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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.
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