Diagnostic dilemma: A baby suddenly started to smell of rotting fish

The patient: A 12-month-old boy in Portugal

The symptoms: Shortly after eating different types of fish, the child would develop an odor of rotting fish emanating from his body. The smell was noxious and powerful, especially around his head and hands. He was 10 months old the first time this happened. (The child had been exclusively breastfed as an infant and began eating solid foods at 7 months of age.) His mother temporarily put him on a fish-free diet, but after she reintroduced fish to his meals two months later, the odor returned.

What happened next: When doctors examined the child, they did not detect a fishy smell, though they noted that he had not eaten fish before his medical visit, according to a report of the case. They detected no abnormalities in the boy's neurodevelopment or growth, and laboratory tests showed that his kidneys, liver and thyroid were functioning normally.

The diagnosis: Doctors suspected that the child had developed a rare metabolic disorder called trimethylaminuria, also known as "fish odor syndrome," in which breath, saliva, sweat and urine smell like decaying fish. Rotten fish gets its distinctive smell from a molecule called trimethylamine, and the human body produces trimethylamine from nitrogen-rich foods, such as fish. An enzyme called flavin-containing monooxygenase 3 (FMO3) breaks down trimethylamine in the body, changing it to the odorless compound trimethylamine N-oxide. But if the enzyme isn't working as it should, trimethylamine accumulates in the body and can make a person produce a foul, rotting-fish aroma.

A metabolic pediatrician conducted a molecular analysis of the child's FMO3 gene, which makes the FMO3 enzyme, and found a combination of genetic variants that could disrupt the production of the enzyme, leading to a mild or temporary type of trimethylaminuria.

The treatment: The doctors recommended that the child's parents reintroduce fish to the child's diet in small increments and that they manage any lingering smell with a low-pH soap, which reduces body odors. Over time, the rotten smell after fish meals weakened. By the time the child was 19 months old, he was eating fish five or six times per week without experiencing any malodorous symptoms afterward, and the symptoms had not returned by the time he was 3 years old.

What makes the case unique: There is no treatment or cure for trimethylaminuria; its primary cause is genetic, and in adults the condition is frequently chronic. In this case, the child's temporary bout of trimethylaminuria was likely shaped by a combination of factors, including inherited variants of the FMO3 gene and the immaturity of his metabolism, his physicians wrote in the report.

As the boy's metabolic system matured, the FMO3 enzyme improved its ability to break down trimethylamine. This child's case and others suggest that the causes of trimethylaminuria are more nuanced than once thought and that the disorder varies in severity, according to the report.

Trimethylaminuria is an uncommon disorder, with just a few hundred cases described in medical literature. However, the condition may be underdiagnosed, as people with trimethylaminuria often link their symptoms to personal hygiene habits and do not seek medical treatment for the unusual smell.