331 A seizure is a paroxysmal alteration in neurologic function resulting from abnormal excessive neuronal electrical activity. The pathophysiologic basis of seizures is loss of normal regulation of neuronal excitation and inhibition, resulting in a state of relative hyperexcitability. Epilepsy is a chronic condition characterized by recurrent seizures unprovoked by an acute systemic or neurologic insult. MRI plays a pivotal role in every aspect of the diagnosis and management of seizures. MR techniques are crucial for a complete workup of a patient with seizures because the ultimate goal of treatment is relief from symptoms, whether it be medical or surgical treatment and in the latter case prevention of disastrous postresection complications, such as speech and memory loss. The International classification of epilepsies and epileptic syndromes classifies clinical epilepsy into two broad categories, idiopathic (primary) and symptomatic (secondary) disorders [1]. Primary epilepsies are genetically transmitted seizures that are not associated with other neurological disturbances or structural pathology and are usually benign. Secondary epilepsies, in contrast, are seizures resulting from a specific pathologic substrate that can be genetic or acquired. There is a third group which includes epilepsy syndromes of uncertain or mixed classification such as neonatal seizures, febrile seizures etc. Both primary and secondary epilepsies are further divided into generalized disorders (where the brain is diffusely and bilaterally involved) and localization-related or focal disorders (seizures originate from a localized cortical region). The common causes of seizures can be roughly classified as 1) Idiopathic (no identifiable cause) - Usually begin between 5-20 years. MRI In Seizure Disorder - A Pictorial Essay V ARORA, IBS NIJJAR, DS MAHAJAN, PS SANDHU, JP SINGH, R CHOPRA Ind J Radiol Imag 2005 15:3:331-340 Key words : - Seizure disorder - MRI From Nijjar Scan & Diagnostic Centre, Amritsar Request for Reprints: Dr. Vijinder Arora, C/o Nijjar Scan & Diagnostic Centre, 51/1, Court Road, Amritsar - 143001 (Pb) Received 22 September 2004; Accepted 10 March 2005 09-135 - Can occur at any age. - No other neurologic abnormality. - Often family history of epilepsy or seizures. 2) Mesial Temporal sclerosis: In addition to being the lesion most commonly associated with complex partial seizures (60-85% cases) [2], it is also the most common structural abnormality in human epilepsy [3]. It is characterized by pyramidal cell loss and astrogliosis in the mesial temporal lobe, hippocampal formation, amygdala, parahippocampal gyrus and the entorhinal cortex [4]. Although MTS is a bilateral process, one side is affected more than the other in 80% of patients. Usually, the more severely affected side is the origin of the patients seizures. Cases in which both sides are equally affected generally respond less favorably to temporal lobectomy. Fig1) Bilateral hippocampal sclerosis: Bilateral small and hyperintense hippocampus.
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A seizure is a paroxysmal alteration in neurologic function
resulting from abnormal excessive neuronal electrical activity. The
pathophysiologic
basis of seizures is loss of normal regulation of neuronal
excitation and inhibition, resulting in a state of relative
hyperexcitability. Epilepsy is a chronic condition characterized by
recurrent seizures unprovoked by an acute systemic or neurologic
insult.
MRI plays a pivotal role in every aspect of the diagnosis and
management of seizures.
MR techniques are crucial for a complete workup of a patient with
seizures because the ultimate goal of treatment is relief from
symptoms, whether it be medical or surgical treatment and in the
latter case prevention of disastrous postresection complications,
such as speech and memory loss.
The International classification of epilepsies and epileptic
syndromes classifies clinical epilepsy into two broad categories,
idiopathic (primary) and symptomatic (secondary) disorders [1].
Primary epilepsies are genetically transmitted seizures that are
not associated with other neurological disturbances or structural
pathology and are usually benign. Secondary epilepsies, in
contrast, are seizures resulting from a specific pathologic
substrate that can be genetic or acquired. There is a third group
which includes epilepsy syndromes of uncertain or mixed
classification such as neonatal seizures, febrile seizures
etc.
