Materials and methods

In this retrospective, observational study, medical records from all patients admitted to hospitals in Skåne county with the ICD-code G00 (bacterial meningitis) between 2000 and 2017 were retrieved. Exclusion criteria were neonatal, viral, fungal or non-infectious meningitis, borrelia, nosocomial, postoperative, or ventricular shunt-related infections.
Information about gender, age, otoscopy results, CT/MRI signs of middle ear infection, microbiological results, subjective hearing loss and hearing tests were extracted from medical charts. Where pure tone audiograms were available, pure tone averages (PTA4; defined as the average of hearing thresholds at 500, 1000, 2000 and 4000 Hz) were extracted, as was presence of conductive hearing loss (defined as an air-bone gap of ≥10 dB on at least two adjacent frequencies). Hearing loss was defined as PTA4 ≥ 25 on either ear, and further subdivided into mild (25-40 dB HL), moderate (41-70 dB HL) and severe (>70 dB HL). High frequency hearing was evaluated by calculating the average hearing thresholds at 6000 and 8000 Hz.
By including data over 18 years in one of the most populous counties in Sweden, we hoped to be able to estimate the incidence of bacterial meningitis in Sweden over a long period of time. The proportion of otogenic meningitis cases, time trends, microbiology and outcomes other than hearing loss will be reported separately.
The study was approved by the Ethics Review Authority. The STROBE guideline was used for the preparation of the manuscript.