Prevalence of BHD – an Epidemiological Study

The prevalence of Birt-Hogg-Dubé (BHD) is currently unknown and is particularly difficult to ascertain considering the wide range and variability of symptoms associated with BHD, even within the same family. Furthermore, the only definitive way of diagnosing BHD is through genetic testing for folliculin mutations, therefore many people go undiagnosed.

Around one in four people with BHD experience one or more spontaneous pneumothorax (SP) in their life and it is thought that BHD is the cause of 5-10% of SP. To determine the prevalence of BHD, researchers from the University of Lausanne conducted a meta-analysis of previously published studies on the prevalence of SP among BHD patients and the general population using a statistical method called Bayes equation¹. To estimate the prevalence of BHD they calculated the following 3 components:

Firstly, Muller et al., determined the prevalence of BHD in apparent primary SP (PSP) to be 9%. The calculation of the second component, prevalence of PSP in the general population, was a little more complicated as this factor is not directly measurable. Instead, they used a formula to estimate the prevalence based on the incidence and average duration of PSP. The overall incidence rate of PSP was 8.69 per 100,000 person-years. From this, Muller et al., determined the overall prevalence of PSP in the general population (using all available data and a duration of 30 days) as 0.77/100,000 people. Finally, the third component identifying the prevalence of PSP in BHD individuals, was determined to be 43%.

Combining the above components in the Bayes equation, and assuming the most accurate data came from studies post-2000, they estimated the prevalence of BHD to be around 2 cases per million people, with no difference in gender.

This is the first study of its kind to determine the prevalence of BHD, a statistic the field would greatly benefit from. However, this study is not without caveats and limitations, some of which the authors addressed in their discussion. There are only four articles in the literature, with a total of 827 cases of PSP, where BHD has been identified.  However, this does suggest that BHD isn’t rare in PSP and that lung abnormalities and a genetic cause in PSP should be investigated, especially in cases where there is a family history.

It is also important to note that the prevalence of BHD estimated here is based entirely on a single symptom of BHD, pneumothorax. This is a major caveat of this study as not every BHD patient experiences a pneumothorax.

Although this study does provide useful information on the prevalence of BHD based on pneumothorax, the limitations and caveats in this study, combined with the wide range of symptoms an individual with BHD can present with implies that prevalence of BHD will be higher than the figure estimated here. Further investigation into the prevalence of BHD that would encompass all patients with BHD, such as genetic screening for mutations in folliculin, is warranted and would be of great interest to researchers, clinicians and patients.

References

1. Muller, M.-E., Daccord, C., Taffé, P. & Lazor, R. Prevalence of Birt-Hogg-Dubé Syndrome Determined Through Epidemiological Data on Spontaneous Pneumothorax and Bayes Theorem. Front. Med. 8, (2021).