The mastoid air cells are a network of small, interconnected, air-filled pockets located within the skull behind the ear. These cells are a unique feature of the mastoid process, the prominent bony projection of the temporal bone you can feel directly behind your earlobe. This complex structure is an important component of the skull that connects directly to the middle ear. Understanding their structure and function is particularly relevant when considering ear health and certain types of infection.
Anatomy and Location within the Temporal Bone
The mastoid air cells are housed inside the mastoid portion of the temporal bone, which is a thick segment of the skull. This bone is characterized by a high degree of “pneumatization,” meaning it is filled with air spaces rather than solid bone marrow. The entire system is often described as having a honeycomb-like appearance due to the multiple bony septa, or walls, that separate the individual air cells.
Development of this pneumatic structure begins in infancy, as pneumatization gradually expands the air cells into the surrounding bone. At birth, the mastoid area contains only a single large cavity called the mastoid antrum. The full extent of the air cells develops significantly between the ages of two and five and typically completes by age six. The cells themselves are lined with a delicate mucous membrane that is structurally continuous with the lining of the middle ear cavity.
A key anatomical feature is the connection of the mastoid air cells to the middle ear through the mastoid antrum and a narrow passage called the aditus ad antrum. This connection means that the air within the mastoid system is shared with the middle ear space, allowing for free air exchange. The size and extent of this air cell system can vary significantly from person to person, sometimes being extensive and sometimes minimal or even absent.
Physiological Roles of the Air Cells
The primary physiological purpose of the mastoid air cells is to act as a gas reservoir, which helps to maintain pressure equilibrium within the middle ear. The middle ear constantly exchanges gas with the surrounding tissues, which naturally causes a pressure drop. The large volume of the mastoid air cell system acts as a buffer, slowing the rate of pressure change and reducing the frequency with which the Eustachian tube must open.
This buffering capacity is directly related to the volume of the air cells; a larger, more pneumatized mastoid is more effective at diluting pressure changes than a smaller one. The extensive mucosal lining within the cells also contributes to the regulation of temperature and gas pressure. The mastoid air cells also play a role in the acoustic properties of the ear, contributing to sound dampening or resonance that modifies sound transmission.
The extensive air-filled spaces also serve a structural function by reducing the overall weight of the skull. The air cells provide a lighter yet structurally sound framework for the mastoid process. Another function is protecting the sensitive inner ear structures from rapid external temperature changes.
Mastoiditis and Clinical Significance
The interconnected, honeycomb structure of the mastoid air cells becomes a clinical concern when an infection from the middle ear spreads into this area, a condition known as mastoiditis. This infection most often begins as a complication of an untreated or severe acute middle ear infection, or otitis media. Bacteria migrate from the middle ear through the connecting passages, causing the air cells to fill with infected material and pus.
The multiple bony partitions that define the air cells make treatment challenging because they create small compartments where bacteria can hide, preventing antibiotics from reaching the entire infected area effectively. Symptoms of mastoiditis typically appear days or weeks after the initial ear infection. These symptoms include persistent, throbbing pain, ear drainage, and noticeable redness and swelling behind the ear. This swelling can sometimes be severe enough to cause the ear to protrude outward.
If the infection progresses, it can lead to serious complications because the mastoid bone is in close proximity to sensitive structures. Untreated mastoiditis can result in the destruction of the mastoid bone and can spread to the inner ear, causing hearing loss or vertigo. The infection can also spread into the cranial cavity, potentially leading to a brain abscess, meningitis, or a blood clot, which are all life-threatening conditions.
Initial treatment involves strong antibiotic therapy, often administered intravenously in a hospital setting to ensure the medicine reaches the deep infection. If antibiotics fail to resolve the issue or if complications arise, surgical intervention may be necessary. This procedure, called a mastoidectomy, involves drilling into the mastoid bone to remove the infected air cells and drain the pus, preventing the infection from spreading further into the skull.