Metachromatic leukodystrophy (MLD) is Alzheimer’s on speed. Children born with the most common form of the disease will die by age five, due to atrophy of brain tissue. The incidence in the general population is estimated to be 1 in 40,000 to 160,000 births.

Unlike Alzheimer’s, MLD has a known genetic cause. It has an autosomal recessive inheritance pattern, which means a child must inherit a defective, recessive gene from each of two carrier parents to be symptomatic. That suggests the frequency of carriers in the population is 1 in 200 to 1 in 400. But for couples who are both carriers, the odds of a child with MLD are 1 in 4.

Post-natal testing does not work on MLD. The enzyme test that reveals the disease has a 7-15 percent false positive rate due to other, less severe conditions affecting the same body chemistry. Unless a specific genetic test is done on the parents or the pre-natal fetus, the first time most affected couples will know is when their child exhibits unmistakable symptoms. By that point, the disease in untreatable and irreversible. And there are 1 in 4 odds that any future child of theirs will die the same way (see illustration).