Both primary and secondary epilepsies are further divided into
generalized disorders (where the brain is diffusely and bilaterally
involved) and localization-related or focal disorders (seizures
originate from a localized cortical region).
The common causes of seizures can be roughly classified as
1) Idiopathic (no identifiable cause) - Usually begin between 5-20
years.
MRI In Seizure Disorder - A Pictorial Essay V ARORA, IBS NIJJAR, DS
MAHAJAN, PS SANDHU, JP SINGH, R CHOPRA
Ind J Radiol Imag 2005 15:3:331-340
Key words : - Seizure disorder - MRI
From Nijjar Scan & Diagnostic Centre, Amritsar
Request for Reprints: Dr. Vijinder Arora, C/o Nijjar Scan &
Diagnostic Centre, 51/1, Court Road, Amritsar - 143001 (Pb)
Received 22 September 2004; Accepted 10 March 2005
09-135
- No other neurologic abnormality.
- Often family history of epilepsy or seizures.
2) Mesial Temporal sclerosis: In addition to being the lesion most
commonly associated with complex partial seizures (60-85% cases)
[2], it is also the most common structural abnormality in human
epilepsy [3]. It is characterized by pyramidal cell loss and
astrogliosis in the mesial temporal lobe, hippocampal formation,
amygdala, parahippocampal gyrus and the entorhinal cortex
[4].
Although MTS is a bilateral process, one side is affected more than
the other in 80% of patients. Usually, the more severely affected
side is the origin of the patients seizures. Cases in which both
sides are equally affected generally respond less favorably to
temporal lobectomy.
Fig1) Bilateral hippocampal sclerosis: Bilateral small and
hyperintense hippocampus.
332
MR features of MTS Primary signs
- A small atrophic unilateral hippocampus (fig.2).
- Hyperintensity on both T2 W and FLAIR images (Fluid attenuated
inversion recovery images)(fig.3).
- Loss of the hippocampal internal architecture and that of normal
digitations of the head (fig.3).
- Visual assessment of size, architecture and signal intensity
changes is quite sensitive, with the eye being able to detect
asymmetry of 14% or more [5].
Secondary signs
- Unilateral atrophy of the mamillary body [6], fornix columns
(circuit of paper) [7], and the amygdala.
- Increased T2 W signal in the anterior temporal lobe white matter
[8] with loss of grey-white demarcation in the ipsilateral anterior
temporal lobe (fig.4).
- Unilateral dilatation of the temporal horn (a less reliable
secondary sign) [4]
- Unilateral atrophy of the collateral white matter bundle
[8].
Fig2) Small hippocampus on right side.
Fig3) Loss of normal hippocampal digitations on left side.
Fig4) Loss of grey-white matter differentiation in left anterior
temporal lobe.
Volumetric MR Imaging:
Quantitative evaluation of hippocampal volume has been found to
marginally increase the sensitivity over visual analysis in
detection of hippocampal sclerosis [9].
Measuring size can be accomplished by manually tracing the
hippocampus. The normal, ipsilateral hippocampal volume is
approximately 2.8.Ml [4].
T2 Relaxometry:
It is used to quantify the T2 signal in the hippocampus. It
332 V Arora et al IJRI, 15:3, August 2005
333
Other techniques useful in MTS are diffusion tensor imaging and
functional MR imaging.
3) Developmental Disorders: These can be further divided into four
groups [12] on the basis of
- Abnormal cell proliferation. - Abnormal neuronal migration. -
Abnormal cortical organization. - Unclassified miscellaneous
group.
Some of the common abnormalities are: -
Focal Cortical Dysplasia is among the most common causes of
epilepsy attributable to focal cerebral dysgenesis. 60% FCD is
found in temporal lobes [13].
measures the decay in signal intensity at different TE s in a
series of T2W images acquired in the same slice. In MTS, the
relaxation time has proven to be lengthened by 10 milliseconds
[4].
Fig5) Hippocampal sclerosis right side with dilated right temporal
horn.
MR Spectroscopy:
Proton MRS has been widely used in providing insight into the
metabolic alterations in epilepsy.
