Labyrinth

Is secreted by Stria vascularis [Ref: Ds. of ENT by P.L. Dhingra 5/e p12] Repeat from May 09

There are 2 main fluids in the inner ear:

• Endolymph

– It fills the entire membranous labyrinth

– Resembles intracellular_. fluid, being rich in K ions.

–  It is secreted by the secretory cells of the stria vascularis of the choclea and by the dark cells (present in the utricle and near the ampullated ends of semicircular ducts).

• Perilymph

–  It fills the space between the bony and the membranous labyrinth.

–  It resembles extra cellular fluid being rich in Na ions.

– It communicates with CSF through the aqueduct of cochlea which opens into the scala tympani near the round window.

– There are 2 views regarding its formation:

i)    It is a filterate of blood serum and is formed by capillaries of the spiral ligament

ii)     It is a direct continuation of CSF and reaches the labyrinth via aqueduct of cochlea.

Most potential route for transmission of Meningitis from CNS to Inner ear is Cochlear Aqueduct.

The scala tympani is closed by secondary tympanic membrane; it is also connected with the subarachnoid space through the aqueduct of cochlea.

Hence the infections from meninges can spread to inner ear.

A positive Rinnie’s test is seen in normal people and in sensorineural deafness, and a positive fistula test implies that the labyrinth is still functioning.

In a positive fistula test, a pressure change in the external ear canal is transmitted to the labyrinth by an abnormal opening thereby producing nystagmus and vertigo.

It also implies that the labyrinth is still functioning, and is absent when labyrinth is dead.

The abnormal opening caused by a cholesteatoma eroding into the horizontal semicircular canal, following fenestrating operations and rupture of round window membrane are associated with positive fistula test.

Hypermobile stapes footplate is associated with false positive fistula test.

Cochlear aqueduct: connects scala tympani to CSF

The labyrinth maintains connections with the central nervous system (CNS) and subarachnoid space by way of the internal auditory canal and cochlear aqueduct.

Bacteria may gain access to the membranous labyrinth by these pathways or through congenital or acquired defects of the bony labyrinth.

The labyrinthine artery (auditory artery, internal auditory artery), a long slender branch of the anterior inferior cerebellar artery (85%-100% cases) or basilar artery.

D. i.e. Anterior inferior cerebellar artery

Internal ear derives its blood supply from labyrinthine (internal auditory) artery which is usually a branch of anterior inferior cerebellar artery or sometimes of basilar artery.

Cochlear aqueduct is most potential route for spread of infection from CNS to inner earQ

Cochlear aqueduct & vestibular aqueduct are 2 bony channels (canals) that connect inner ear (cochlea & vestibule respectively) to intracranial sub arachnoid space. Vestibular aqueduct contains endolymphatic duct filled with endolymph, which does not communicate freely with CSF as it ends in a cul de sac and forms a closed space. Whereas perilymph filled cochlear aqueduct is in direct continuation with the CSF which makes it a most potential route of infection.

Hyrtle’s fissure is present in early embryonic life that normally obliterates by 24 weeks of gestation. When persistent it provides a connection from middle ear to subarachnoid space.

Bony labyrinth is an example of cartilaginous bone (i.e. a bone which ossification cartilage).

Cochlear aqueduct (Aqueduct of Cochlea) is a connection between scala tympani (containing perilymph) and the subarachnoid space (containing CSF). On occassions, particularly in young children, the Cochlear aqueduct is large and open.

Infection can spread to the inner ear from the infected CSF or vice versa, via the cochlear aqueduct resulting in severe profound hearing loss (meningitic labyrinthitis).

 Labyrinthine artery is a branch of anteroinferior cerebellar artery but can sometimes arise from basilar artery.

It supplies whole of the inner ear.

Trautmann’s triangle is bounded by the bony labyrinth anteriorly, sigmoid sinus posteriorly and the dura or superior petrosal sinus superiorly

In severe SNHL: Rinne’s test is false negative and because labyrinth is dead. Fistula test is negative.

False negative Rinne test occurs in case of severe SNHL because patient does not perceive any sound of tuning fork by air conduction but responds to bone conduction due to intracranial transmission of sound from opposite healthy ear.

Fistula Test

The basis of this test is to induce nystagmus by producing pressure changes in the external canal which are then transmitted to the labyrinth. Stimulation of the labyrinth results in nystagmus and vertigo. Normally the test is negative because the pressure changes in the EAC cannot be transmitted to the labyrinth.

Positive Fistula Test is seen in:

• Erosion of horizontal semicircular canal (Cholesteatoma or fenestration operation)
• Abnormal opening in oval window (post stapedectomy fistula) or round window (rupture of round window).
• A positive fistula test also implies that the labyrinth is still functioning.
• False-negative fistula test: Dead labyrinth, cholesteatoma covering site of fistula.
• False-positive fistula test (Positive fistula test without Fistula): Congenital syphilis, 25% cases of Meniere’s disease (Hennebert’s sign.)
• OAE is considered as best screening test as it is less time consuming, easy to perform, child does not need to be sedated and results are available immediately
• Absent OAE indicates cochlear lesion.
• If OAE are absent child is taken up for BERA which is confirmatory.

• In destructive lesions eg. trauma to labyrinth, the nystagmus is towards the opposite side.
• In irritative lesions eg. serous labyrinthitis, fistula of labyrinth, the nystagmus is towards ipsilateral side.

Extra cranial complications of CSOM are:

• Petrositis (gradenigo syndrome)
• Labyrinthitis
• Osteomyelitis of temporal bone
• Septicemia / pyaemia
• Otogenic Tetanus.
• F. Facial nerve palsy
• Acute Mastoiditis: – Postaural sub periosteal abscess

– Zygomatic abscess

-Luc’s abscess

-Citelli abscess

–  Bezold abscess

Cochlear aqueduct is a bony canal that connects the cochlea to the intracranial subrachnoid space.

Perilymph within the cochlear aqueduct is in direct continuation with the CSF and hence Cochlear Aqueduct is the most important route for meningitis to spread to the inner ear.

Ans. C: It moves when perilymph moves

Into the cupula are projected hundreds of cilia from hair cells located on the ampullary crest.

When the head suddenly begins to rotate in any direction (angular acceleration); the endolymph in the semicircular canals, because of its inertia, tend to remain stationary while the semicircular canals turn. This cause relative fluid flow in the ducts in the direction opposite to the head rotation.

Similarly When the rotation stops suddenly: the endolymph continues to rotate. And this time cupula bends in opposite direction, causing hair cells to stop discharging entirely.

The ossicles (auditory ossicles) are the three smallest bones in the body, the malleus, the incus, and the stapes.

They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea).

The inner ear within the petrous part of temporal bone consists of a membranous labyrinth enclosed in a bony (osseous) labyrinth. So, inner ear has two parts : ‑

1)   Bony labyrinth :- Cochlea, Vestibule, Semicircular canals.

2)   Membranous labyrinth :- Cochlear duct, utricle, Saccules, three semicircular ducts, and endolymphatic duct & sac.

Ans. is ‘c’ i.e., Left inner ear pathology