Improving identification of idiopathic intracranial hypertension patients in Swedish patient register

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Idiopathic intracranial hypertension (IIH) is a rare disorder mainly affecting young, obese females. The incidence of IIH is estimated to be between 0.9 and 2.36 per 100 000 individuals.1 In our recent population‐based study validating registered diagnosis of IIH by medical record review, we found a slightly lower incidence rate of 0.65 per 100 000 in Stockholm County, Sweden.5 The disorder is characterized by high intracranial pressure with headache and visual disturbances being the most common symptoms. The pathophysiology behind this disorder is not fully understood.6 Some potential risk factors for IIH have been proposed; for example, treatment with tetracyclines, vitamin A, and lithium, medical disorders such as hypoparathyroidism, Addison′s disease, iron‐deficiency anemia, and uremia.7 Further carefully designed epidemiological studies with enough statistical power are needed to find whether these associations are true or spurious. The use of register data could clarify some of the remaining uncertainty surrounding these associations.
Misdiagnosis of IIH is shown to be common. Research conducted by Fisayo8 found that 40% of patients with a prior IIH diagnosis did not fulfill the criteria on follow‐up. The most common diagnostic error was inaccurate ophthalmoscopic examination in headache patients. Koerner et al9 also showed a positive predictive value (PPV) of 55% when validating registered diagnosis coding of IIH in inpatient and emergency hospital settings. A recent validation study conducted as part of this project showed a PPV of 65.2% (CI: 58.4‐71.4) on registered diagnosis code for IIH (G93.2) in the Swedish National Patient Register (NPR) in Stockholm County registered between 2006 and 2013.5
A useful tool for more accurately selecting correct IIH patients if using registry data would be beneficial. The aim of this study was to produce an efficient algorithm for identifying patients with a correct diagnosis of IIH in the NPR using variables possible to extract from Swedish registries.

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