Incidental detection of hippocampal sclerosis.

The MR examination of medial temporal structures has recently been pushed to the limits of current technology, with some hope of uncovering the secrets of intractable epilepsy. The specific MR imaging features of one important source of seizures, mesial temporal sclerosis, have been well described. These include a small hippocampus with abnormal signal on T2-weighted scans, but many other findings in the temporal lobes and limbic system have been reported. These careful investigations of brain anatomy have occurred without a critical examination of the hippocampus of nonepileptics to determine if these findings might be encountered as incidental findings in healthy patients, or at least in patients without epilepsy. This is not a simple task because high-resolution imaging in the coronal plane is essential for this diagnosis. In this issue of the AJNR, Moore et al (page 1609) report on the temporal lobe findings in 207 cases without known epilepsy in an attempt to determine the positive predictive value of the MR findings of mesial sclerosis. In this retrospective study of patients with hearing loss, the authors identified two patients out of the 207 with abnormal hippocampal formations. In both cases, further investigation of the clinical history uncovered a history of epilepsy. The other 205 patients had normal medial temporal structures; ie, they found no cases with hippocampal abnormalities without epilepsy. The authors conclude that the findings of mesial sclerosis are ''uncommon and significant.'' Although their observations are impressive, one should be aware of two important caveats. The first concern is how representative is the study sample? There can be little argument that this finding is uncommon. Of the patients referred from our epilepsy service with a suspected epileptogenic focus, MR studies are abnormal in approximately 25% of cases. With this experience in patients at risk, there can be little doubt that the occurrence of hippocam-pal abnormalities will be considerably less among nonepileptics. With an uncommon imaging finding, an important question is the number and composition of the study cases needed in order to reach a valid conclusion. It is essential that the cases in the study group are representative of the patient population (1). Ideally, this control group should include cases with a different disease process in the same anatomic location as well as patients with the same disease (epilepsy) but of nontemporal lobe origin. The outcome might have been different if the authors had decided to study patients with …