Tympanic Membrane (Eardrum)

Overview

• Definition: A thin, semi-transparent membrane separating the external ear from the middle ear.
• Structure: Oval, trilaminar, 0.1 mm thick, 8-10 mm in diameter.
• Position: Located at an acute angle of ~55 degrees.

Anatomy

Subdivisions

• Pars Tensa
  • Forms most of the tympanic membrane.
  • Thickened periphery forming a fibrocartilaginous rim (annulus tympanicus).
  • Notch above the rim; anterior and posterior malleolar folds attach to the lateral process of the malleus.
  • Handle of the malleus attached to the inner surface.
• Pars Flaccida (Shrapnell's Membrane)
  • Small triangular region above the lateral process of the malleus.
  • Thin, lax, pink in color.

Surfaces

• Lateral Surface: Concave towards the meatus, directed downward, forward, and laterally.
• Medial Surface: Convex, bulges into the middle ear, maximum convexity point known as the umbo.

Structure

• Layers:
  1. Outer Cuticular Layer: Hairless keratinized squamous epithelium.
  2. Intermediate Fibers Layer: Outer radiating and inner circular fibers.
  3. Inner Mucosal Layer: Single layer of ciliated columnar epithelium.

Blood Supply

• Outer Surface: Deep auricular artery (branch of the maxillary artery).
• Inner Surface: Anterior tympanic artery (branch of the maxillary artery) and posterior tympanic artery (branch of the stylomastoid artery).

Venous Drainage

• Outer Surface: Drains into external jugular vein.
• Inner Surface: Drains into transverse sinus and pterygoid venous plexus.

Nerve Supply

• Lateral Surface:
  • Anterior half: Auriculotemporal nerve.
  • Posterior half: Auricular branch of the vagus nerve.
• Medial Surface: Tympanic branch of glossopharyngeal nerve via tympanic plexus.

Development

• Develops from the tubo-tympanic recess, first ectodermal cleft, and intervening mesoderm.
• Consists of three layers derived from ectoderm, mesoderm, and endoderm:
  • Cuticular layer (ectoderm)
  • Intermediate layer (mesoderm)
  • Mucous layer (endoderm)

Clinical Correlation

• Examination: Inspection with an otoscope reveals middle ear condition.
  • Features: Color, curvature, lesions, malleus position.
  • Illuminated examination shows a cone of light in the anterior inferior quadrant from the umbo.
• Visible Structures:
  • Handle of the malleus as a yellow streak.
  • Lateral process of the malleus as a white prominence.
  • Long process of the incus as white streaks.
  • Cone of light at five o'clock position in the anterior inferior quadrant.
• Clinical Divisions: Four quadrants using imaginary lines through the umbo.
• Perforation: Can be caused by injury or otitis media.
• Myringotomy: Incision in the posterior inferior quadrant to drain pus, avoiding injury to the chorda tympani nerve.