Causes

Acute otitis media (middle ear infection) is usually due to a combination of factors that increase susceptibility to infections by specific organisms in the middle ear. The infection typically evolves as follows:

  • The primary setting for ear infections is in a child's Eustachian tube, which runs from the middle ear to the nose and upper throat. The Eustachian tube is shorter and smaller in children than adults, and therefore more vulnerable to blockage.
  • Changes in middle ear pressure occur in about two-thirds of children with colds. Colds are caused by viruses, such as the rhinovirus. Viruses are considered important in many ear infections, but usually only set the scene for a bacterial infection.
  • However, many bacteria normally thrive in the passages of the nose and throat. Most are not harmful. In fact, some can even block harmful bacteria from getting out of control. An additional defense system in the airways, such as mucus, prevents the harmful bacteria from spreading and infecting deeper passages, such as those in the ear.
  • If a cold does occur, the virus can cause the membranes along the walls of the inner passages to swell and obstruct the airways. If this inflammation blocks the narrow Eustachian tube, the middle ear may not drain properly. Fluid builds up. The defense systems described above become inefficient, and the fluid becomes a breeding ground for bacteria and subsequent infection.

Respiratory viruses may also contribute directly to the infection. Allergens can also produce inflammation and blockage in the Eustachian tube, which creates an environment favorable to bacteria.

Infections and Triggers

Bacteria. Certain bacteria are the primary causes of acute otitis media (AOM). They are detected in about 60% of cases. The bacteria most commonly causing ear infections are:

  • Streptococcus pneumoniae (also called S. pneumoniae or pneumococcus) is the most common bacterial cause of acute otitis media, causing about 40 - 80% of cases in the U.S.
  • Haemophilus influenzae, the next most common culprit, is responsible for 20 - 30% of acute infections.
  • Moraxellacatarrhalis is responsible for 10 - 20% of infections.
  • Other bacteria include Streptococcus pyogenes and Staphylococcus aureus.

About 15% of AOM-causing bacteria are now believed to be resistant to the first-choice antibiotics.

Viruses. Rhinovirus is a common virus which causes a cold, and plays a leading role in the development of ear infections. It is not the direct infecting organism, however. But other viruses, such as respiratory syncytial virus (RSV, a virus responsible for childhood respiratory infections) and influenza (flu), may be the actual causes of some ear infections. Increasing evidence suggests that such viruses play a greater role in ear infections than previously believed. The theory rests on the significantly lower rates of ear infections in children who have received flu vaccines.

Allergies. Allergies can cause inflammation in the airways, and contribute to ear infections.

Inborn Conditions that Predispose a Child to Middle Ear Infections

Genetic Factors. Several studies suggest that multiple genetic factors may make a child more susceptible to acute otitis media.

  • Genetic susceptibility to certain bacteria may result in development of persistent and recurrent acute otitis media.
  • Abnormalities in genes that affect the defense systems (cilia and mucus production) and the anatomy of the skull and passages would also increase the risk for ear infections.
  • Abnormalities in genes that regulate a powerful immune factor called interleukin 1 have been identified in some patients with recurrent acute otitis media who did not have any allergic disorders. Interleukin-1 plays a major role in producing inflammation in tissues and cells during heightened immune activity. Abnormalities in interleukin production may possibly result in a persistent inflammatory response.

Researchers are hoping that these findings may encourage primary care doctors to closely monitor children who have a family history of unusually frequent or severe upper respiratory tract infections.

Medical or Physical Conditions that Affect the Middle Ear. Any medical or physical condition that reduces the ear's defense system can increase the risk for ear infections. Children with shorter than normal and relatively horizontal Eustachian tubes are at particular risk for initial and recurrent infections. Inborn structural abnormalities, such as cleft palate, or genetic conditions, such as Kartagener's syndrome in which the cilia (hair-like structures) in the ear are immobile and cause fluid build up, also increase the risk.

Causes of Otitis Media with Effusion (OME)

OME may occur spontaneously following an episode of acute otitis media. Susceptibility to OME may also be due to an abnormal or malfunctioning Eustachian tube that causes a negative pressure in the middle ear, which allows fluid to leak in through capillaries. Problems in the Eustachian tube can be due to viral infections, second-hand smoke, injury, birth defects (such as cleft palate), or genetic diseases that affect the defense systems, such as Kartagener's syndrome.

The Incidence of Ear Infections and Other Airway Problems

Increased diagnosis of other disorders and infections of the upper and lower airways, such as asthma, allergies, and sinusitis, have paralleled the rise in ear infections. For example, the same bacteria are often responsible for both ear infections and sinusitis. In one study, 38% of children with ear infections also had sinusitis, and other studies have reported that nearly half of children with OME have concurrent sinusitis. While these studies may have overestimated the extent of clinically important sinus disease, the association is significant, even if the causal relationships are unclear. Researchers are looking for common risk factors, such as:

  • Increase in Day Care Center Attendance. Although ear infections themselves are not contagious, the respiratory infections that precipitate them can pose a risk for children with close and frequent exposure to other children. Some experts believe that the increase in ear and other infections may be due to the higher attendance of very small children, including infants, in day care centers beginning in the 1970s. For children who had the condition for a long time, however, neither day care attendance nor any other risk factor, including a history of upper respiratory tract infections or family history of OME, appeared to be relevant. Attendance in day care centers, then, may explain part, but not all, of the current increase in ear infections and other upper airway disorders.
  • Increase in Allergies. Some experts believe that an increase in allergies is also partially responsible for the higher number of ear infections, which is unlikely to be related to day care attendance. Studies indicate that 40 - 50% of children over 3 years old who have chronic otitis media also have allergic rhinitis (hay fever). Allergies are also associated with asthma and sinusitis.