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The Temporal Bone

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The Temporal Bone Anatomic Moment The Temporal Bone Joel D. Swartz, David L. Daniels, H. Ric Harnsberger, Katherine A. Shaffer, and Leighton Mark Despite the advent of thin-section high-reso- the inner ear structures and the external lution computed tomography and, more re- environment. cently, unique magnetic resonance imaging se- Virtually all textbooks define the inner ear as quences with thin sections, T2 weighting, and a membranous labyrinth housed within an os- maximum-intensity projection techniques, seous labyrinth. The membranous labyrinth three-dimensional neuroanatomy of the tempo- contains endolymph, a fluid rich in potassium ral bone and related structures remains some- and low in sodium, similar to intracellular fluid. what of an enigma to many medical specialists, Interposed between the membranous labyrinth neuroradiologists included. Therefore, we are and the osseous labyrinth resides a supportive undertaking a series of anatomic moments in perilymphatic labyrinth. Perilymph is similar to the hope of solidifying the most important ana- cerebrospinal fluid and other extracellular tissue tomic concepts as they relate to this region. Our fluid. approach will be organized so as to consider the The osseous labyrinth consists of the bony temporal bone to represent the complex inter- edifice for the vestibule, semicircular canals, relationship of three systems: hearing, balance, cochlea, and vestibular aqueduct. The mem- and related neuroanatomic and neurovascular branous labyrinth consists of the utricle and structures. saccule (located within the vestibule), the semi- The ear originated in fish as a water motion circular ducts, the cochlear duct, and the en- detection system (1). The vestibular (balance) dolymphatic duct. The latter is a channel within mechanism becomes more complex as we the vestibular aqueduct with communications to scale the embryologic ladder and endolymph the utricle and saccule. The endolymphatic duct replaces sea water as the essential fluid. The terminates in the endolymphatic sac, which re- semicircular canals and utricle are phylogeneti- sides within a bony fovea on the posterior sur- cally older than the saccule and cochlear duct. face of the petrous bone. Perilymph is inter- As such, hearing developed as we evolved from posed between the inner surface of the aquatic to terrestrial organisms. Considering semicircular canals and ducts, between the in- these basic facts, it is not surprising that devel- ner lining of the bony vestibule and the utricle opmental malformations are substantially more and saccule, and between the endolymphatic common in the neolabyrinth (cochlear duct and duct and the vestibular aqueduct. saccule) than in the paleolabyrinth (semicircu- The cochlea has been described countless lar canals and utricle). times in the literature as containing between 21⁄2 When considering evolutionary mechanisms, and 23⁄4 turns, with the apex directed anteriolat- one also should appreciate that the tympano- erally. The uncoiled cochlea is 30 to 32 mm in mastoid compartment represents the functional length and consists of three parallel fluid-con- adaptation of the aquatic gill and that the ossic- taining channels. The central channel is the ular chain probably developed because sea membranous cochlear duct (scala media), water was no longer available to serve as which is filled with endolymph and contains the a conductive medium (1). Thus, the tym- primary sense organ for hearing, the organ of panomastoid compartment essentially repre- Corti, firmly embedded within the basilar mem- sents a reestablishment of a continuity between brane (1, 3). The cochlear duct is separated From the Department of Radiology, Germantown Hospital and Medical Center, Philadelphia, Pa (J.D.S.); the Section of Neuroradiology, Medical College of Wisconsin, Milwaukee (D.L.D., K.A.S., L.M.); and the Department of Radiology, the University of Utah, Salt Lake City (H.R.H.). Address reprint requests to David L. Daniels, MD, Neuroradiology Section, Department of Radiology/DH151, Doyne Clinic, Medical College of Wisconsin, 8700 W Wisconsin Ave, Milwaukee, WI 53226. Index terms: Temporal bone, anatomy; Anatomic moments AJNR 16:2081–2084, Nov 1995 0195-6108/95/1610–2081 q American Society of Neuroradiology 2081 2082 SWARTZ AJNR: 16, November 1995 Fig 1. Schematic diagram of the endolymph-containing membranous labyrinth (blue) within the osseous labyrinth. The space (white) surrounding the membranous labyrinth contains perilymph. Modified from Williams et al (4) and Ferner (5). AJNR: 16, November 1995 ANATOMIC MOMENT 2083 Fig 2. A–D, Schematic diagrams of the petrous bone and osseous labyrinth. Endolymph and perilymph are differentiated from the petrous bone, which has a high density on computed tomography (B) and negligible signal on T1-weighted (T1-W)(C) and T2-weighted (T2-W)(D) MR. In D, endolymph and perilymph have high signal intensity. 2084 SWARTZ AJNR: 16, November 1995 from the perilymph-containing scala tympani References by this membrane and from the perilymph-con- 1. Bergeron RT. In: Bergeron RT, Som PM, eds. Head and Neck taining scala vestibuli by Reissner’s membrane. Imaging, III. St Louis: Mosby; 1992 With computed tomography, the bony laby- 2. Schuknecht HF, Gulya AJ. Anatomy of the Temporal Bone with rinth is differentiated from the membranous lab- Surgical Implications. Philadelphia: Lea and Febiger; 1986 yrinth and labyrinthine fluid by its bone density. 3. Daube JR, Reagan TJ, Sandok BA, Westmoreland BF. Medical Neurosciences: An Approach to Anatomy, Pathology and Physiol- The bony labyrinth perhaps has the highest at- ogy by Systems and Levels. New York, NY: Little Brown and Co, tenuation numbers of any area of the body. With 1986; 357–365 magnetic resonance, endolymph and peril- 4. Williams PL, Warwick R, Dyson M, Bannister LH, eds. Gray’s Anat- ymph fluid signal allows differentiation from the omy, 37th ed. London: Churchill Livingstone; 1989 surrounding low signal of the bony labyrinth. 5. Ferner H, ed. Pernkopf Atlas of Topographical and Applied Human Anatomy. Baltimore, Md: Urban and Schwarzenberg; 1980 The initial contributions to this series of ana- tomic moments will focus primarily on ana- tomic and physiologic issues as they pertain specifically to hearing or balance..
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