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Ear function and CT. appearance

Most of the digital images were originally generated on an Amiga A500 for a technologists course in 1988 under the title Lend me your ears...

• Design requirements
• Protective mechanisms
• Transducer mechanisms
• Structures of the middle and inner ear
• The appearance of a few pathologies

The anatomy of the ear reflects the development of structures in the neck. There are some arches of tissue, rather like gill-slits in a fish. It is from the first two of these arches that the middle ear structures are derived. For those unfamiliar with embryology, the development of an individual from a sheet of tissue on an egg involves some pretty complex folding and cell migration. Regard it as celestial origami.

Incidentally, the inner ear mechanism is a fluid containing tubular arrangement analogous to the lateral line system in fishes. As the branchial tissue migrates into the skull, (or more accurately, as the skull grows around the other elements) there is distortion of the simple arches, each with their own vessel nerve and skeletal elements. These are 'used' to make the ear elements.

Design requirements

We know the starting materials so let us look at the design requirements:
1. We need an efficient system to conduct the sound into the ear.
2. A light-weight mechanism to conduct the sound to a fluid containing detector / transducer. We can't avoid using fluid, since the nerves need fluid and nutrients.
3. A quality transducer with a high density path to the brain.

The ability of any structure to conduct sound depends on its density, elasticity and shape. Mothers already know that the adult external auditory meatus has a resonance best suited to the transmission of the noise of a baby's cry. Those unfamiliar with resonance are advised to blow air across the lip of a bottle and to vary the amount of water while listening for the change in pitch.

The external auditory meatus slopes down to the outside, presumably to keep it clear. The external canal is lined by a combination of epithelium and periosteum. This improves the efficiency of sound transmission. Ask any Racing driver about the difference between low and high hysteresis car tyres. Grip can be improved on a dry road if the tyre will absorb as much of the impact energy as possible. The opposite is needed in the external auditory canal.

The light fragile bones of the middle ear are protected from the outside by the typanum. The air space in the middle ear is very necessary. Fluid would reduce and 'damp' the vibrations (in all meanings of the word). The Eustachian tube intermittently connects with the air-filled pharynx to keep the middle ear air-filled and is the remnant of the first pharyngeal pouch, related to the inside of the first branchial cleft which goes to form the external auditory meatus.

The typanum is a layer of skin under tension, which collects the sound energy to transfer it to the small bones in the middle ear. The shape of the tympanum reflects the shape of a cone in a loud speaker. The cone shape makes it more sensitive to higher frequencies. Its arrangement is for maximum efficiency of sound transfer to the inner ear. The system of long handled levers will magnify the force and reduce the movement. This is essential because of the incompressibility of the fluid in the inner ear transducer, the cochlea, when compared to the air. The arrangement allows a better matching of the different impedances, which would otherwise cause reflection and loss of most of the sound energy.

The bones (or ossicles) are malleus, incus and stapes which are jointed together to comprise a single mechanism of complex lever and piston. The malleolus lies antero-laterally. The incus lies more postero-medially between malleolus and incus. The bones are as light as may be possible. For example, the stapes (piston) combines the economy and grace of a gothic arch. The upper ends of malleolus and incus are accomodated in a space, which expands the middle ear cleft superiorly. This region is called the attic and leads back into the Mastoid air-cells.

The normal stapes is not usually visible on most CT sections, but its replacement by a piston in this patient illustrates the position of the stapes and its relation to malleus and incus.

Protective mechanisms

The eustachian tube and tensor tympani muscle canal run together to the middle ear cleft. This and the stapedius muscle both restrict movement of drum and ossicles. They also help to dislocate the small joints to protect the inner ear from loud noise.

The muscles and ligaments restrict and confine the effect of the bones to act as a lever. They can also restrict the movements to reduce the chance of damage to the inner ear. The complex lever is capable of dislocation at the joints between the ossicles. This dislocation is secondary to the arrangement of the three bones and the misalignment of the forces operating at each end of the chain, best seen in this coronal view. Dislocation secondary to the large vibrations of loud noises allows a mechanism for the protection of the sensitive elements of the middle ear. Walkman owners and users of loud amplifiers wear out the joints between these bones earlier than the rest of us.

The movement is not simply in and out of the oval window like a piston. The incompressibility of fluid explains the functional reason for the round window. There is some 'give' in the membrane which closes it. The eccentric arrangement of the ossicles produces a rocking movement which is greater for the louder sound, thus reducing the transmission of forces to the inner ear.

Simple thoughts on Transducer mechanisms

I am grateful that the microstructure of the inner ear cannot be seen on CT. It comprises a spiral tube with a central line of transducers, throughout its length. Each hair cell in the spiral organ has very low inertia and detects small movement, signaling this to the brain stem. I don't know exactly how it works, but I can guess why it isn't simple.

Blowing over the top of different sizes of bottles is the best way to understand the principle of the Helmholtz resonator. The sound is reflected from the end of the bottle, and is added to the sound energy that enters. The frequency that best matches the transmission of sound in the bottle and which is augmented by the geometry is the note that is heard, the standing wave. The wavelength matches twice the length of the resonating chamber or the length of a vibrating string.

