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Temporal Lobe Epilepsy

Cerebral cavernous malformations (CCMs) may appear anywhere in the brain, but they are most often found in the supratentorial region. This area includes the frontal, parietal, temporal, and occipital lobes as well as the thalamus, hypothalamus, and basal ganglia. Research has shown that 50 to 70% of supratentorial CCMs result in seizure disorders, and seizure is often the sole symptom in lesions of this location.[1] CCM-related seizures have a higher incidence than seizures induced by either arteriovenous malformations (AVMs) or gliomas. Additionally, CCM seizures are more likely resistant (“intractable”) to medical therapy with antiseizure drugs.

The right and left temporal lobes are particularly vulnerable to seizure activity. There are a number of different types of seizure - the type of seizure most commonly associated with temporal lobe cavernous malformations are “simple partial” and “complex partial” seizures.

Simple partial – a type of seizure in which a person remains aware but is unable to stop their experience or behavior. The seizure behavior depends on the area of the brain affected. The seizure can result in such things as intense feelings, uncontrolled movements, vision problems, or speech problems. Simple partial seizures centered in the temporal lobe most commonly result in unexplained intense feelings.

Complex partial – a type of seizure in which there is a loss of awareness and the person engages in “automatisms” – behaviors such a chewing, swallowing, picking at clothes, or scratching. Seizures may “generalize” with frank loss of consciousness and convulsions, as a partial seizure spreads throughout the brain.

In each individual there are zones in the brain, called epileptogenic zones, which can cause seizure when irritated. The exact location of these zones is unique to each individual and is usually fairly small. Within each person, the locations of the zones remain stable over time. In other words, if an electroencephalogram (EEG) or magnetoencephalogram (MEG) were used to define the basic area of a person’s epileptogenic zone, this zone would not change or move over time.[2] Limitations of EEGs/MEGs, specifically placement of electrodes, make it impossible to define the exact boundaries of this zone within any give person.

Cavernous malformations can irritate epileptogenic zones, including those in the temporal lobes, in two different ways. First, cavernous malformations may exert pressure against an epileptogenic zone. Second, hemorrhages in cavernous malformations produce deposits of a substance called hemosiderin. Hemosiderin, a blood breakdown product, is a form of iron. These iron deposits do not go away, even if the cavernous malformation shrinks. If a hemosiderin deposit is in an epilotogenic zone, it can cause seizures.[3]

Once a seizure begins, it is not confined to the epileptogenic zone. Seizure activity spreads to a larger region which can be mapped using an EEG. Mapping these areas is helpful in generalizing the location of the epileptogenic zone and thus the origin of the epileptic seizure.[1] It’s somewhat analogous to earthquake mapping where a seizure is akin to an earthquake of the brain. While an earthquake may be felt for hundreds of miles, the true epicenter of the quake may be determined from measurement locations quite distant from the epicenter itself.

Sometimes, epilepsy can result from sources other than cavernous malformation. Defining the boundaries of the epileptogenic zones can set the stage for determining whether a lesion, or some other process, is responsible for epilepsy. It’s not as cut and dried as finding a lesion in one area and simply attributing epilepsy to the presence of the lesion alone. Other factors can play a role. These factors include such things as the number of lesions a person or the susceptibility/predisposition of the person to epilepsy. This susceptibility can vary greatly from person to person. For some people, the source of their epilepsy may never be determined.

In the absence of surgical removal of the lesion, the most effective treatment method is through the use of anticonvulsants. The prospect for seizure control is excellent in most cases. Anti-convulsants have developed to the point where it is easier to achieve seizure control with fewer side effects than in the past. Also, CCM induced epilepsy that is completely controlled by medication probably does not warrant surgery unless other factors (mass effect, hemorrhage, lesion history/stability, etc.) necessitate it.[4]

For those patients with truly intractable seizures or those who do not respond fully to medications, surgery is the primary solution. Unlike most other CCM removal procedures, surgeries performed to stop CCM related seizures include removing a portion of healthy brain tissue stained by hemosiderin irritant. Without removing this stained tissue, seizure control is not nearly as assured. Of course, removal of healthy tissue cannot occur if the tissue is involved in functioning, or “eloquent”. Functional MRI or other diagnostic “brain mapping” may be used to delineate eloquent from non-eloquent tissue.[5]

Of note, venous angiomas, regardless of location, rarely result in seizure activity. If a person who previously had been seizure free suddenly experiences partial seizures, an MRI in conjunction with EEG/MEG is strongly recommended to ensure that a cavernous malformation is not the root cause of the problem.

Acknowledgments

Our thanks to Dr. Issam Awad, head of our scientific advisory board, for his review of the information in the previous articles.

References 

[1] Awad IA, Rosenfeld J, Ahl J, et al. Intractable epilepsy and structural lesions of the brain: mapping, resection strategies, and seizure outcome Epilepsia.1991; 32:179-186.

[2] Awad IA, Nayel M. surgery: introduction and overview.Clin Neurosurg. 1992;38:493-513.

[3] Awad, IA, Robinson JR. Cavernous Malformations and Epilepsy. Cavernous Malformations. 1993:49-63.

[4] Robinson JR, Awad IA, Little JR. Natural history of the cavernous angioma J Neurosurg. 1991;75:709-714.