The protympanum, a final common pathway between the tympanic cavity and external environment, is gaining relevance due to the ease and completeness of visualization with angled endoscopes. Two primary conformations are described, quadrangular and triangular, and new anatomic structures such as the protiniculum, subtensor recess, and protympanic spine are defined. Surgical relevance of the protympanum is described with respect to ventilation, cholesteatoma, cerebrospinal fluid leak, otic neuralgia, and surgical access to the eustachian tube.
Key points
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The protympanum, a final common pathway between the tympanic cavity and external environment, is gaining relevance due to the ease and completeness of visualization with angled endoscopes.
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Two primary conformations are described, quadrangular and triangular, and new anatomic structures such as the protiniculum, subtensor recess (SbTR), caroticocochlear recess, and protympanic spine are defined.
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The SbTR, an area of pneumatization inferomedial to the tensor tympani canal (TC) is shown to have 3 conformations: flat TC/absent SbTr (A), raised TC/shallow SbTR (B), and raised TC/deep SbTR (C).
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The protiniculum, a consistent bony ridge from the promontory to the lateral wall demarcating the transition between the hypotympanum and protympanum, is shown to have 3 conformations: ridge (A), bridge (B), and absent (C).
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Surgical relevance of the protympanum is described with respect to ventilation, cholesteatoma, cerebrospinal fluid leak, otic neuralgia, and balloon dilatation of the eustachian tube.
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Introduction
The protympanum, or bony portion of the eustachian tube (ET), is a middle ear space that lies anterior to the mesotympanum. The space is also confluent with the epitympanum superiorly, hypotympanum inferiorly, and the cartilaginous ET anteriorly.
The protympanum has been infrequently examined in the past due to its difficulty to view using the operating microscope. However, the area is now gaining relevance with endoscopic ear surgery because it can easily be seen with angled scopes. With surgery moving toward evaluation and restoration of ventilation pathways, the protympanum serves as the final common pathway between the tympanic cavity and external environment, drawing comparisons to the ventilatory function of the larynx in the airway.
Introduction
The protympanum, or bony portion of the eustachian tube (ET), is a middle ear space that lies anterior to the mesotympanum. The space is also confluent with the epitympanum superiorly, hypotympanum inferiorly, and the cartilaginous ET anteriorly.
The protympanum has been infrequently examined in the past due to its difficulty to view using the operating microscope. However, the area is now gaining relevance with endoscopic ear surgery because it can easily be seen with angled scopes. With surgery moving toward evaluation and restoration of ventilation pathways, the protympanum serves as the final common pathway between the tympanic cavity and external environment, drawing comparisons to the ventilatory function of the larynx in the airway.
Embryology
True ossification of the protympanum only commences at the 18th fetal week because of its dependence on the bone growth of the otic capsule. Earlier studies reported a contribution to protympanum development from the tympanic part of the temporal bone, while more recent studies point to development solely from the petrous part of the temporal bone.
From the 21st fetal week, 2 bony laminae develop around the carotid artery, eventually forming the carotid canal. The superior lamina is better developed, forming 2 prolongations: a superior, which is longer and connects to the tympanic annulus posteriorly, forming the lateral wall of the protympanum; and an inferior prolongation, which is shorter and forms the lateral wall of the carotid canal. Similarly, from the 23rd fetal week, the canal for the tensor tympani muscle forms from superior and inferior laminae. In addition, the tegmen tympani and promontory help to form the superior and medial walls of the protympanum, respectively.
Boundaries
The boundaries of the protympanum had been previously defined as the most anterior extent of horizontal and vertical tangents through the margins of the osseous tympanic ring. The boundaries can now be defined more clearly with angled endoscopes ( Fig. 1 ).