There are two reasons why the ear is not a simple set of resonators:

The first is related to its complex structure, having two windows, one piston and a central membranous compartment. The distribution of motion and sound energy in the cochlea is continually changing with the reflection and generation of a travelling wave of energy. The transducers are therefore continually in motion. The energy from the lower pitched sound is greater near the apex of the coil of the cochlea and the higher pitches produce more energy near the base.

The second is the nature of the nervous system. Standing waves in resonators would not be appropriate. The nervous system fatigues quickly so a constant indicator of a stimulus would soon be ignored. With the exception of heat and tissue damage, a single nerve cell only sends a pulse when the stimulus changes. A monotone whistle in our ears continues. The neurological mechanism is therefore very sophisticated. That is why they still include a loudspeaker on ultrasound machines.

Structures of the middle and inner ear

Axial cuts are collected in the plane of the acantho-meatal line in these examples. It is considered better to reduce the lens and corneal dose and increase the angle of cut from this standard axial view. All petrous temporal bone CT. is collected, using contiguous 2mm. cuts, or a smaller slice width. Spiral CT with 2mm or below is now widely available and allows reconstructions in different planes to clarify anatomical anomalies.

This image displays both axial and coronal cuts of the middle ear to show the Scutum ( Latin for the spur that it resembles in the coronal view). The region of the attic and scutum is very important, since it is both a common site for the complications of infection and is hard to see with the otoscope. This area is easier to interpret with a coronal plane of the CT cut.

Consideration of the embryological development of the ear may clarify the course of the major vessels and facial nerve, distorted as the cranium grows in utero. This means that the jugular vein and carotid artery will diverge as the axial cuts go towards the vertex of the skull.

In all of the following examples, the lines around the CT images are the same colour as the planes they are taken from. The orientation and view of the bony parts of the inner ear is taken as viewed through the external meatus, across the typanic membrane. Axial cuts of cochlea.

The axial cuts of the semi-circular canals can be seen in the same way. Ignore the facial nerve canal for the time being.

The context of the inner ear structures can be seen in a larger image of skull base.

Coronal cuts show both the scutum and the 2.5 times turn of the coils of the cochlea to advantage. You can now see why it resembles a snail shell.
c= cochlea, i = incus, l = lateral semicircular canal, o = oval window, p = posterior semicircular canal, s = superior semicircular canal, v = vestibule.

The facial nerve is bent in two planes. The roman numerals indicate the Cranial Nerve numbers. The facial nerve is a branchial nerve of the 2nd arch structure and is distorted by the evolution of the anatomy.

It passes back through the medial wall of the middle ear. Its bony separation from the cavity of the middle ear is so thin as to be invisible on most CT images. Here is how the facial nerve appears in axial images. Even the petrosal nerve may be seen in the higher cuts near the highest coil of the cochlea in the axial cuts. The nerve passes forward eventually to the spheno-palatine ganglion.

The coronal images show how the facial nerve passes back, below the lateral semicircular canal.

or try this set of axial images of both ears:

axial images of both ears

7. (top cut)		Superior Semicircular Canal SCC.
6.		Posterior and superior SCC. genu of Facial nerve.
5.		lateral SCC. facial nerve. Tegmen typani malleus incus.
4.		Cochlea and Vestibule , scutum, long process of malleus and incus.
3.		Tensor Tympani, Lowest turn of cochlea.
2.		EAM. top of Temporo-mandibular joint.
1. (bottom cut)		External Auditory meatus and TMJ.

The cochlear aqueduct and vestibular aqueduct are additional normal structures, which can be seen. Both aqueducts pass medially to the inner surface of the petrous temporal bone and are potential communicators between endolymph and the cerebro-spinal fluid. The vestibular aqueduct houses the saccus endolymphaticus.

The appearance of a few pathologies

Atresia of the external auditory meatus is a predictable, but rare inherited abnormality.

Destruction of the scutum is an indicator of Cholesteatoma, a complication of chronic otitis media

Otosclerosis is an acquired condition. It may be generalised and classically presents as perilabyrinthine otosclerosis, but may be more localised. More correctly the condition perhaps should be called a peri-labyrinthitis or spongiosis. It is characterised by a thickened, increased bone density and fixity of the stapes with resulting deafness.

Accoustic neuromas can expand the internal auditory meatus and fill the cerebello-pontine angle. Subtle pathology may be missed. Air meatography was an attempt to show such pathology before the advent of the more superior Gadolinium enhanced MRI.

The technique involved lumbar puncture and the trickling of a small amount of air up the theca to the sub-arachnoid space in the posterior fossa. The head was tilted to allow the air to rise into the internal auditory meatus. The presence of air in the internal auditory meatus outlines vessels and nerves, revealing the neuroma in this instance.

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For further reading try Ear Anatomy.com Ian Maddison. Nov 1994, revised July 1998