Workbook questions 8 - Hearing Flashcards
A mother took her 27 month old child complaining that the child was not speaking as he should for his age. The GP examined the child and also arranged for his hearing be tested by an audiologist. Examination revealed that the child’s tympanic membranes were very turgid and immobile as a result of fluid accumulation within both middle ear compartments. Hearing test results showed the boy’s hearing to be below normal. The child was diagnosed as suffering from hearing loss as a result of congestion in the middle ear. The GP suspected eustachian tube dysfunction as the most probable cause of the child’s problems.
Q8-1 How would you best classify this child’s hearing loss?
Conductive hearing Loss
What are the key contents of the middle ear in as far as the sense of hearing is concerned?
The inner surface of the tympanic membrane, synovial joints of the the ossicular chain, the outer face of the oval window, tensor tympani muscle, stapedius muscle, The middle ear chamber is filled with air.
What role does the middle ear play in the transduction of air pressure waves into electrical impulses associated with the sense of hearing?
The middle ear is itself not directly involved in the actual transduction of sound energy into electrical impulses. However, the middle ear is involved in ensuring that sound pressure energy is not lost as it exerts pressure on the outer face of the tympanic membrane. Instead, the middle ear acts as a necessary transition between the outer ear (via the tympanic membrane) and the inner ear (via the oval window). Taken together, the apparatus of the tympanic membrane, ossicle chain, oval window, tensor tympani muscle and stapedius muscle, act as an impedence-matching device for sound energy travelling firstly in air (outer ear) and endolymph of the inner ear (Cochlear)
List 4 general classes of reversible and common causes of deafness
Infection within the middle ear;
Fluid accumulation due to poor drainage of the middle ear,
Trauma of the ossicles of the ear,
Fixation of any joints of the ossicle chain,
Entry of liquids into the in the middle ear such as in swimming,
Wax build up in the external auditory canal,
Deposition of foreign objects into the external auditory canal,
Tissue growth into the external and/or middle ear, Rupture/perforation of the tympanic membrane
The thing common to all these causes of hearing loss is that they each contribute to conductive hearing loss, which is by its nature, fully reversible if the offending apparatus is removed.
The child’s hearing problems arose from failure of sound energy to reach the neural apparatus of the ear. What is the justification for such a statement?.
If any fluid other than air accumulates in the middle ear, transmission of sound waves from air to these fluids will compromise (i.e.dampen down) the expected movements/displacements of the ossicle chain and in turn those of the oval window, resulting in difficulties with hearing. The “dampening” process develops when sound energy that carries or transmits signals from the sound source is absorbed and dissipated by the water fluid filling the middle ear cavity. This in turn, weakens the displacement of the ossicle chain and with it failure of the ossicles to impress upon the oval window and the endolymph. This can be summarised as conductive hearing loss
Q8
Comment on the anatomical layers and neural innervation of the tympanic membrane?
The tympanic membrane has three layers, as follows; the outer epithelial surface, the middle fibrous layer and the inner epithelial surface. The External Surface is innervated by;
a) Auriculotemporal nerve, a branch of the mandibular division of the fifth nerve and sometimes
b) a small branch of the Vagus (CN X).
The Internal Surface is innervated by the Glossopharyngeal nerve (CN IX).
(Note the separate embryonic derivations of the internal and external epithelia surfaces. The fibrous middle layer forms the skeleton of the tympanic membrane)
Liquid accumulation in the external auditory meatus (e.g. from swimming) or the middle ear is likely to lead to impairment of hearing. Making use of basic features of fluids, explain how this impairment arises
The middle ear is a fluid filled compartment. Normally it contains air. In some cases, liquids in the form of mucus, exudate from tissues or blood can accumulate within the middle ear. If any fluid other than air accumulates in the middle ear, these fluids are likely to absorb sound energy of displacement of the tympanic membrane, thereby dampening the expected displacements of the ossicle chain. If the middle ear is full of liquid, this will have a hydrostatic pressure that in turn will push against the tympanic membrane from within the middle ear, thereby resisting any possible displacement of the tympanic membrane mediated through air of the external auditory canal.. Thus, there will be increased resistance or impedence to sound energy impinging upon the tympanic membrane. This increased impedence will lead to the sound energy being absorbed by the new fluid of the middle ear and thus muffling of the transmitted sounds as movements of the ossicle chain would be reduced or completely eliminated.
Assuming this child had gone on to develop infections of the ears as a result of fluid accumulation in either of the middle ears, what important anatomical structures of the middle ear are likely to be affected if such an infection is not treated successfully?
Epithelium lining the middle ear and mastoid air-cells (mastoid antrum); 2) The Vestibuloochlear nerve; 3) The temporal bone in the vicinity of the ear
Assuming the ear infection of .this case ( Above) was not treated successfully and it spread intracranially, which fossa of the cranium would you expect to be implicated here? Justify your answer.
The middle cranial fossa as this communicates with the external environment via the auditory meati.
ssuming the ear infection of this case ( Above) was not treated successfully and it spread intracranially, how is it most likely to reach the posterior cranial fossa?
The most likely way is for the infection to travel into the brain via the trunking of the vestibulochoclhear nerve. This could lead to death of the nerve itself (leading to a neuroma of the vestibulocochlear nerve) or infection of the temporal lobe of the cerebral cortex.
The infection could spread from the middle ear to the mastoid antrum via the epithelium lining the middle ear as this is continuous with that of the mastoid antrum. Under these conditions, such an infection might eventually break into the mastoid process of the temporal bone and then to the sigmoid sinus which is found in the posterior cranial fossa.
If the infection breaks into any process of the temporal bone, this will most probably then pass to the periosteum of the bone and then the meninges which are continuous with the periosteum of the temporal bone.
Assuming the ear infection of .this case ( Above) was not treated successfully and it spread intracranially, why would this potentially lead to an emergency?
If the infection spreads to the meninges, this immediately becomes an emergency as the meninges are continuous and widely distributed intracranially and therefore would facilitate easy spread of the infection to intracranial structures.Please not also that the brain does not have an immune defence mechanism to talk of. Thus, any infections breaking out intracranially would lead to catastrophe.
If the infection spreads into the sigmoid sinus, this could then use the vascular system as a channel of spread to remote sites.
Identify 4 general functions of the auditory (or Eustachian) tube in the normal functions of the ear?
Allows equilibration of pressure between middle ear and the atmosphere
Allows ventilation of the middle ear
Drainage of mucous secretions from the middle ear to the nasopharynx
The opening of the Eustachian tube in the nasopharynx is guarded by a lymphoid organ that prevents spread of infection from the nasopharynx to the middle ear
What are the consequences of compromising Eustachian tube function?
Failure to equilibrate pressures between the middle ear and outside leading to pressure build-up, hence pain most probably in the inner surface of the tympanic membrane
Failure to drain secretions or fluids from the middle ear will lead to fluid accumulation in the ear and a compromise to hearing ability. This could eventually promote infections to establish within the middle ear leading to further threats to the health of intracranial structures.
What muscle opens the auditory (Eustachian) tube and when does that happen?
Tensor Veli Palatini &/or The salpingopharyngeus muscle
What is the origin and pathway of the sympathetic supply to the face?
Pre-ganglionic sympathetic fibres destined to the face and head arise from the lateral horn of spinal segments T1 / T3, (see fig 6-2). They enter the sympathetic chain at these levels but the post-ganglionic fibres exit the sympathetic chain in the neck and are distributed with the carotid artery and its branches to innervate structures in the face & head.
