Labyrinth

INNER EAR or LABYRINTH

• Inner ear is present in Petrous part of temporal bone .
• Inner ear bony labyrinth is an example of cartilaginous bone (i.e. a bone which ossification cartilage).
• It consists of Bony and Membranous Labyrinth.
• Membranous Labyrinth is filled with Endolymph
• Space between membranous and bony labyrinth filled with – perilymph

BONY LABYRINTH

Vestibule

• In its lateral wall lies the oval window
• Medial wall has two recesses
• Spherical recess – lodges saccule
• Elliptical recess – lodges utricle
• Posterosuperior part receives five openings of semicircular canals

Semicircular canals

• Lateral, posterior and superior semicircular canals
• Lie in planes at right angles to one another
• Responds to angular acceleration and deceleration

Cochlea

• Coiled tube making 2.5 to 2.75 turns round a central pyramid of bone called modiolus
• Three compartments – Scala vestibule, Scala tympani and Scala media
• The scala tympani is closed by secondary tympanic membrane; it is also connected with the subarachnoid space through the aqueduct of cochlea.
• Infection of CNS spreads in inner ear through Cochlear aqueduct.

MEMBRANOUS LABYRINTH

Cochlear ducts

• Also called the membranous cochlea or scala media
• Triangular in cross section and the three walls are formed by
• Basilar membrane
• Reissner’s membrane
• Stria vascularis
• Utricule and Saccule: Utricle responds to linear acceleration and deceleration or gravitational pull.
• Semicircular ducts
• Endolymphatic duct and sac.

 INNER EAR FLUIDS

2 main fluids in the inner ear:

• Endolymph

• It fills the entire membranous labyrinth
• It is secreted by the secretory cells of the stria vascularis of the cochlea and by the dark cells (present in the utricle and near the ampullated ends of semicircular ducts). 
• Endolymph has high concentration of K+ and low concentration of Na+.
• Cells in the stria vascularis have high concentration of Na+ K+ ATPase activity. 

• Perilymph

• It fills the space between the bony and the membranous labyrinth.
• 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: Filterate of blood serum,formed by capillaries of the spiral ligament or a direct continuation of CSF reaching the labyrinth via aqueduct of cochlea.  

BLOOD SUPPLY AND LYMPHATIC DRAINAGE OF INNER EAR

• 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.
• No lymphatics

CLINICAL SIGNIFICANCE OF INNER EAR

• The commonest genetic defect of inner ear causing deaf­ness is Scheibe aplasia.
• Trautmann’s triangle is bounded by the bony labyrinth anteriorly, sigmoid sinus posteriorly and the dura or superior petrosal sinus superiorly.
• This triangle is a potential weak spot through which infections of temporal bone may traverse and affect cerebellum.
• This can be used as an approach to posterior cranial fossa lesions.
• Labyrinthis can occur due to extracranial complication of CSOM.
• Fluctuating recurring variable sensorineural deafness is seen in Perilabyrinthine fistula .
• 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.

Positive Fistula Test is seen in:

• Perilabyrinthine Fistula due to 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.)

Nystagmus:

• 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.

Exam Question

• Infection of CNS spreads in inner ear through Cochlear aqueduct.
• Endolymph in the inner ear is secreted by Stria vascularis.
• 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.
• Endolymph has high concentration of K+ and low concentration of Na+.
• Inner ear is present in Petrous part of temporal bone .
• Inner ear bony labyrinth is an example of cartilaginous bone (i.e. a bone which ossification cartilage).
• The commonest genetic defect of inner ear causing deaf­ness is Scheibe aplasia.
• Trautmann’s triangle is bounded by the bony labyrinth anteriorly, sigmoid sinus posteriorly and the dura or superior petrosal sinus superiorly.
• Labyrinthis can occur due to extracranial complication of CSOM.
• Fluctuating recurring variable sensorineural deafness is seen in Perilabyrinthine fistula .

Positive Fistula Test is seen in:

• Perilabyrinthine Fistula due to 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.
• In destructive lesions of labyrinth, the nystagmus is towards the opposite side