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A variety of different syndromes share the common feature of affecting mitochondria. Mitochondria are tiny structures inside cells which carry out the metabolic burning of fuels such as sugars and fats to provide the cell with energy. If some of the mitochondria are abnormal or lack important enzymes, then the energy source of the cell is affected. The range of symptoms of any particular syndrome depends upon the degree of deficit of the mitochondria, which may range from slight to total, and the distribution of affected mitochondria within the body. The muscles are commonly affected in such syndromes. Furthermore, episodes of respiratory insufficiency (inadequate breathing) have been encountered in such patients. These episodes were severe enough to require mechanical breathing support for the patient. While it might be imagined that the respiratory insufficiency would result from deficits of the muscles involved with breathing, the recent cases of three patients illustrates that respiratory insufficiency in such patients may also arise from a direct failure of the brain to adequately control breathing. Three patients with a familiar muscle disorder stemming from abnormal mitochondria, and two of their affected children, were tested for respiratory drive. In normal respiration, too much carbon dioxide or too little oxygen in the bloodstream causes cells in the brainstem to signal an increased rate of breathing. This may be tested by the so-called 'bag-in-a-box method'. The patient exhales into a bag and rebreathes the same air. As the amount of carbon dioxide goes up, breathing should increase. Similarly, if carbon dioxide is chemically removed from the bag as it is produced, the breathing rate should increase as the oxygen in the bag is consumed. When the increases measured for the patients were compared with those of normal subjects, it was found that the control of breathing was abnormal in these patients. Numerous case reports have suggested that mechanical respiration has been needed in many different types of muscle disease resulting from abnormal mitochondria, not just the sort tested here. The results suggest that in many cases, the respiratory insufficiency may result from the failure of the brainstem to properly respond to changes rather than from the failure of the breathing muscles themselves. (Consumer Summary produced by Reliance Medical Information, Inc.)