N-acetyl - aspartate (NAA) is a marker of metabolically active
neurons, and decreased NAA/ creatinine or decreased NAA /
creatinine +choline ratios signify neuronal loss and / or metabolic
dysfunction. A decrease in these ratios has been shown to
lateralize temporal lobe epilepsy in 65% to 90 % of patients with
bilateral temporal lobe structural abnormalities on MR [10]. In
cases of temporal lobe epilepsy with normal MR studies, NAA ratios
can provide lateralizing evidence in at least 20% of patients [11]
(fig.6).
Fig6) MR spectroscopy in left hippocampal sclerosis showing reduced
NAA/ Creatine + Choline ratio on left side.
Fig7) Focal cortical dysplasia right side: Expanded gyrus with
thickened cortex and abnormal sulcus.
Focal Cortical Dysplasia with balloon cells dysplasia
- Expanded gyrus with abnormally oriented sulci and thickened
cortex [13](fig.7). Normal cortical thickness is 4mm [4]. Cortical
white matter junction is often indistinct and the subcortical white
matter may exhibit T1 & T2 prolongation as compared with white
matter with mature myelination.
In focal transmantle dysplasia, abnormal signal is seen to extend
from the cortex to, or almost to, the ventricle [14] (fig.8).
MRI in Seizure Disorder 333IJRI, 15:3, August 2005
334
- Subcortical heterotopia may occur at any location from the
periventricular
- white matter to the grey-white matter junction.
- Band heterotopia is found almost exclusively in females, although
rare
- cases in males have been reported [16]. On imaging, white matter
zone
- separates a thin rim of cortex from a broad band of subcortical
grey matter
- (fig.10). In FCD without balloon cells, the lesion may be evident
only as areas of blurring of the cortical white matter
junction.
Grey Matter Heterotopia: -
It is the focal collection of ectopic neurons in the cerebral
hemispheres and can be of three broad types
- Subependymal heterotopia appearing as ovoid lesions within
the
- subependymal region. They are isointense with grey matter on all
imaging
- sequences [15](fig.9).
Fig8) Focal transmantle dysplasia left frontal lobe: Thickened
cortex with abnormal signal extending from the cortex to the left
frontal horn.
Fig9) Subependymal heterotopia along the right occipital
horn.
Fig10) Band heterotopia right temporal lobe.
Fig11) Lissencephaly.
335
Lissencephaly: -
It includes a group of disorders characterized by a generalized
paucity of gyral and sulcal formation known as agyria and
pachygyria. Imaging studies reveal a range from the severe cases
showing completely smooth brain with sulcal fissures being only the
definable fissures to milder cases, in which a few shallow sulci
surround broad flat gyri (fig.11).
Polymicrogyria: -
This refers to an abnormal appearance of the cortex with multiple
abnormally small convolutions and too few sulci. A variety of
etiologic factors has been linked to polymicrogyria including
prenatal cytomegalovirus infection, cerebral ischaemia and genetic
disorders [17,18]. It maybe unilateral or bilateral, symptomatic or
asymptomatic and associated with other anomalies or isolated. On
MR, polymicrogyria is seen as thickened cortex with poorly
developed sulci and irregular margin of the cortical white matter
junction (fig.12). In bilateral perisylvian polymicrogyria, the
opercula are dysplastic and incomplete and sylvian fissure is wide
and under- developed, with the sagittal images showing posterior
extension of the sylvian fissure and apparent thickening of the
cortex (fig.13). Schizencephaly:
Schizencephaly describes grey matter lined clefts that extend
through the entire cerebral hemisphere from the lateral ventricle
to the cerebral cortex [19,20].
Fig12) Polymicrogyria: Thick cortex with poorly developed sulci
right side.
Fig13) Perisylvian polymicrogyria.
336
Schizencephalic clefts maybe "closed" in which only a double layer
of cortex is seen extending from the surface to the ventricle.
Alternatively, they maybe "open" in which there is wide
communication between the subarachnoid space and ventricle through
a broad hemispheric cleft (fig.14). These clefts are lined by grey
matter. A key feature is the presence of a ventricular dimple. This
is almost always present in cases with closed or minimally open
lips (fig.15).
