U14C1 Otitis Media Flashcards
How does the ear develop embryologically?
- The growth and development of the face is formed from the growth of pharyngeal arches, transient segmented structures that form the embryonic head.
- Cranial nerves innervate the arches early in development, and this innervation is ‘carried with’ the tissues as the migrate during morphogenesis of the face
The middle ear develops from a combination of pharyngeal arches 1 and 2, and a combination of tissues:
- Ectoderm → external auditory meatus + bones of the inner ear
- Mesoderm → otic capsule (of course, the neurons inside are ectodermal!)
- Endoderm → Eustachian tube
What is the physiology of hearing?
External ear
Sound waves exist as vibrations in air which is transmitted by pinna through the external acoustic meatus. It is then transmitted along to meet tympanic memb – lateral/external portion (which marks end of external ear) – moving tympanic memb medially
Middle ear
Transmits vibrations along ossicular chain (x3 ossicles = small bones of ear):
- Handle of malleus attaches to medial/internal portion of tympanic memb – this bone moves medially
- Transmits to head of malleus – moves this laterally
- Transmits to long process of incus which articulates with malleus – moves this medially
- Transmits this to head of stapes which articulates with incus – moves this medially & stapes attaches to oval window of cochlea (a bone in inner ear) - this moves oval window medially
= end of vibration movements (which were large amplitude low force airborne waves)
Inner ear (labyrinth)
- This movement of oval window of cochlea causes fluid wave of perilymph in scala vestibuli towards scala tympani (within cochlea bone) & endolymph in cochlear duct (organ of hearing)
- Causes basilar memb (separating scala tympani & cochlear duct) to vibrate (basilar memb has a topographic map of sound frequencies - base of basilar memb corresponds to high frequency sounds & apex to low frequency sounds
- This disruption of basilar memb is detected by mechanoreceptors (hair cells) in Organ of Corti (which is within basilar memb). Organ of Corti are hair cells (mechanoreceptors) with stereocilia arranged in height order, tethered to tectorial memb
- This causes bending of stereocilia of these hair cells – away from tallest stereocilia – causes opening of K+ channels = K+ influx from endolymph into stereocilia) = depolarisation of stereocilia (converting energy of vibrations into electrical AP)
- This depolarisation causes opening of voltage gated Ca2+ channels
- This causes vesicle fusion & exocytosis of neurotransmitters which excite afferent nerve – cochlear nerve
- Cochlear nerve passes through spiral ganglion & then ascends to ventral cochlear nucleus of medulla where it synapses
- 2nd order fibres then synapse bilaterally in superior olives of medulla
- 3rd order fibres ascend in lateral lemniscus tract to synapse in inferior colliculi of midbrain
- 4th order fibres ascend to MGN of thalamus and synapse
- 5th order fibres ascend to primary auditory cortex where are interpreted as sounds of specific frequencies (decibels)
When there is no sound – stereocilia stand straight – maintaining a low level of constant depolarisation
What are the types of hearing loss?
Conductive and sensorineural. Conductive is due to
damage to the outer or middle ears resulting in lack of
conduction of sound to the cochlea. Sensorineural loss is
due to a defect in the sensory hair cell detection within the
cochlea, or the neural coding of this detection by primary
sensory neurons located in the spiral ganglion that
project down the cochlear nerve.
What are the risk factors for otitis media?
- Age: Children, especially those under 3 years old, are more prone to otitis media due to the anatomy of their Eustachian tubes. They are shorter and narrower which makes children more prone to blockages and infections because it’s easier for fluids to accumulate in the smaller tubes.The angle at which the Eustachian tube meets the nasopharynx is more horizontal in infants and young children, which makes it harder for fluids to drain properly. It’s also closer to the nasopharynx, making it easier for bacteria and viruses to travel from the throat to the middle ear.
- Seasonal factors: It’s more common during the colder months, possibly due to increased respiratory infections.
- Exposure to smoke: Passive smoking or exposure to secondhand smoke can increase the risk.
- Attendance in daycare: Children in daycare settings may have a higher risk due to increased exposure to viruses and bacteria.
- Bottle feeding: Bottle feeding while lying down can increase the risk of fluid buildup in the middle ear.
- Craniofacial abnormalities: Conditions affecting the structure of the face or skull can increase the risk.
- Allergies: Allergic conditions like hay fever can increase the risk of otitis media.
- Genetic factors: Family history of ear infections may predispose individuals to otitis media.
- Immune system deficiencies: Weakened immune systems can make individuals more susceptible to infections, including otitis media.
- Cleft palate: Structural abnormalities in the palate can affect the Eustachian tube function, increasing the risk of middle ear infections.
What are the symptoms of cholesteatoma?
What are the symptoms of otitis media with effusion?
What are the signs of OME on investigation?
What are the differential diagnoses of an ear infection?
What are the diagnostic tests for OME?
- Pure tone audiogram
- Otoscopy
- CT
How is a pure tone audiogram interpreted?
Measures bone and air conduction hearing thresholds across a range of frequencies. It is performed by presenting pure tones with an audiometer through headphones - this is measuring air conduction. Bone conduction can be tested by placing a device behind the ear to transmit sound via mastoid bone. Across all frequencies (500 – 8000 Hz the hearing range in 0-20 dB and patients press a button when they hear the sound. It takes place in a soundproof room usually done by an audiologist. Masking is when a distraction noise (usually vibration) is used to ensure the correct ear is being tested only
What are examples of abnormal pure tone audiogram?
How is rinnes and webers interpreted?
What does an otoscopy show?
External auditory canal inspection:
- Excessive ear wax – most common cause of conductive hearing loss
- Erythema and oedema – associated with otitis externa
- Discharge – may suggest otitis externa or media (with associated tympanic membrane perforation)
- Foreign bodies – cotton buds, insects etc.
Tympanic membrane – 4 quadrants
- Colour-
Healthy – pearly grey and translucent
Erythema (redness) suggests inflammation e.g. in otitis media
- Shape
Healthy – relatively flat
If bulging, suggests increased middle ear pressure e.g. acute otitis media with effusion (often seen a meniscus line)
If retracted, suggests reduced middle ear pressure e.g. 2º to URTI
- Light reflex
Healthy - cone-shaped reflection of light in anterior inferior quadrant
Distorted or absent light reflex e.g. in otitis media due to bulging of tympanic membrane
- Perforation
Due to infections e.g. OM with effusion, trauma, cholesteatoma, insertion of grommets
Cholesteatoma typically perforates superior portion
- Scarring
AKA tympanosclerosis and can result in conductive hearing loss
How is OME managed?
-
oral amoxicillin
-Surgery- A small incision (myringotomy) is made across the tympanic membrane to allow for drainage then a grommet is placed through the tympanic membrane to allow air to pass through into the middle ear and equalise the air pressure. A grommet/tympanostomy tube is a plastic ventilatory tube that has a hollow passage in the centre that allows air to enter the middle ear so maintains airflow to the middle ear to equalise the air pressure. Eventually the grommets fall out as the eardrum grows and heals itself -
Tympanoplasty- perforation is repaired with cartilage from the ear or lining from nearby muscle
-hearing aids
What are the types of hearing aids?
