The incidence of small subpleural lung cysts was analyzed for autopsies of 98 patients with Down syndrome (DS): 9 fetuses or stillborns and 89 liveborns. Such cysts were not seen in any of the fetuses or stillborn infants; they were identified in 18 (20%) liveborn DS patients but were present in only 1 of 11 DS patients who survived less than 4 weeks. Such cysts were not found in lungs of 61 non-Down patients with ostium atrioventricular commune (AVC), the most frequent congenital cardiac lesion of DS, or in those of 60 non-DS patients with bronchopulmonary dysplasia. Although subpleural lung cysts were more frequent in DS patients with than in those without congenital heart disease, the difference in incidence of such cysts between 25 liveborn DS patients with AVC and 61 non-DS patients with AVC was highly significant (chi 2 - 19.46, P less than .001). The data suggest that the lung cysts in DS are an intrinsic, albeit not invariant, feature of the disease and not necessarily secondary to congenital heart disease or sequelae thereof. We suggest that they may result from reduced postnatal production of peripheral small air passages and alveoli, which in turn may reflect the slow rate of cell proliferation that appears to be a general feature of DS.

The size of cardiac muscle fibers was determined for 15 patients with Down syndrome who did not have congenital heart disease and for 15 age- and sex-matched control patients by counting the number of cardiac muscle cells in specified areas of microscopic sections (method of Black-Schaffer and Turner). Mean ratio of muscle cells per unit area for the Down patients versus controls was 84.9%, with mean cross-sectional area of Down fibers 117.8% of control value and calculated mean volume of Down cells 127.7% of control value. Mean weights of Down hearts related to normal for age, versus control hearts, was 79.8% of control values. Calculated ratio of total cardiac muscle cell number in Down hearts was 62.5% of the value for controls. This value corresponds closely to the 62.4%, which can be calculated from Naeye's data on cardiac muscle fiber size in Down syndrome based on a point-count method, and is also close to published values for liver cell size and number and skeletal muscle nuclear number in Down syndrome. This study demonstrates that the Black-Schaffer and Turner method gives results equivalent to those of the point-count method for studies of cardiac muscle fiber size, and further supports the suggestion that chromosome 21 controls the size and number of at least certain cell types.

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