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Superior : tegmen tympani and entire tensor tympani canal, merging posteriorly with and including the supratubal recess if present, with the boundary defined here by the tensor fold;
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Inferior : from the protiniculum (an oblique bony ridge demarcating the transition from hypotympanum) posteriorly, extending anteriorly with the possible presence of protympanic air cells, an anterior extension of the hypotympanic cell complex;
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Anterior : confluent with the junctional and then cartilaginous portion of the ET;
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Posterior : confluent with the mesotympanum;
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Medial : lateral wall of the carotid canal, extending from the caroticocochlear recess anteriorly, with caroticotympanic vessels and nerves including anterior branches from Jacobson’s (tympanic branch of glossopharyngeal) nerve. More anteriorly, variations of false passages occur depending on pneumatization patterns.
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Lateral : bony wall separating the space from the mandibular fossa and extending to the anterior annulus, from the level of the protiniculum inferiorly to the anterior limit of the notch of Rivinus at the anterior tympanic spine.
Superior Boundary
The predominant feature of the superior boundary is tensor tympani canal. The region superolateral to the tensor canal forms the supratubal recess. The supratubal recess has a variable size or is absent, depending on the orientation of the tensor tympani fold. Önal and colleagues described 2 principal configurations of the anterior epitympanic space: type I (83%) consists of an oblique (and occasionally vertical) tensor tympani fold that creates a smaller anterior epitympanic space and the presence of a supratubal recess; type II (17%) consists of a more horizontally placed tensor fold, which does not allow for the presence of a supratubal recess. The superior boundary of the supratubal recess (if present) is the tensor fold, which is occasionally incomplete.
The tubular tensor tympani canal sits either flat or raised on the superior boundary. When an area of pneumatization is inferomedial to the tensor tympani canal, it is called the subtensor recess (SbTR). To keep endoscopic anatomic nomenclature consistent with previous descriptions of the retrotympanum, there are 3 apparent conformations possible based on the depth of the SbTR when present ( Fig. 2 ):
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Type A : Flat tensor canal, absent SbTR;
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Type B : Raised tensor canal, shallow SbTR, easily visible fundus;
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Type C : Raised tensor canal, deep SbTR, difficult to see limits of fundus.
Demarcation between a type B and type C SbTR is when the fundus of the SbTR extends superior to the midpoint of the tensor tympani canal.
Inferior Boundary
A protympanic crest of a variable site, size, direction, and shape was described by Abou-Bieh and colleagues, who found it present in 79% of temporal bones studied either directly or radiologically.
In temporal bones studied endoscopically by the authors, a bony ridge was commonly found extending from the promontory on the medial wall, across the inferior wall and merging with the lateral wall. The medial aspect of the ridge consistently marked the end of the most anterior hypotympanic air cell, and thus, the start of the protympanum.
This bony ridge has therefore been named the protiniculum (from the Latin protinus : forward, farther on), in keeping with previously described nomenclature of promontorial bony ridges in the middle ear. The protiniculum has 3 conformations ( Fig. 3 ):
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Type A : Ridge, with no air cells passing medially;
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Type B : Bridge, with hypotympanic air cells extending inferiorly into the protympanum;
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Type C : Absent, no discernible protiniculum, the hypotympanum fusing with the protympanum.
Anterior Boundary
The anterior boundary of the protympanum is a transition point into the junctional and then cartilaginous portions of the ET. It can be reliably found by following the junction of the superior and lateral walls of the epitympanum anteriorly, with the tensor tympani muscle medially, which consistently seems to ensure that false passages are not entered.
The exact point of transition at the anterior boundary however is difficult to define endoscopically. As the protympanum continues toward the anterior boundary, the tensor tympani muscle rotates 90° around the superior wall in a clockwise (right) or counterclockwise (left) fashion, taking it from a medial to a lateral position, consistent with the theory that the tensor veli palatini muscle, which attaches to the entire lateral cartilaginous portion of the ET, forms a functional unit with the tensor tympani.
