Retrotympanum

The retrotympanum is the posterior compartment of the middle ear cavity (posterior to the tympanic annulus) and contains a number of important recesses.

Gross anatomy

The midline of the retrotympanum is defined by the pyramidal eminence and styloid eminence ¹. The pyramidal eminence is the most reliably visible landmark in the retrotympanum and contains the stapedius muscle ². The pyramidal and styloid eminences are sometimes connected by a bony crest called the pyramidal ridge. The styloid eminence is the upper end of the styloid process. Together, these structures separate the medial retrotympanum and lateral retrotympanum.

The medial retrotympanum contains 4 sinuses and 3 bony crests, listed superior to inferior ^1-3:

• sinuses
  1. posterior sinus (posterior tympanic sinus of Proctor)
     • posterior to oval window
     • inferior to facial nerve (Fallopian) canal
     • superior to ponticulus
  2. sinus tympani (tympanic recess)
     • inferior to ponticulus
     • superior to subiculum
  3. subtympanic sinus (sinus subtympanicus)
     • inferior to subiculum
     • superior to funiculus
  4. hypotympanum
     • inferior to funiculus
• bony crests
  1. ponticulus
     • separates posterior sinus and sinus tympani
     • extends between cochlear promontory and pyramidal eminence
  2. subiculum
     • separates sinus tympani and subtympanic sinus
     • extends between posterior pillar of round window niche and styloid eminence
  3. funiculus
     • separates subtympanic sinus and hypotympanum
     • extends from anterior pillar of round window niche toward jugular bulb along cochlear promontory

The lateral retrotympanum contains 3 sinuses and 2 bony crests, listed superior to inferior ^1-3:

• sinuses
  1. facial sinus/recess (suprapyramidal fossa of Sappey)
     • lateral to facial nerve (Fallopian) canal
     • inferior to fossa incudis
     • superior to pyramidal eminence and chordiculus
  2. lateral tympanic sinus (fossa or fossula of Grivot)
     • inferior to chordiculus
     • superior to styloid ridge
  3. hypotympanum
     • inferior to styloid ridge
• bony crests
  1. chordiculus (chordal ridge)
     • separates facial sinus and lateral tympanic sinus
     • extends between chordal eminence (where chorda tympani enters tympanic cavity) and pyramidal eminence
  2. styloid ridge
     • separates lateral tympanic sinus and hypotympanum
     • extends between chordal eminence and styloid eminence

The bony crests may take the form of a ridge (a continuous piece of bone completely dividing two sinuses) or bridge (a piece of bone with a gap between it and the wall allowing for communication between adjacent sinuses) ¹. They may be incomplete (not completely spanning from origin to destination) or absent altogether ².

Radiographic features

CT

Consistent evaluation of the retrotympanum is aided by reformats of each temporal bone in standard planes ¹:

• axial images in the plane of the lateral semicircular canal
• sagittal images
• for medial retrotympanum: sagittal oblique images in the Pöschl projection (parallel to the superior semicircular canal)
• for lateral retrotympanum: coronal oblique images in the Stenvers projection (perpendicular to the superior semicircular canal)