The tympanic membrane, commonly known as the eardrum, is a delicate, thin sheet of tissue separating the outer ear and the middle ear cavity. It is roughly cone-shaped and pulled inward toward the middle ear space. Its primary function is to capture sound waves entering the ear canal, causing vibrations. These vibrations are then transferred to the tiny bones of the middle ear, initiating the process of hearing.
Location and Structure
The eardrum is positioned at the medial end of the external auditory canal, forming the lateral wall of the air-filled middle ear cavity. It is a slightly oval structure, about one centimeter in diameter, and is angled obliquely within the ear canal, facing forward and downward. This positioning helps protect the middle ear.
Structurally, the membrane is divided into two parts: the pars tensa and the pars flaccida. The pars tensa is the largest and most taut portion, responsible for the majority of sound transmission due to its three layers. The smaller pars flaccida sits near the top of the membrane and is looser, lacking the organized fibrous layer found in the pars tensa.
The manubrium, or handle, of the malleus—the first of the three tiny middle ear bones—is firmly attached to the inner surface of the eardrum. This attachment pulls the membrane inward, creating a central depression known as the umbo. The malleus handle is a constant visual landmark that helps orient an observer looking into the ear.
The Appearance of a Healthy Eardrum
When viewed through an otoscope, a healthy eardrum is generally pearly gray or translucent. The surface is smooth and slightly glossy, giving it a highly reflective sheen. This translucency allows the examiner to see structures within the middle ear behind it.
The most characteristic feature is the “cone of light,” or light reflex. This is a bright, triangular reflection caused by the otoscope’s light source bouncing off the membrane’s concave, reflective surface. The apex of this reflection originates at the umbo and extends outward into the anteroinferior quadrant. This distinct light pattern indicates the eardrum is properly positioned and taut.
Behind the translucent membrane, the whitish, linear shaft of the malleus handle is clearly visible. The edges of the eardrum are bordered by a dense, whitish ring called the annulus. The superiorly located pars flaccida may occasionally appear slightly pinkish compared to the pearly gray pars tensa, but it remains intact and mobile.
How the Eardrum Reacts to Injury or Illness
Changes in the eardrum’s appearance are often the first sign of injury or disease in the middle ear. Acute otitis media, a common ear infection, causes the membrane to become intensely red, replacing the pearly gray color. Pressure from fluid and pus accumulating in the middle ear forces the membrane to bulge outward, distorting or eliminating the cone of light.
When fluid persists in the middle ear without active infection, the eardrum takes on a dull, amber or yellowish-gray hue. This fluid may appear as an air-fluid level or small bubbles trapped behind the tissue. In these cases, the membrane is often retracted inward due to negative pressure, and its mobility is significantly decreased.
Physical trauma or severe infection can lead to an eardrum perforation. This injury immediately relieves the pressure and pain associated with a bulging eardrum, often resulting in drainage of pus or mucus from the ear canal. A perforation compromises the membrane’s integrity, allowing a direct view into the middle ear cavity and sometimes revealing the underlying ossicles.