since the 19th century , it has been a controversial topic whether epilepsy is a progressive disorder . repeated and continuous epileptic seizures that are generated from a primary focus can induce secondary epileptogenic zones in previously unaffected brain areas . this secondary epileptogenesis concept was first described by morrel in animal studies.1 ' mirror focus ' is a special form of secondary epileptogenesis ; it means that the secondary epileptogenic zone is observed in a contralateral homotopic area to the primary epileptic focus.2 some previous studies reported mirror focus for temporal lobe epilepsy with a tumor or focal lesion.3,4 we have observed a patient with mirror focus in left occipital lobe epilepsy . she was right handed and her epileptic seizures consisted of a visual aura with flashes , unresponsiveness and oro - ailimentary automatisms like lip smacking . her seizures were not completely controlled by a polytherapy with carbamazepine , lamotrigine , and valproic acid . in a neurological examination , no lateralizing or focal neurological deficits were noted . initially she was admitted to a long - term video eeg monitoring unit ( emu ) for presurgical evaluation . during 9 day stay , interictal epileptileptifom discharges were seen frequently in the left occipital area ( fig . ictal events were 31 simple partial seizures ( sps ) and 11 complex partial seizures ( cps ) . among the 31 sps , 12 visual auras occurred on her right side , whose ictal eeg showed left occipital discharges in 9 sps and the remaining 3 auras showed no eeg changes . the other 19 visual auras occurred on her left side , whose ictal eeg revealed left occipital discharges in 2 sps and the remaining 17 auras showed no eeg changes . among the 11 cps , right versive seizures are in 7 seizures , dialeptic seizures in 3 seizures , and left versive seizure in 1 seizure . substracted ictal spect co - registered to mri ( siscom ) demonstrated a hyperperfused area in the left posterior temporal area . brain 18f - fdg pet demonstrated bilateral posterior temporo - occipital hypometabolism , but the left temporo - occipital hypometabolism were more prominent than the right one . those results of the first presurgical evaluation demonstrated the focal epilepsy was originated from left occipital area . invasive presurgical evaluation , after placement of subdural electrodes over the left temporo - occipital areas , was planned for resective epilepsy surgery . she underwent subdural grids and strips around the left occipital area , and her seizures were recorded again in the emu . the 3 cps were right versive seizures and ictal eeg showed a start from the left occipital area . 19 visual auras occurred on her left side and none of them showed eeg changes . 15 visual auras occurred on her right side ; for 2 of these auras , the eeg showed changes in the left occipital area , and the remaining 13 auras showed no eeg changes . interictal epileptiform discharges were seen only in the left occipital areas . after the functional mapping using evoked potentials and brain stimulation , she has undergone neocortical resection of left occipital area including ictal onset zone . the pathology of the specimen demonstrated focal cortical dysplasia type i. she had no seizures for 10 months after epilepsy surgery , but then her seizures started again with seizure frequency similar to that before the operation . in contrast to the first long - term video monitoring , interictal epileptiform discharges were seen every minute , all in the right occipital area ( fig . ictal events were recorded in 17 seizures . among them , 12 were sps and 5 were cps . all sps consisted of visual auras that occurred on the left , right or whole fields . among the sps , electro - clinical correlations showed right occipital origin seizures were 1 out of 4 left visual auras , 0 out of 2 right visual auras , and 5 out of 6 whole field visual auras . but left occipital origin sps were not seen . among 5 cps brain 18f - fdg pet demonstrated left posterior temporo - occipital metabolism defects due to the previous operation . we concluded that the epilepsy classification was right occipital lobe epilepsy after epilepsy surgery , which was the mirror focus , compared with the previous left occipital focus . she was right handed and her epileptic seizures consisted of a visual aura with flashes , unresponsiveness and oro - ailimentary automatisms like lip smacking . her seizures were not completely controlled by a polytherapy with carbamazepine , lamotrigine , and valproic acid . in a neurological examination , no lateralizing or focal neurological deficits were noted . initially she was admitted to a long - term video eeg monitoring unit ( emu ) for presurgical evaluation . during 9 day stay , interictal epileptileptifom discharges were seen frequently in the left occipital area ( fig . ictal events were 31 simple partial seizures ( sps ) and 11 complex partial seizures ( cps ) . among the 31 sps , 12 visual auras occurred on her right side , whose ictal eeg showed left occipital discharges in 9 sps and the remaining 3 auras showed no eeg changes . the other 19 visual auras occurred on her left side , whose ictal eeg revealed left occipital discharges in 2 sps and the remaining 17 auras showed no eeg changes . among the 11 cps , right versive seizures are in 7 seizures , dialeptic seizures in 3 seizures , and left versive seizure in 1 seizure . substracted ictal spect co - registered to mri ( siscom ) demonstrated a hyperperfused area in the left posterior temporal area . brain 18f - fdg pet demonstrated bilateral posterior temporo - occipital hypometabolism , but the left temporo - occipital hypometabolism were more prominent than the right one . those results of the first presurgical evaluation demonstrated the focal epilepsy was originated from left occipital area . invasive presurgical evaluation , after placement of subdural electrodes over the left temporo - occipital areas , was planned for resective epilepsy surgery . she underwent subdural grids and strips around the left occipital area , and her seizures were recorded again in the emu . the 3 cps were right versive seizures and ictal eeg showed a start from the left occipital area . 19 visual auras occurred on her left side and none of them showed eeg changes . 