easy child/difficult child 2's MRI showed small lesions in her temporal lobes, which according to her neurologist, probably occurred as a result of a blood supply loss in utero. Her two follow-up EEGs -- a short sleep-deprived EEG and a longer 72-hour EEG -- were negative for seizure activity. Her neuro said that the lesions make easy child/difficult child 2 more susceptible to seizures prior to puberty (she hasn't even started puberty), but if she got through puberty without seizures, she would likely not develop them after that point. In addition, the neuro said the lesions could be contributing to her mood instability symptoms. The treatment with anticonvulsants (like Lamictal in her case) would be appropriate whether she had mood instability or seizures.
Given that your difficult child presented with symptoms so suddenly and that she's not responding well to medications, I think your psychiatrist is wise to recommend these tests. It is never wrong to rule out physical causes before accepting a psychiatric diagnosis.