Diminished radial count is found only postnatally in Down's syndrome.

Acinar complexity was assessed using the radial count in 23 patients with Down's syndrome. Late intrauterine growth of the lung is not impaired, and acinar complexity is normal. Thereafter, decreased acinar complexity occurs and is grossly and microscopically apparent by 4 months of age. The gross appearance is characteristic, consisting of a diffuse and uniform porosity of the cut surface of the lung. Microscopically, this appearance is due to the presence of dilated alveoli and alveolar ducts. Children and adults with Down's syndrome have a significantly reduced number of alveoli, which are enlarged. In the majority of cases, the alveoli have a double capillary network that has not been described in any other lung condition. No correlation could be established between the radial count and the presence or absence of double capillary network, hypertensive pulmonary vascular disease, or congenital heart disease. The unique appearance of the lung in Down's syndrome results from failure of the lung to develop properly in the postnatal period, presumably genetically determined.