Causes of abdominal swelling (6F's)

This illustrated article is a part of my ‘Medical Mentalism’ series, exploring important causes of common symptoms. Click here if you would like to support the series.

The six F’s summarise causes of abdominal swelling: Fluid, Fat, Faeces, Flatus, Fetus, and Flippin’ big mass (or Fatal mass). It’s a mnemonic taught in medical school to group diagnoses when a patient presents with this symptom. While it can be a helpful framework to jog memory, none of the F’s are diagnoses in themselves. Let’s look a little closer at each:

Fetus

Pregnancy is important to consider; first it can be the reason for abdominal swelling; second, radiation exposure can be harmful to the developing fetus if scanning; and thirdly, it can affect which treatment options are safe to offer a patient. An ectopic pregnancy describes when a fertilised egg has implanted outside of the womb, such as in a fallopian tube. If the pregnancy continues, it can rupture these structures which are not designed to grow with a fetus, which can threaten life and future fertility.

Fat

Medical folk reading this might hear the following (arguably) more polite variations used in front of patients:

• elevated BMI (body mass index)

• excess adipose tissue

• subcutaneous tissue +++

• increased body habitus

• BMI >40 or morbidly obese

• ‘increased cardiothoracic impedance’ which means that a defibrillator may struggle to deliver a shock to the heart

Weight gain has its own differential diagnosis worth knowing as much for empathy reasons as for clinical acumen. For most of our history, humans have been evolving to survive in environments of food scarcity. We are adapted to ‘store fat now to survive later’ because our genes do not expect us to have access to regular meals. They haven’t caught up to the surplus, calorie-dense world we now find ourselves. We also live longer than we were built to survive, and (never mind natural variation), there is a ‘slowing-down’ in our metabolisms with age.

Mental and physical health conditions and treatments can affect hunger, metabolism, and how our body stores fat. Our employment can dictate working patterns, for example long-haul drivers and night-shift workers have different opportunities for exercise and access to fresh healthy foods than someone who works from home. Socioeconomic deprivation can affect what food is affordable and readily available. There are so many factors to consider here, so if the cause of abdominal swelling is fat, please don’t stop your care there.

Faeces

Constipation can fill the length of our intestines with faeces. If left unresolved, this can become solid and ‘impacted’ where the bowel is no longer able to move it along. This is a serious problem which can require surgical intervention. The underlying causes of constipation boil down to a problem with the faeces (such as dehydration or a low-fibre diet), the structure of the bowels (e.g. an obstructing mass like cancer), or their transiting action (e.g. resisting the urge to have a bowel movement, or opioid use).

Fluid

Fluid can accumulate in the abdomen in several conditions, many driven by pathology of the liver (e.g. cirrhosis) and kidney (e.g. nephrotic syndrome). Bleeding is a more acute cause of fluid accumulation, such as a ruptured spleen or aortic aneurysm. Ultrasound scans and ‘tapping’ the fluid to determine its composition are common investigations to help find the diagnosis.

Flatus

Increased ‘wind’ in the digestive tract can be the result of additional introduction (e.g. by medics when ventilating the stomach instead of the airways), production (e.g. by bacteria in an infection), or obstruction (e.g. volvulus or severe constipation).

Flippin’ big mass

Also known as ‘fatal mass’ this F refers to organomegaly - enlargement of abdominal organs. Some example causes include polycystic kidney disease (PKD), polycystic ovaries, urinary retention, volvulus (twist in the bowel), intussusception (‘telescoping’ of the bowel), hepatomegaly from cirrhosis.

Fin

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