The degree of olfactory function and damage to olfactory bulbs and tracts were quantitatively determined in 268 patients with head trauma (HT) presenting with complaints of olfactory disturbance at the University of Pennsylvania Smell and Taste Center, Philadelphia, PA. Anosmia occurred in 179 patients (67%), microsmia (lessened sense of smell) in 20%, and 12% had normal sense of smell. Olfactory test scores after head trauma were not related to age or sex. Occipital or temporo-parietal impacts caused more cases of olfactory dysfunction than frontal impacts. Of 66 retested after periods ranging from 1 month to 13 years, 24 (36%) showed slight improvement, 45% were unchanged, and 18% were worse. Parosmia, or perversion of smell, occurring in 40% of patients immediately after the trauma, decreased to 15% during an 8 year follow-up. Volumes of olfactory bulbs and tracts measured by MRI were smaller than controls in male, but not female, patients. 
COMMENT. Patients with head trauma who lose their sense of smell rarely regain normal olfactory function, whereas those who complain of distortions of smell usually recover over time. Head trauma male patients with olfactory dysfunction have MRI evidence of greatly reduced olfactory bulb and tract volumes. The apparent selective sparing of female olfactory structures may be explained by lesser severity of trauma or a protective effect of estrogens.