Mastoid pneumatization in patients with unilateral aural atresia.

Debate continues regarding theories of the control of temporal bone pneumatization. The "hereditary theory" holds that mastoid size is independent of status of the mesotympanum. The "environmental theory" holds that the size of the mastoid air cell system is determined by the degree of pathological involvement of the middle ear during childhood: that is, an abnormal middle ear prompts a small mastoid. This report concerns the volume of temporal bone pneumatization in patients with unilateral aural atresias, involving a malformed mesotympanum and absence of the external ear canal. Computed tomograms of the temporal bones were available for 16 patients. The mastoid pneumatization volumes of the atretic/non-atretic sides were approximately equal (Spearman's r = 0.80, P < 0.01) but not symmetrical. In 12 patients, the atretic side had less pneumatization than did the non-atretic side; in 3, the atretic side had more pneumatization; and, in 1 patient, neither mastoid had any pneumatization. Such a non-symmetrical pattern is unlikely by chance (P < 0.04). These data can be interpreted to endorse both the hereditary and environmental theories of temporal bone pneumatization.