Doctors in China and the United States performed a meta analysis of 24 studies from the past 48 years that met the eligibility criteria. Studies had to have been performed on humans, with a sufficient number of both control (healthy) and affected participants. They also needed to have included a clear definition of what they considered recurrent or chronic otitis media and must have contained enough data to draw conclusions from the study.
The analysis began with a lengthy list of factors commonly believed to affect the risk of recurrent ear infections and searched for data related to each item.
Many of the factors purported to have an effect on ear infection risk didn't stand up to the scrutiny, including whether a child had been breastfed as an infant and the mother's educational level. The authors describe the differences in breastfeeding between the control group and patients with chronic ear infections as "unremarkable."
The authors found some factors - sex, attending day care centers and having large families with more siblings - with results that weren't statistically significant by common measures, but that they believe warrant further study.
The meta analysis didn't find enough data to draw conclusions about the effects of genetic predisposition, nutritional factors, medication use during pregnancy, being of a certain ethnicity, pharyngeal reflux and overweight status.
The doctors concluded that several factors, which they describe as "closely interrelated," clearly increase the risk of repeated ear infections.
They found that having already had ear infections was a risk factor for developing recurring ones, as was having upper respiratory infections and snoring, defined as "the presence of loud snoring at least three times per week." The study found a wide range of significant results for patients with allergies, saying, "The prevalence of ... allergic rhinitis, in patients with [recurrent ear infections] ranges from 24 percent to 89 percent."
Exposure to second hand smoke and low socioeconomic status were also found to increase the risk of repeated ear infections. The authors caution that many unidentified risk factors remain and that further study is needed to discover them.