation of the Prussak space occurs through the communication with the mesotympanum, the only ventilation route that is separated from the epitympanic upper unit. This ventilation route of the inferior epitympanic compartment through the posterior pocket of von Tröltsch is rough and narrow, especially compared with the ventilation route through tympanic isthmus, which aerates the upper epitympanic compartment and is wider. For these reasons, the possibility of anatomic reduction of the passage until the closing of the posterior pocket is plausible, especially the presence of thick and viscous secretions within the Prussak space that could cause a chronic sectorial dysventilation associated with a retraction of the Sharpnell membrane and its adhesion with the malleus neck.

For these reasons, although the Prussak space is anatomically inseparable from the epitympanum, in terms of ventilation and drainage, it represents an independent unit.

This space may have a block and/or an obliteration without any involvement of the compartments above the epitympanic diaphragm, like the anterior and posterior epitympanum, the aditus, and mastoid cells.

Palva dissected subjects on ventilation tubes for epitympanic retraction, showed that, despite the surgical treatment, an attical dysventilation was still present.

Given these phenomena, Palva assumed that behind the genesis of attical cholesteatoma there was a progressive closure of the ventilation route of the inferior epitympanic unit (lower unit) initially derived from mucous tissue inflammation in the posterior pocket and Prussack space, then from the granulation tissue formation that progressively causes a total block to the passage of air from this route. These events could lead to retraction of the Sharpnell membrane toward the malleus neck. The positioning of a ventilation tube causes an improvement in mesotympanic and hypotympanic ventilation, but does not address the blockage in the posterior pocket, and the process of retraction would be irreversible. It is still debated whether these phenomena are sufficient to cause an attical cholesteatoma.