Temporal Lobe Epilepsy
There are 72 studies on brain SPECT imaging and temporal lobe epilepsy (TLE) in the literature involving over 2100 patients (see Table 4 for a summary of some of the studies 96-107).
Temporal Lobe Epilepsy (TLE) is one of the most frequently encountered chronic epileptic disorders and has been associated with numerous psychiatric symptoms, such as depressed mood, anergia, irritability, euphoric mood, atypical pain, insomnia, fear, and anxiety(88). The medial aspects of the temporal lobes are frequently involved in TLE and they are difficult to evaluate with routine EEG studies and therefore may be missed. Ictal SPECT and, to a lesser degree, interictal and peri-ictal SPECT have been used to accurately identify seizure foci. SPECT findings in epilepsy most often reveal focal decreased perfusion in the interictal phase and focal increased perfusion in the ictal phase of a seizure(89-94). In a review of 30 studies, Devous et al.(89) found that SPECT localization in patients with temporal lobe seizures were 0.44 (interictal), 0.75 (postictal) and 0.97 (ictal). False-positive rates were low.
Shen et. al.(94) utilized brain SPECT imaging to evaluate 34 patients who eventually underwent a temporal lobectomy for medically intractable complex partial seizures. Consistent with previous findings, patients in the interictal phase revealed decreased regional cerebral perfusion in the temporal lobe. These abnormalities corresponded with the eventual site of surgery in 73% of their patients. Ninety-three percent of their patients also displayed increased perfusion on SPECT during the ictal phase, which also corresponded with the eventual site of surgery. Sixty-nine percent of the patients had both increased cerebral perfusion on the ictal scan and decreased cerebral perfusion on the interictal scan, which corresponded to the eventual site of surgery. Duncan et. al.(95) report similar sequences of ictal (increased) and post-ictal (decreased) temporal lobe perfusion in their study of 28 patients with medically intractable complex partial seizures. Subsequent temporal lobe surgery on the hemisphere indicated by SPECT was again successful (i.e., 90% reduction in seizure frequency) in 96% (27/28) of their patients.
Brain SPECT findings in epilepsy research have correlated well with other techniques and may contribute additional information. A number of authors have evaluated the utility of brain SPECT and various structural techniques for the localization of seizure foci(96-102). Otsubo et. al.(90) reviewed EEG, CT, MRI, and SPECT imaging data obtained from 28 children with intractable seizures. In their study, brain SPECT accurately localized epileptogenic abnormalities found with MRI, CT, and EEG. However, the SPECT studies additionally showed interictal decreases of rCBF that corresponded with the temporal epileptogenic zone in 68% (15/22) of the patients. Uvebrant et. al.(102) compared the value of SPECT to neuropediatric evaluations in their study of 79 children. Of the 56 children investigated for epilepsy, SPECT yielded clinically relevant information consistent with a diagnosis of epilepsy in 79% of their cases. MRI and CT yielded clinically relevant information in 49% and 36% of their cases, respectively. Although neither functional nor structural imaging can clarify the etiology of epilepsy, SPECT may provide information that associates the distribution of functional lesions with the behavioral manifestations associated with epilepsy. Patients with epilepsy may have functional pathophysiological deficits of otherwise grossly intact brain tissue(97-99). In 45 patients Matsuda et al.(98) found that SPECT gave useful information about interictal cerebral blood flow around an epileptic focus and about the effects of antiepileptic drugs on brain function in TLE. Ictal SPECT may also be helpful for subclassification of temporal lobe seizures(102), whereas clinical features are relatively unhelpful. Perfusion patterns provide insight into preferential pathways of seizure propagation in the subtypes of TLE.
One of the major drawbacks of SPECT is that several studies report that interictal PET may be as sensitive as ictal SPECT(104-105). It is easier to perform studies in the interictal phase where PET is available. Yet, it is clear that SPECT may add to the clinical evaluation of temporal lobe function by identifying an area(s) of abnormality, showing deficits not seen by EEG, and possibly by shedding light on why anticonvulsants are useful for broad neuropsychiatric indications.