Medial and Posterior Boundary
In a series of 1000 formalin-fixed temporal bones, carotid canal dehiscence on the medial wall of the protympanum has been identified in up to 7.7% of temporal bones and was more common in patients younger than 2 years and older than 40 years. The dehiscence generally arises from failure of the laminae to fuse congenitally, but microdehiscences may be secondary to bony resorption later in life. In a smaller series of 150 temporal bones, the mean thickness of the thinnest bone overlying the carotid artery was 1.5 mm (range 0–3 mm), and bulging of the carotid artery into the protympanum was barely indicated in 31%, moderately noticeable in 56%, and markedly noticeable in 13%.
Jacobson’s nerve or the tympanic branch of the glossopharyngeal nerve is a bundle of predominantly secretomotor and sensory fibers. The nerve leaves the inferior ganglion above the jugular foramen and traverses the inferior tympanic canaliculus to enter the middle ear either through or just anterior to the finiculus. The first branches of the tympanic nerve tend to occur above the round window. The nerve traverses anteromedially to form the posterior boundary of the protympanum.
The tympanic nerve is the main contributing nerve to the tympanic plexus, which lies on the promontory. The plexus is usually submucosal; however, the nerves may lie deeper and groove the bone and rarely are embedded in the bone of the promontory. The tympanic nerve provides sensation to the protympanum and ET as well delivers parasympathetic fibers that arise in the inferior salivary nucleus to the same region. The nerve exits the middle ear space, medial to the tensor tympani tendon, and becomes the lesser superficial petrosal nerve carrying the visceral motor parasympathetic fibers from the tympanic plexus to the parotid gland via the otic ganglion. Further parasympathetic supply reaches the tympanic plexus via the nervus intermedius and the facial nerve at the geniculate ganglion.
The caroticotympanic nerves and arteries exit through channels in the bone overlying the carotid canal on the medial aspect of the protympanum. The caroticotympanic nerves carry sympathetic fibers posteriorly and cross the caroticocochlear recess, between the anterior aspect of the promontory and the carotid canal, to reach the tympanic plexus. Postganglionic oculosympathetic palsy (Horner syndrome) has been reported secondary to middle ear infections, likely through an effect on these nerves.
Occasionally, a rough spine composed of either bony ridges or spicules is seen over the carotid artery prominence (see Fig. 1 ). This rough spine has been named the protympanic spine and is likely related to a fusion of the 2 laminae of the carotid canal in embryologic development. If so, it may indicate a decreased likelihood of carotid canal dehiscence in the protympanum.
Lateral Boundary
The lateral lamina separates the protympanum from the mandibular fossa. The lamina is the only boundary of the protympanum that has some contribution from the tympanic part of the temporal bone on the posterior aspect. It is more commonly convex toward the lumen, but may also be concave. A convex conformation appears to result in a narrower lumen and may obstruct view of the anterior boundary, even when using an angled endoscope. In the posterior aspect of the lateral lamina, just medial to the annular sulcus lies a shallow, relatively short and smooth recess, named the pretympanic recess.
Superolaterally is the opening of the petrotympanic (Glaserian) fissure, between the bony annulus of the tympanic and petrous parts of the temporal bone, and containing the anterior malleolar ligament (AML) and discomalleolar ligaments (DML), anterior tympanic artery, and chorda tympani nerve. The AML extends from the neck of the malleus, traverses the fissure, attaches to the capsule of the temporomandibular joint, and is closely associated with the smaller DML. The anterior tympanic artery arises from the first (mandibular) part of the maxillary artery, traverses the fissure, and gives rise to a superior, posterior, and ossicular branch. The chorda tympani nerve exits the middle ear medial to the AML and through the petrotympanic fissure in the separate anterior chordal canal, also known as the iter chordae anterius, Civinini canal, or Huguier canal, traveling superolateral to the ET to reach the lingual nerve. Dehiscence of the anterior chordal canal as well as demonstrated connections between the AML/DML and sphenomandibular ligament may allow for putative anatomic explanation of otomandibular (Costen) syndrome.
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