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Tuesday, June 24, 2008

External ear anatomy

This is the first in the series of posts on ENT,which will deal with Basics as well as high yield facts & clinically important points.But before going to the topic ,i want to explain you the reason behind clubbing of the three regions Ear .Nose & Throat into 1 divison of medicine.

We all know that the cavities of ear ,nose & throat all are interconnected to each other, so due to this, any disease(like infection) in any one region is more likely to affect the other two regions .So when you see a patient with any disease affecting any of them ,we have to search the other 2 also, to exclude any spread or associated signs & symptoms.Hence we have clubbed them into one single division-ENT.

The ear is divided into:

  • external ear
  • middle ear
  • internal ear or labyrinth.

today in this post we are going to deal with only external ear anatomy.majority of it will be represented with help of pictures to understand it & relevant clinical points will be written.

External ear:

  • auricle/pinna
  • external acoustic canal
  • tympanic membrane


auricle:

  • entire auricle(except lobule) & outer part of external auditory canal are made up of a single framework of yellow elastic cartilage.
  • no cartilage b/w tragus & crus of helix-this area is incisura terminalis.
  • so now we have 2 areas devoid of cartilage in pinna:
  1. lobule(consists of fat)
  2. incisura terminalis

clinical aspect:

  • incisura terminalis is site for endaural incision(since no cartilage is damaged) to expose middle ear & this incision can be extended backward to the mastoid.
  • pinna is source for graft materials

external acoustic meatus:

  • length-24mm along posterior wall
  • not a straight tube(S shaped)
  • 2 parts-
  1. outer part-directed upwards,backwards & medially
  2. inner part-directed downward,forward & medially.

clinical:therefore to see tympanic membrane ,pinna is pulled upwards,backwards & laterally to bring the 2parts in alignment.

outer part inner part
made of cartilage bony
length 8mm 16mm length
skin covering is thick thin & continues over tympanic membrane
glands ceruminous & pilosebaceous glands present absent
hair present absent
clinical aspects in all individuals 2 deficiences-fissures of santorini located antero-inferiorly:through them parotid or superficial mastoid infections can spread to canal or vice versa antero-inferior part of bony canal may have a deficiency(foramen of huschke) in children upto age 4 or sometimes in adults,permitting infections to & fro parotid
furuncles(staph infection of hair follicles) can occur(since hair follicles present) don't occur(since hair absent)

note:

  • In the cartilaginous canal, the skin contains sebaceous and apocrine glands with many hair follicles.
  • Together, the hair follicle, sebaceous gland, and apocrine gland are termed the apopilosebaceous unit.
  • Invagination of the epidermis forms the outer wall of the hair follicle. The space between this outer wall and the hair shaft is termed the follicular canal.
  • The excretory ducts of the sebaceous and apocrine alveoli drain into these follicular canals.
  • In the normal ear, the secretions of these glands, combined with the desquamated keratin layer from the stratum corneum, form a water-repellant, acidic, waxy coat of cerumen that serves as a barrier against infection and injury to the skin.
  • Motion of the ear canal provided by ordinary chewing movements together with the process of epithelial proliferation and lateral migration propel the cerumen outward in a self-cleansing manner.

tympanic membrane:

  • obliquely present: posterosuperior part more lateral than its antero-inferior part
  • height=9-10mm
  • width=8-9mm
  • thickness=0.1mm
  • 2parts:
  1. pars tensa
  2. pars flaccida(shrapnel's membrane)

layers of tympanic membrane:3 layers

outer epithelial layer continuous with skin of external auditory canal
middle mucous layer enloses handle of malleus(has 3 types of fibres-radial,circular,parabolic)
inner fibrous layer continuous with mucosa of middle ear

landmarks seen on tympanic membrane:

  • Handle of malleus which slants in posterior direction.
  • Cone of light which gets reflected in anteroinferior quadrant.
  • Umbo - the most prominent part of tympanic membrane
  • Anterior and postrrior malleolar folds - above these the tympanic membrane is called Pars Flaccida or sharpnell's membrane and below these is called pars tensa.
  • Long process of incus and incudo-stapedial joint.
  • Anterior process of malleus.

arterial supply:

  • It receives its arterial blood supply from the superficial temporal and posterior auricular branches of the external carotid artery.
  • The deep auricular branch of the internal maxillary artery serves the more medial canal and lateral TM.

Venous drainage :

  • via the superficial temporal and posterior auricular veins.
  • The posterior auricular usually drains into the external jugular vein, but may also drain to the sigmoid sinus through the mastoid emissary vein.

lymphatic drainage :

  • important with regard to the spread of infection and cancer.
  • Inferiorly, the canal drains into the infra-auricular nodes posterior to the angle of the mandible.
  • The anterosuperior canal empties into the preauricular nodes of the parotid and superior deep cervical nodes.
  • Posteriorly, the lymphatics drain to postauricular and superior deep cervical nodes.
  • Finally, the lymphatics of the antihelix and concha empty into the nodes along the apex of the mastoid process, whereas those of the superior part of the auricle drain into postauricular nodes.

nerve supply:

pinna:

Sensory

  • The auriculotemporal nerve (from cranial nerve V) to the tragus,crus of helix & adjacent part of helix.
  • The lesser occipital nerve (C2) to the upper 1/3 of the medial surface.
  • The greater auricular nerve (C2,3) to the posterior part of the lateral and most(lower 2/3rd) of the medial surfaces.
  • Arnold's nerve(auricular branch of Vagus) to the area around the concha at the external meatus & corresponding area on medial surface.
  • facial nerve -distribute with arnold's nerve,supplies concha & retroauricular groove.

external auditory canal:

Nerve supply: Sensory only

  • Anterior wall & roof:auriculotemporal nerve.
  • Posterior wall & floor: Arnold’s nerve(auricular branch of vagus nerve)
  • posterior wall of auditory canal also receives sensory fibres of facial nerve through auricular branch of vagus

tympanic membrane:

Nerve supply: Sensory only

  • anterior ½ of the lateral surface: The auriculotemporal nerve.
  • posterior ½ of the lateral surface: Arnold’s nerve
  • Medial surface: Jacobson’s nerve.(Tympanic branch of glossopharyngeal nerve)

note:Arnold's cough reflex-due to irritation of auricular branch of vagus nerve.also known as Ear cough.


4 comments:

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