Abstract
Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder characterised by altered stool frequency and/or consistency in addition to abdominal pain. There are four bowel pattern subtypes in IBS: diarrhoea predominant IBS (IBS-D), constipation predominant IBS (IBS-C), alternating bowel habit IBS (IBS-A) and unclassified IBS(IBS-U). IBS is a biopsychosocial syndrome and the predominant psychological treatment is cognitive behavioural therapy (CBT). The objectives of this PhD were (1) to assess whether cognitive and behavioural factors were mediators of treatment effect on the outcomes of symptom severity and work and social adjustment/quality of life, and (2) to identify whether cognitive and behavioural factors were associated with IBS bowel pattern subtypes. Two studies (studies one and two) were conducted to assess objective one, and two studies (studies three and four) were conducted to address objective two. Two data samples were used for analyses conducted in this thesis. Data set 1 was from a previously conducted randomised controlled trial (RCT) including IBS participants meeting Rome I criteria with a GP diagnosis of IBS. Data set 2 was from a new RCT including refractory IBS participants meeting Rome III criteria with a GP diagnosis of IBS.
A systematic review (study one) of mediation analysis conducted in the context of psychological interventions for IBS returned nine studies. The results suggested that illness-related cognitions were important mediators of treatment effect for the outcomes of symptom severity and quality of life. Gastrointestinal (GI) specific anxiety was found to be a mediator more often than general anxiety was. Only two studies assessed the mediating role of behaviours. The study assessing GI related behaviours found them to significantly mediate treatment effect, but the other study assessing general all-or-nothing behaviours did not find significant mediation. Study two utilised data set 1. It assessed simple and sequential mediation models in the context of an RCT assessing the effect of CBT + mebeverine to mebeverine alone on symptom severity and work and social adjustment in IBS participants (n=148). Simple mediation models found that general anxiety, GI related cognitions and GI safety behaviours mediated treatment effect on both outcomes. GI avoidance behaviour was not found to be a significant mediator. Sequential mediation models indicated that change in GI cognitions and GI safety behaviours preceded a reduction in general anxiety, and these sequential paths significantly mediated treatment effect on both outcomes.
Study three and four assessed differences in GI cognitions, GI safety behaviours, GI avoidance behaviours, general anxiety, work and social adjustment and symptom severity between IBS bowel pattern subtypes (IBS-A, IBS-C and IBS-D) using ANOVAs. IBS-U was excluded due to a disproportionately low number of participants in both studies classified as IBS-U. There was a significantly higher level of avoidance behaviours in the IBS-A and IBS-D subtypes than in IBS-C found in both studies. IBS-A was also found to have significantly higher levels of safety behaviours than IBS-D in both studies. However, in study three those with IBS-A also had significantly higher safety behaviours than those with IBS-C, whereas in study four, those with IBS-C (along with IBS-A) had significantly higher safety behaviours than those with IBS-D. In study three there was a non-significant trend towards a greater level of unhelpful GI related cognitions in those with IBS-D compared to the other two subtypes and this was found to be a significant difference in study four.
Together the results of this thesis suggest that cognitive and behavioural factors are important treatment mechanisms in IBS as well as potentially modifiable factors to help treatment targeting for particular IBS subtypes. Future directions for research should involve (1) mediation analysis including all process variables as identified by the CBT models of IBS (2) cluster analysis to identify subgroups in IBS classified by psychological factors in addition to bowel pattern predominance (3) assessment of moderated mediation to assess whether treatment mechanisms vary depending on subgroup membership in IBS.
| Date of Award | 2018 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Trudie Chalder (Supervisor), Rona Moss-Morris (Supervisor) & Kimberley Goldsmith (Supervisor) |