Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 51-year-old woman is evaluated for a 6-month history of diarrhea and bloating. She reports four to six loose stools per day, with occasional nocturnal stools. She has had a few episodes of incontinence secondary to urgency. She has not had melena or hematochezia but notes an occasional oily appearance to the stool. She has lost 6.8 kg (15.0 lb) during this time period. Results of a colonoscopy 1 year ago were normal. She has not had recent travel, antibiotic use, or medication changes. She does not think consumption of dairy products alters her symptoms. She has a history of systemic sclerosis for which she takes omeprazole for symptoms of gastroesophageal reflux disease.

On physical examination, vital signs are normal. BMI is 22. Facial telangiectasias are present, and there is bilateral skin thickening of the hands. The abdomen is mildly distended, and bowel sounds are normal. Rectal examination is normal, with normal resting and squeeze tone. There are no palpable mass lesions.

Laboratory studies:

Hemoglobin	10.8 g/dL (108 g/L)
Mean corpuscular volume	104 fL
Serum electrolytes	Normal
Folate	63 ng/mL (143 nmol/L)
Glucose	Normal
Thyroid-stimulating hormone	Normal
Vitamin B12	118 pg/mL (87 pmol/L)
Tissue transglutaminase antibody	Normal

Stool cultures, including an ova and parasite examination, are normal.

Which of the following is the most likely diagnosis?

A: Celiac disease
B: Irritable bowel syndrome
C: Lactose malabsorption
D: Microscopic colitis
E: Small intestinal bacterial overgrowth

MKSAP Answer and Critique

The correct answer is E: Small intestinal bacterial overgrowth.

This patient has many features of small intestinal bacterial overgrowth (SIBO), including diarrhea, bloating, and weight loss. In addition, she appears to have macrocytic anemia secondary to vitamin B₁₂ deficiency in association with an elevated serum folate level, which is a classic pattern seen in SIBO (bacteria consume vitamin B₁₂ and also synthesize folate). Patients with systemic sclerosis may be particularly at risk for SIBO because of intestinal dysmotility or small-intestinal diverticula. Common risk factors for SIBO include altered gastric acid (achlorhydria, gastrectomy), structural abnormalities (strictures, small-bowel diverticula, blind loops or afferent limbs), and intestinal dysmotility (diabetes mellitus, neuromuscular disorders). The diagnosis of SIBO can be established with hydrogen breath testing or upper endoscopy with small-intestinal cultures, if available.

Although celiac disease may cause diarrhea, weight loss, and bloating, this patient’s normal tissue transglutaminase antibody and elevated serum folate level make this less likely than SIBO, especially given her risk factors for bacterial overgrowth.

Irritable bowel syndrome should not cause weight loss or nocturnal stools. These clinical findings represent alarm features in the evaluation of patients with diarrhea and abdominal pain or bloating, so it would be erroneous to diagnose irritable bowel syndrome with the presence of these findings.

Patients with lactose malabsorption in isolation should not have weight loss, so it would not explain this patient’s clinical picture. Although patients with SIBO may have concomitant lactose intolerance, lactose restriction is discouraged before evaluating and treating the underlying problem, especially because this patient reports tolerance of lactose ingestion.

By definition, microscopic colitis is a disease with histologic changes limited to the colon, unless it occurs in the setting of celiac disease. Therefore, with a colonic disease, features of fat malabsorption and vitamin deficiencies should not be seen. Although patients with microscopic colitis may have mild degrees of weight loss due to volume depletion, this patient’s weight loss is higher than what would typically be seen.

Key Point

• Small intestinal bacterial overgrowth should be considered in patients presenting with diarrhea, bloating, or weight loss; vitamin B₁₂ deficiency or an elevated serum folate level can be laboratory clues to the diagnosis.

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