Diagnostic dilemma: Weakness in a man's leg revealed his abnormally small brain

The patient: A 44-year-old man in France

The symptoms: For two weeks prior to going to the doctor, the man had been feeling mild weakness in his left leg. There weren't any notable abnormalities in his medical history or neurological development, except for when a shunt had to be placed in his head when he was 6 months old. The shunt helped relieve fluid buildup in his brain, which had an unknown cause. When he turned 14 years old, the shunt was removed.

What happened next: When the man reported his leg weakness to medical practitioners, doctors ran CT and MRI scans of his head. They also performed neuropsychological testing, which revealed that the man's IQ was 75, below the average score of 100.

The diagnosis: Doctors determined that the patient was living with a brain that was at least half the average size.

The brain scans also revealed a large buildup of fluid in his brain, which allowed little room for tissue. This left the patient with just a thin sheet of brain tissue pushed up against his skull. The scans showed that the fluid-filled cavities in the brain — called ventricles — were extremely dilated. It's likely that this fluid accumulation drove the man's leg weakness.

The treatment: Doctors drained this excess fluid from the man's skull, which helped the patient regain some of his leg strength. In a case report published in The Lancet, the doctors wrote that a new shunt was also inserted inside the man's skull, and within a few weeks, the patient's neurological examinations — which were used to evaluate his leg weakness — returned to baseline. "The findings on neuropsychological testing and CT did not change," they noted, so the man's IQ remained about the same.

What makes the case unique: Over the course of evolutionary history, humans' brains became notably large compared with those of our relatives, leading to improved cognitive capacity and language acquisition. The exact reasons for this growth are not fully understood but are likely linked to a combination of environmental and dietary factors, among other pressures.

A human's brain also grows significantly within a person's lifetime, with the brain during adulthood typically being dramatically larger than the size of their brain as a newborn. In the man's case, it might have been expected that he would show deficits due to having a smaller-than-average brain. However, despite the smaller size of his brain, the man did not show any deficits and was working as a civil servant.

Had he not developed leg weakness, his small brain may well have gone unnoticed.