GastroenterologyTier 2Disease (DEADMAN)

Congenital disaccharidase deficiencies

Description

Inherited deficiencies of brush-border disaccharidase enzymes (most commonly lactase, less commonly sucrase-isomaltase) impair digestion of specific dietary sugars, causing osmotic diarrhoea and bloating when the relevant sugar is consumed

Congenital lactase deficiency (present from birth) is rare; the far more common primary/adult-type lactase deficiency is a physiological, genetically-programmed decline in lactase expression after weaning, affecting a majority of the world's adult population to some degree, particularly outside populations with a history of dairy farming
Congenital sucrase-isomaltase deficiency is rare, autosomal recessive

Undigested disaccharide (lactose or sucrose) remains in the gut lumen -> osmotically draws water into the bowel -> osmotic diarrhoea; colonic bacterial fermentation of the undigested sugar produces gas (bloating, flatulence) and short-chain fatty acids (further contributing to fluid secretion)

Symptoms (bloating, flatulence, osmotic diarrhoea, abdominal cramping) occur specifically after consuming the relevant sugar, with a temporal relationship the patient can often identify themselves
Hydrogen breath testing (measuring breath hydrogen after an oral sugar load - malabsorbed sugar fermented by colonic bacteria produces hydrogen, detected in exhaled breath) is the standard non-invasive diagnostic test; genetic testing can confirm congenital forms specifically

Dietary avoidance/reduction of the specific poorly-tolerated sugar (lactose or sucrose) is the mainstay, often with a degree of individual tolerance (small amounts tolerated even with significant deficiency, given some residual enzyme activity in most cases)
Enzyme replacement - oral lactase supplements taken with dairy intake (for lactase deficiency), or sacrosidase (for sucrase-isomaltase deficiency) allow continued consumption of the relevant sugar with reduced symptoms

Irritable bowel syndrome (a common differential/coexisting diagnosis, since symptoms overlap substantially), secondary lactase deficiency (transient, following an episode of infectious gastroenteritis or other mucosal injury - distinct from the primary/congenital forms)

Secondary (acquired, transient) disaccharidase deficiency following gut mucosal injury (e.g. post-gastroenteritis) is far more common in clinical practice than the true congenital forms, and typically resolves over weeks as the mucosa recovers - an important distinction when counselling a patient about expected duration of symptoms

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