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
  1. In its lateral wall lies the oval window
  2. Medial wall has two recesses
  3. Spherical recess - lodges saccule
  4. Elliptical recess – lodges utricle
  5. Posterosuperior part receives five openings of semicircular canals
  • Semicircular canals
  1. Lateral, posterior and superior semicircular canals
  2. Lie in planes at right angles to one another
  3. Responds to angular acceleration and deceleration
  • Cochlea
  1. Coiled tube making 2.5 to 2.75 turns round a central pyramid of bone called modiolus
  2. Three compartments – Scala vestibule, Scala tympani and Scala media
  3. The scala tympani is closed by secondary tympanic membrane; it is also connected with the subarachnoid space through the aqueduct of cochlea.
  4. Infection of CNS spreads in inner ear through Cochlear aqueduct.
MEMBRANOUS LABYRINTH
  • Cochlear ducts
  1. Also called the membranous cochlea or scala media
  2. Triangular in cross section and the three walls are formed by
  3. Basilar membrane
  4. Reissner's membrane
  5. 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
  1. It fills the entire membranous labyrinth
  2. 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+.
  3. 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.
  1. This triangle is a potential weak spot through which infections of temporal bone may traverse and affect cerebellum.
  2. 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 
  1. 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.
  2. 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.
  1. False-negative fistula test: Dead labyrinth, cholesteatoma covering site of fistula.
  2. False-positive fistula test (Positive fistula test without Fistula): Congenital syphilis,25% cases of Meniere's disease (Hennebert's sign.)
  • Nystagmus:
  1. In destructive lesions eg. trauma to labyrinth, the nystagmus is towards the opposite side.
  2. 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
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