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Editor,—Lucina wonders why some boys with Duchenne muscular dystrophy (DMD) also have cognitive impairment, and whether it could be related to brain dystrophin.1 Most muscles in DMD show signs of repeated necrosis and repair. Fadic et al, reporting a dystrophic variant recently, were puzzled not to find these signs in the myocardium which showed other signs of the disease.2 I suspect that this is because the heart is not subjected to the disruptive forces that most muscles meet during exercise. I have long wondered why boys with DMD do not have diplopia, and whether the extraocular muscles, which are not subject to disruptive forces, might also lack signs of injury. An analogous situation may be found in the brain lacking dystrophin which might have a low resistance to shear stress at a subcellular level. Insignificant bumps might then cause cumulative damage and loss of intelligence. It would be interesting to know whether affected boys have had more bumps on the head than those whose intelligence is preserved.