Patient: 12-month-old boy living in Portugal
Symptoms: Shortly after eating various types of fish, the child’s body began to smell like rotten fish. The smell was noxious and strong, especially around the head and hands. The first time something like this happened was when he was 10 months old. (The child was exclusively breastfed as an infant and began eating solid foods at 7 months of age.) His mother briefly put him on a fish-free diet, but when she reintroduced fish to his diet 2 months later, the odor returned.
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Diagnosis: Doctors suspected the child had a rare metabolic disease called trimethylaminuria. This is also known as “fish odor syndrome,” where your breath, saliva, sweat, and urine smell like rotting fish. The distinctive odor of rotten fish comes from a molecule called trimethylamine, which your body produces from nitrogen-rich foods such as fish. An enzyme called flavin-containing monooxygenase 3 (FMO3) breaks down trimethylamine in the body, turning it into trimethylamine N-oxide, an odorless compound. However, when the enzymes do not function properly, trimethylamine builds up in the body, causing the fish to smell rotten.
Metabolic pediatricians conducted a molecular analysis of the child’s FMO3 gene, which makes the FMO3 enzyme, and discovered a combination of genetic mutations that can interfere with production of the enzyme and cause mild or temporary trimethylaminuria.
Treatment: Doctors advised the child’s parents to gradually reintroduce fish into the child’s diet and to manage residual odor with a low-pH soap that reduces body odor. Over time, the rotten smell after fishmeal weakened. By the time the child was 19 months old, he was eating fish 5 to 6 times a week, but the odor symptoms did not occur again, and by the time he was 3 years old, the symptoms had not returned.
What is unique about this case is that there is no treatment or cure for trimethylaminuria. Its main cause is genetic, and in adults, the condition is often chronic. In this case, the child’s temporary bouts of trimethylaminuria were likely formed by a combination of factors, including genetic mutations in the FMO3 gene and metabolic immaturity, the doctors wrote in their report.
As the boy’s metabolic system matured, the FMO3 enzyme improved its ability to break down trimethylamine. The report said this child’s case and others suggest that the causes of trimethylaminuria are more subtle than previously thought and that the disorder can vary in severity.
Trimethylaminuria is a rare disease, with only a few hundred cases described in the medical literature. However, the condition may be underdiagnosed because people with trimethylaminuria often associate symptoms with personal hygiene practices and do not seek treatment because of unusual odors.
This article is for informational purposes only and does not provide medical advice.
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