Labyrinth

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.  
Fluids Endolymph Perilymph CSF
Sodium(mEq/L) 5 140 152
Potassium(mEq/L) 144 10 4
Protein(mg/dL) 126 200-400 20-50
Glucose(mg/dL) 10-40 85 70

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
Don’t Forget to Solve all the previous Year Question asked on Labyrinth

Leave a Reply

%d bloggers like this:
Malcare WordPress Security