15 visual auras occurred on her right side ; for 2 of these auras , the eeg showed changes in the left occipital area , and the remaining 13 auras showed no eeg changes . after the functional mapping using evoked potentials and brain stimulation , she has undergone neocortical resection of left occipital area including ictal onset zone . the pathology of the specimen demonstrated focal cortical dysplasia type i. she had no seizures for 10 months after epilepsy surgery , but then her seizures started again with seizure frequency similar to that before the operation . in contrast to the first long - term video monitoring , interictal epileptiform discharges were seen every minute , all in the right occipital area ( fig . ictal events were recorded in 17 seizures . among them , 12 were sps and 5 were cps . all sps consisted of visual auras that occurred on the left , right or whole fields . among the sps , electro - clinical correlations showed right occipital origin seizures were 1 out of 4 left visual auras , 0 out of 2 right visual auras , and 5 out of 6 whole field visual auras . but left occipital origin sps were not seen . among 5 cps siscom demonstrated a hyperperfused area in the right occipital areas ( fig . 2c ) . brain 18f - fdg pet demonstrated left posterior temporo - occipital metabolism defects due to the previous operation . we concluded that the epilepsy classification was right occipital lobe epilepsy after epilepsy surgery , which was the mirror focus , compared with the previous left occipital focus . this is the case report about mirror focus in a patient with pharmacoresistant occipital lobe epilepsy with normal brain mri . in the first study , our case s seizure focus was left occipital lobe , based on the full presurgical evaluation including video eeg monitoring , interictal spect and 18f - fdg pet . invasive presurgical evaluation and immediate surgical outcome demonstrated the left occipital focus . however , her epileptic seizures relapsed 10 months after the surgery . in the second study , the patient s redeveloped focus was right occipital lobe , which is contralateral homotopic area , a mirror focus . there is a possibility that right occipital focus can be considered as the remained focus in case with initially bilateral occipital epilepsy . however , the clinical course and initial evaluation of definite unilateral occipital lobe epilepsy supports the development of mirror focus in the contra - lateral hemisphere . to our knowledge , we can suggest two reasons why the mirror focus is not reported in occipital lobe epilepsy . the frequency of occipital lobe epilepsy varies from 2% to 13% of symptomatic partial epilepsies.6,7 second , the connection between left and right hemispheric homotopic area is not direct in occipital lobes . frontal and parietal lobes are densely and closed connected via corpus callosum , and temporal lobes are via anterior and posterior commissure . morrell defined the three different stages of the secondary epileptogenesis process.8 the first stage is the dependent stage , in which interictal epileptiform discharges are observed synchronously with primary focus . the second stage is the intermediate stage , in which interictal epileptiform discharges and occasional seizures occur independent of the primary focus . the third stage is the independent stage , in which interictal epileptiform discharges and seizures occur independent of the primary focus . removal of the primary focus is followed by persistence of interictal epileptiform discharges and seizures . our case showed the persistence of secondary interictal and ictal focus after removal of the primary focus . that means that the right occipital mirror focus in our case was in the third independent stage . goddard described the  kindling phenomenon ( repeated stimulations lower the seizure threshold and induce more frequent seizures).9 both mirror focus and kindling phenomenon are applied in animal models , but these ideas are disputed in human beings . these issues are very important in epilepsy patients , in order to decide on early aggressive medical treatments and plan epileptic surgeries . usually occipital lobe epilepsy can have spread into frontal area through supra - svlvian pathways and temporal lobe through intra - sylvian pathway . also our case suggests the possibility that contralateral spread can occur and make a mirror focus in contralateral homotopic area . if the formation of mirror focus is in occipital lobe epilepsy , it can cause fatal visual field dysfunction as well as progressive disabling seizures which can not be surgically removed . also the progression of epilepsy and timely surgery of focus should be deliberately considered in a patient with occipital lobe epilepsy . generally when we designate a lesion as a mirror focus in the third independent stage , interictal epileptiform discharges or seizures pesistently exist  before  and after resection of the primary focus . but any epileptiformdischarges or seizures of right occipital origin were not recorded in the long - term presurgical evaluation of this patient . given that postoperative seizures unexpectedly developed from 10 months after epilepsy surgery , this was a latent mirror focus which was newly developed after the removal of the primary focus .
mirror focus is one of the evidence of progression in epilepsy , and also has practical points for curative resective epilepsy surgery . 
 the mirror foci are related to the kindling phenomena that occur through interhemispheric callosal or commissural connections . 
 a mirror focus means the secondary epileptogenic foci develop in the contralateral hemispheric homotopic area . 
 thus mirror foci are mostly reported in patients with temporal or frontal lobe epilepsy , but not in occipital lobe epilepsy . 
 we have observed occipital lobe epilepsy with mirror focus . before epilepsy surgery , the subject s seizure onset zone was observed in the left occipital area by ictal studies . 
 her seizures abated for 10 months after the resection of left occipital epileptogenic focus , but recurred then . 
 the recurred seizures were originated from the right occipital area which was in the homotopic contralateral area . 
 this case can be an evidence that occipital lobe epilepsy may have mirror foci , even though each occipital lobe has any direct interhemispheric callosal connections between them .