Review for 10.1186/s12876-023-02861-w (2023)

Title: Associations and gastrointestinal symptoms in women with endometriosis in comparison
to women with irritable bowel syndrome: A  study based on a population cohort
Version: 1 Date: 04 May 2023
Reviewer's report:
As can be expected from two disease conditions located in the same area of the body and whose
etiology is not fully explained, the symptoms of endometriosis and irritable bowel syndrome
(IBS) do largely overlap. One is a functional symptom conglomeration (IBS), the other a
condition with distinct pathology requiring operative diagnosis for accurate verification, but
with a large psychosomatic component when only symptoms are considered. That there are
associations to be seen between the two is thus not surprising and has been described repeatedly
before.  Misdiagnosis is as the authors say commonly seen, in both directions. The reviewer can
not see how the authors can set forth the hypothesis "that there are clinical differences between
endometriosis and IBS".  If a hypothesis on this is to be tested, should it not be that there are no
major differences ? The motivations set in the Introduction point that way.
Here this issue to compare gastrointestinal symptoms between endometriosis and IBS is raised
again using a  material consisting of a cohort of children and grandchildren from a sub-study
population and data come from a questionnaire that half of them answered. Even though
the original study with 41% participation may not fully reflect the background population ,
the selected study material may be fairly well representative of a more educated and health-
conscious part of the population. The  women included had replied  positively to a question
enquiring about gastrointestinal symptoms in the preceding 2 weeks (only), and then they scored
different symptoms relating to potential IBS. Local and a nationwide Swedish diagnostic register
provided data on endometriosis and a host of life-style information and drug use. The study thus
has a basis that has weaknesses. Accuracy of the register information on the dependent or effect
variables may vary in accuracy in relation to the independent life-style variables. Therefore
associations  must be looked upon as indicators and can not be assigned too much importance,
particularly where they are borderline, such as when the lower odds approach 1.0. In the reviewer
´s opinion the authors could be more careful in how they deal with this, not least considering  the
rather small or even just plainly small number of individuals who in the end provided information
(Tables 1-3). An emphasis on p-values is not an ideal approach here.  The use of 2-3 decimals
in the text and tables  can be questioned.
Of 2200 women selected into the group whose information was used, only 72 or 3.27% had an
endometriosis diagnosis . This proportion of  women with endometriosis is in the higher range
of what is seen in the most reliable population-based studies on the prevalence of endometriosis,
which is not surprising when the nature of the study material is considered. Yet the authors cite a
review originating from a group of authors in Iran where the overall prevalence of endometriosis
is said to be 18%.  This so-called "systematic" review is in the reviewer´s opinion seriously
flawed. For IBS the authors cite a prevalence from 1% up to 25%, with a pooled prevalence of
3.8%. This does not  make sense  in comparison with 18% for endometriosis in the reviewer
´s opinion.  The authors´ comment on a "low" prevalence of endometriosis in their own study
(paragraph 2 in the Discussion) is thus based on a misunderstanding from their use of  the flawed
reference 4.
Of the 72 women with endometriosis 61 had answered questions on self-reported IBS, and
only a third had provided the VAS scores on IBS. Of the original MOS material of over
5000 individuals the actual number thus dwindled drastically when it comes to comparing
endometriosis and IBS symptoms, even if data on self-reported IBS were more copious.
Therefore data on the age of women with endometriosis (52-53 years median) are not meaningful
nor are those  on women without endometriosis (almost 10 years younger).  In the adjusted OR
this is reflected in wide OR and none of the results are new, neither on sick leave, BMI or other
life or life-style variables. The adjusted association between endometriosis and self-reported IBS
is the one item that has most value as a result from the study. The results on IBS associations
are also known from other sources.
On the whole the study has useful points on the similarity of symptoms and the risks of
endometriosis being labelled IBS which may lead to diagnostic delays. This is articulated well
in paragraphs 3 and 4 of the Discussion.  To speculate that non-significant results might become
significant  in a larger group as done towards the end of the Discussion, is inappropriate.
The reference-use for this article  is not ideal, including  referring repeatedly to the senior author
´s own previous studies.
The tables are bulky but useful. Tables 4 and 5 are better as an overview.
By the way, studies or reviews do not find or show anything by themselves, but something may
be shown or found in a study or a review. The scientific English is could (only a few faults).
On the whole the study has useful points, but is flawed in it´s presentation. The use of references
must be reconsidered.