Beyond IBS: When the Brain Takes Over
Centrally mediated abdominal pain syndrome (CAPS), formerly known as functional abdominal pain syndrome, sits at the extreme end of the DGBI severity spectrum. Unlike IBS, where symptoms are clearly related to bowel habit changes, CAPS is characterised by continuous or nearly continuous abdominal pain that has little or no relationship to eating, defecation, or menstruation. The pain is real, severe, and disabling — but its primary driver is central nervous system amplification rather than peripheral gut pathology.
Rome IV criteria require: continuous or nearly continuous abdominal pain, no or only occasional relationship of pain to physiological events (eating, defecation), pain that limits daily functioning, and the pain is not feigned. Symptoms must be present for the last 3 months with onset at least 6 months before diagnosis.
The Descending Inhibition Failure
In healthy individuals, the brain modulates pain signals through a descending inhibitory system — serotonergic and noradrenergic neurons from the periaqueductal grey and rostral ventromedial medulla project to the spinal cord dorsal horn, dampening incoming nociceptive signals. In CAPS, this inhibitory system is dysfunctional: functional MRI studies show reduced activation of descending inhibitory pathways and enhanced activation of pain amplification centres (anterior cingulate cortex, prefrontal cortex).
This central augmentation explains why CAPS patients experience pain disproportionate to any peripheral stimulus, why standard GI investigations are normal, and why GI-specific treatments (antispasmodics, PPIs, dietary modification) provide little relief. The peripheral gut is not the primary problem — the volume knob in the brain is turned up.
The Opioid Trap
CAPS patients are at high risk of opioid-induced hyperalgesia — a paradox where chronic opioid use actually increases pain sensitivity. The mechanism involves mu-opioid receptor desensitisation and compensatory upregulation of pronociceptive pathways (NMDA receptors, dynorphin). Patients develop tolerance, require escalating doses, and experience worsening pain — creating a vicious cycle that is extremely difficult to break. Narcotic bowel syndrome (opioid-induced GI hyperalgesia) compounds the problem by adding nausea, bloating, and abdominal distension.
Treatment Approach
Effective CAPS management centres on central neuromodulation. First-line pharmacotherapy includes tricyclic antidepressants (amitriptyline, desipramine at 25-75mg) or SNRIs (duloxetine 60mg, venlafaxine) — used for their direct analgesic properties on descending inhibitory pathways, not as antidepressants. Augmentation with pregabalin or gabapentin (targeting calcium channel-mediated central sensitisation) may be added.
Psychological therapies are not optional extras — they are core treatment. Cognitive behavioural therapy, acceptance and commitment therapy, and gut-directed hypnotherapy all target the central pain processing dysfunction that defines CAPS. The evidence base is strongest for CBT, with demonstrated improvements in pain severity, disability, and quality of life.
Opioid detoxification, when applicable, must be gradual and supported. The pain typically worsens before it improves as hyperalgesia reverses. Multidisciplinary pain management — combining pharmacotherapy, psychology, physical rehabilitation, and sometimes inpatient pain programmes — offers the best outcomes for this challenging condition.