The Pit and the Pyramid

The middle ear has more nooks and crannies than a gnome's cave (gnomes live in caves, right?), and these are places where all kind of nastiness can hide out.  One example of such a hiding place is the sinus tympani, tucked up in the superior, posterior, and medial wall of the middle ear behind the pyramidal eminence.  The importance of the sinus tympani for radiologists is because this little nook is not always well visualized by ENT endoscopy, and it's a little cranny where acquired cholestomas, specifically the pars tensa cholestetomas, like to originate.

Hi res axial CT of the IAC

Hi res coronal CT of the IAC

There is variation in the depth of the sinus tympani from nearly flat with virtually no sinus at all, to a deep, hidden pocket... which of course is the more dangerous end of the spectrum.  The images below show how the sinus tympani is not always completely visualized during endoscopy.

In these endoscopic views, the sinus tympani (st) is not completely visualized.  Note also the pyramidal eminence (pe) and the stapedius muscle tendon (ts) exiting to attach to the stapes (s).

So why is the pyramidal eminence blocking the view? It's because it wraps around the tiny stapedius muscle (the smallest skeletal muscle in the human body). The stapedius, not surprisingly, attaches to the stapes bone, where it dampens vibrations in the stapes and decreases the volume of incoming sound.  Since it is innervated by a tiny branch from the nearby CN VII, pathologies affecting this cranial nerve (such as Bell's palsy) can result in a loss of sound dampening or hyperacusis.  Below is a schematic of the middle ear (in a non-radiologic orientation) which demonstrates the relationship of the sinus tympani, pyramidal eminence (unmarked) with its stapedius muscle exiting, and the stapes.

1. Abdel Baki F, Badr El Dine M, et al. "Sinus Tympani Endoscopic Anatomy" Otolaryngology - Head and Nexk Surgery  127: 3 158-162 (Sep 2002)
2. Tomura N, Sashi R, et al. "Noraml Variation of the Temporal Bone on High-Resolution CT: Their Incidence and Clinical Significance."  Clinical Radiology 50, 144-148 (1995)
3. Marchioni D, Alicandri-Ciufelli M, et al. "Pyramidal eminence and subpyramidal space: an endoscopic anatomical study"  Laryngoscope. 2010 Mar;120(3):557-64.