External Auditory Canal

External Auditory Canal


EXTERNAL ACOUSTIC CANAL

  • Length of external auditory canal is 24 mm.
  • Outer part – directed upwards, backwards and medially
  • Inner part – directed downwards, forwards and medially
  • To see the tympanic membrane – pinna has to be pulled – upwards, backwards and laterally

Cartilaginous part

  • Outer one-third (8mm)
  • Two deficiencies – fissures of Santorini – through them the parotid and superficial mastoid infections can appear in the canal or vice versa
  • Skin is thick and contains ceruminous and pilosebaceous glands which secrete wax
  • Hair is only confined to the outer canal and therefore furuncles are seen only in the outer third of the canal.
  • The cartilaginous part of external auditory canal has 2 deficiencies – the “fissures of santorini” through which infections can pass from external ear to parotid and vice versa

Bony part

  •  Inner two-thirds (16mm)
  •  Devoid of hair and ceruminous glands
  •  Isthmus – bony projection lateral to TM – foreign bodies lodged medial to isthmus are difficult to remove
  •  Antero-inferior part of bony canal may present a deficiency – foramen of Hushke – in children up to 4 years – permitting infections to and from the parotid.

NERVE SUPLY OF EXTERNAL AUDITORY CANAL

  • Anterior and superior walls – auriculotemporal branch of mandibular nerve
  • Posterior and inferior walls – auricular branch of vagus – Arnold’s nerve or Alderman’s nerve.
  • Mechanical stimulation of ear canal while cleansing it  can cause stimulation of the auricular branch of vagus nerve and may induce cough ,vomitting, or even death due to sudden cardiac inhibition.
  • Part of posterior wall – sensory branch of facial nerve – Nerve of Wrisberg 

LYMPHATIC DRAINAGE OF EXTERNAL AUDITORY CANAL

  • Pre auricular and parotid nodes 

DEVELOPMENT OF EXTERNAL AUDITORY CANAL

  • External Auditory canal is formed from first pharyngeal groove/Cleft

CLINICAL SIGNIFICANCE

Malignant Otitis Externa

  • Ear pain and drainage in an elderly diabetic patient must raise concern about malignant external otitis. Associated with swelling and inflammation of the external auditory meatus .
  • Granulation tissue may be present in the external auditory canal.This infection usually occurs in older diabetics and is almost always caused by P. aeruginosa.
  • Severe Hearing Loss is absent.ESR is used for follow-up.
  • Other features -Facial palsy, not responding to antibiotics, increased uptake on technetium bone scan .
  • Antibiotic of Choice-Antipseudomonal penicillin or cephalosporin (3rd generation piperacillin or ceftazidime) with an aminoglycoside/fluoroquinolone

Isotretinoin Teratogenicity

  • Newborn developed cleft lip and cleft palate and agenesis of the external ear canal.
  • Malformations typically involve the cranium and face, heart, central nervous system, and thymus.
  • The craniofacial malformation most strongly associated with isotretinoin—microtia or anotia—is bilateral, but often asymmetrical. These defects frequently appear in conjunction with agenesis or stenoses of the external ear canal.

Microtia

  • The congenital anomaly of the external ear which could be a small, malformed or absent external ear.
  • Corrective surgery for a new born with  microtia  is usually performed at 5-7 years of age.
  • Costal cartilage is harvested for ear reconstruction which is sufficiently developed by the age of 5-7 yrs.

Mac Ewen’s triangle

  • Mastoid antrum is marked externally on the surface by suprameatal (Mac Ewen’s) triangle.
  • Formed by Posterosuperior segment of bony external auditory canal,Temporal line(Suprameatal crest) and Tangent drawn to the external auditory meatus to Suprameatal Crest.
  • Lateral wall of Mastoid Antrum is related to External Auditory Canal

Keratosis Obturans

  • Also known as canal wall cholesteatoma.
  • Due to due to defective epithelial migration from the tympanic membrane to posterior meatal wall, which results in collection of pearly white desquamated epithelial debris with cholesterol in deep meatus.

Exostosis

  • Exostosis of External Auditory canal occurs due to recurrent proplonged cold water exposure.
Exam Question
 
  • Malignant Otitis externa-Ear pain and drainage in an elderly diabetic patient must raise concern about malignant external otitis.It is associated with tenderness in the pinna and  swelling and inflammation of the external auditory meatus.Granulation tissue may be present in the external auditory canal.This infection is almost always caused by P. aeruginosa.
  • Other features -Facial palsy, not responding to antibiotics, increased uptake on technetium bone scan.
  • Severe Hearing Loss is absent.ESR is used for follow-up.
  • Antibiotic of Choice-Antipseudomonal penicillin or cephalosporin (3rd generation piperacillin or ceftazidime) with an aminoglycoside/fluoroquinolone . 
  • Length of external auditory canal is 24 mm.
  • Use of Isotretinoin may lead to teratogenic effects like Cleft lip,Cleft Palate and Agenesis of the External ear canal. 
  • Corrective surgery for a new born with microtia  is usually performed at 5-7 years of age. 
  •  Mechanical stimulation of ear canal while cleansing it  can cause stimulation of the auricular branch of vagus nerve and may induce cough.
  • The cartilaginous part of external auditory canal has 2 deficiencies – the “fissures of santorini” through which infections can pass from external ear to parotid and vice versa.
  • Mac Even’s Triangle is formed by Posterosuperior segment of bony external auditory canal,Temporal line(Suprameatal crest) and Tangent drawn to the external auditory meatus to Suprameatal Crest.
  • External Auditory canal is formed from first pharyngeal groove/Cleft. 
  • Keratosis Obturans occurs due to defective epithelial migration from the tympanic membrane to posterior meatal wall, which results in collection of pearly white desquamated epithelial debris with cholesterol in deep meatus.
  • Lateral wall of Mastoid Antrum is related to External Auditory Canal.
  •  Exostosis of External Auditory canal occurs due to recurrent proplonged cold water exposure.
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