BECKER MD--A LESS SEVERE DISEASE

A word needs to be said here about Becker muscular dystrophy (named after the German doctor Peter Emil Becker, who first described this variant of Duchenne dystrophy in the 1950s).

BMD is a much milder version of DMD. Its onset is usually quite a bit later in life, and its course is far less predictable. Even in late adulthood, many people with Becker have little trouble getting around and doing daily activities. Only some need a wheelchair.

Until the late 1980s, it wasn't clear whether BMD was related to the same gene as DMD or a different gene. The molecular genetics advances of the late 1980s settled the question once and for all: BMD results from mutations in the same gene as Duchenne, but different mutations. These genetic mutations result in a dystrophin protein that's abnormal in either quantity or quality, or both; in other words, there isn't enough of it or what there is isn't quite up to the job of the normal protein. But it's clearly far better than not having any dystrophin, which is the case in Duchenne.

If your son has been given a diagnosis of BMD, the predictions of his course are far less certain, though more optimistic, than in Duchenne. A diagnosis of BMD means that the child has signs and symptoms of muscle degeneration similar to those of Duchenne, but less severe and over a much longer period of time. It means that some dystrophin has been identified in the muscle biopsy. (If a muscle biopsy hasn't been done and the diagnosis is uncertain, insist on one.)

It can't be said for certain which genetic mutations will lead to Duchenne and which to BMD (which is why doctors like to rely on the protein they see on a muscle biopsy and on clinical signs rather than on DNA results alone), but scientists are now fairly certain that BMD results when certain small sections of the dystrophin gene are missing. These sections lie between points labeled exon 45 and exon 59 and don't change the way the rest of the gene's DNA is processed. Missing or duplicated sections in other parts of the large dystrophin gene, especially when they affect the way large parts of the gene are "read" as the DNA is processed, lead to the more severe Duchenne type of dystrophy.

To distinguish Becker from Duchenne dystrophy by symptoms, doctors use the somewhat arbitrary criterion of ability to walk unaided at the age of 16 as the dividing line between the two disorders. This usually poses no ambiguity, since most children with Duchenne lose their walking ability long before 16.

Contrary to what might seem logical, Duchenne and Becker dystrophies don't form a smooth continuum, with a lot of children falling between the extremes of early loss of function and long retention of function. Instead, Duchenne and Becker dystrophies form what scientists call a bimodal distribution, meaning there are two clusters of loss of function, one clearly forming the Duchenne pattern and the other clearly forming the Becker pattern, with very few children in between.

There are, of course, some children in between, and doctors tend to call these "Duchenne outliers" or "severe Beckers."

One important point about BMD is that it's possible to have quite mild skeletal muscle involvement along with severe cardiac muscle involvement. In some people, this situation seems to result from a specific genetic mutation that has a preference for heart dystrophin over skeletal muscle dystrophin. As one young man who eventually underwent heart transplant surgery said, "It was as if the genetic mutation just landed on the heart."

Cardiac problems are by no means universal in BMD, but they can't be dismissed just because the boy is doing well otherwise. Sometimes serious heart problems "sneak up" on a young man with BMD whose skeletal muscles are giving him few, if any, problems. Therefore, the heart should be checked regularly, and treated if necessary. In a few cases, heart transplants have been successfully done for advanced cardiac muscle destruction.

In BMD, there is a great deal of variation in both the timing and the extent of loss of muscle function. Often, the diagnosis isn't made until adolescence or even adulthood, possibly when a young man can't keep up in physical education classes or sometimes military training.

As with Duchenne, the pattern of muscle loss in BMD usually begins with the muscles of the hips and pelvic area, the thighs and the shoulders. However, the problems begin much later, often in the teens or 20s, when the young man begins walking with a waddling gait, walking on his toes or sticking out his abdomen. Some men will require wheelchairs by their 30s or later, while some will manage with minor aids, such as canes.

Most young men with BMD have normal intelligence, but there is a significant minority with learning disabilities (more than in the general population). Occasionally, the learning disabilities may be the problem that brings the young person to a doctor's attention, and only then is the muscular dystrophy noticed.

It used to be believed that BMD was much less common than Duchenne, but experts now believe these conditions are about equal in the population, with BMD being severely underdiagnosed. It's often overlooked entirely or misdiagnosed as limb-girdle muscular dystrophy or spinal muscular atrophy (a neurologic disorder). These disorders have different inheritance patterns from BMD. For this reason, it's important to have both genetic testing and a muscle biopsy before assuming that the problem is actually BMD.

Since men with BMD often live well into adulthood and many have children, it's important for them to be aware of their X-linked muscular dystrophy, with its 100 percent risk of making all female children carriers and its lack of risk in passing the faulty gene to male children. As in Duchenne dystrophy, there are rare instances in which girls develop the disorder.

In this guide, we aren't going to say much about BMD, since its course is so different from that of Duchenne, despite its being linked to the same gene. Most of the time, a young man with BMD will be able to manage for himself much as any other young man does. The degree of disability varies, but all young men with BMD should be monitored regularly for cardiac involvement.

Next... CHAPTER 3: Sorting Out The White Coats