4) Neoplasms And Vascular Malformations:
MR has nearly 100% sensitivity for detecting epileptogenic
neoplastic and vascular lesions [21]. Most epileptogenic neoplasms
occur in the temporal lobe, in or adjacent to the cerebral cortex.
Indolent tumours such as ganglioglioma, dysembryoplastic
neuroepithelial tumours, and low grade gliomas are often associated
with chronic intractable seizures (fig.16). In the elderly
population, cerebral metastasis is the most frequent neoplastic
lesion associated with late onset seizures [22](fig.17).
Seizure is the principal clinical manifestation of vascular
malformations, occurring in 24% to 69% of arterio-venous
malformations [23] and 34% to 51% of cavernous haemangiomas (also
referred to as cavernous angiomas or cavernous malformations)[24].
High flow vascular lesions appear as curvilinear signal voids
(fig.18). Cavernous haemangiomas demonstrate a stereotypical
appearance of central hyperintensity due to haemoglobin products,
surrounded by a hypointense rim resulting from haemosiderin
(fig.19).
Fig15) Minimally open lip Schizencephaly with a ventricular
dimple.
Fig16) Patient with right temporal lobe tumour presenting with
seizures.
Fig17) a) Left frontal lobe enhancing lesion in a fifty two year
old man with history of single seizure. b) MR Spectro reveals high
choline suggestive of mitotic lesion. The possibility of metastasis
was kept. On further investigation patient was proved to have Ca
lung.
336 V Arora et al IJRI, 15:3, August 2005
337
5) Infections:
Seizures are common with acute cerebral infections (viral
encephalitis and bacterial and aseptic meningitis) as well as those
with brain abscesses, parasitic infections, aspergillosis, and
other fungal infections. Patients developing acute central nervous
infection before four years of age have a higher propensity to
develop hippocampal sclerosis [4]. Chronic epilepsy however may
result from post-inflammatory glial scarring.
In certain developing regions of the world, neurocysticercosis has
been reported to be the most common cause of new onset partial
seizures. Inflammation surrounding the cysticercosis manifests as
acute seizure disorder. In the inflammatory stage provoked by the
dying parasite, the cerebral lesions of cysticercus appear as small
enhancing rings on CT and MR with variable degree of oedema in
surrounding brain [25](fig.20). In vivo proton MR spectroscopy has
been described in a case of a large cysticercus cyst [26]. The
metabolites observed are lactate, succinate, acetate, alanine and
an unassigned resonance at 3.3ppm.
In the developing world, tuberculomas account for 15-50% of the
intra-cranial tumours seen [27]. Symptoms are often limited to
seizures and correlates of intra-cranial pressure. The MR features
of the individual tuberculoma depend on whether the granuloma is
non-caseating or caseating with a solid center, or caseating with a
liquid center [27]. The non-caseating granuloma is usually
iso/hypointense on
Fig18) Right temporal lobe arteriovenous malformation.
Fig19) Cavernous haemangioma left caudate nucleus.
Fig20) Neurocysticercosis: Multiple cystic lesions with hypointense
speck representing scolex.
MRI in Seizure Disorder 337IJRI, 15:3, August 2005
338
T1W and hyperintense on T2W images. These granulomas show
homogeneous enhancement after injection of contrast agent. The
caseating solid granulomas appear relatively isointense/hypointense
on T1W images with isointense/hyperintense rim and isointense to
hypointense on T2W images(fig.21). These lesions show rim
enhancement on post-contrast T1W imaging. The granulomas with
central liquefaction of caseous material appear centrally
hypointense on T1W and hyperintense on T2W images and show rim
enhancement after contrast administration(fig.22). MR Spectroscopy
has been found to be specific for intra- cranial tuberculomas when
combined with imaging. Intracranial tuberculomas are characterized
by a spectral pattern that primarily involves long chain lipids,
with a 0.9 to 1.6ppm peak range, associated with a virtual absence
of all brain metabolites normally present [27](fig.23).
6) Neurocutaneous Syndromes (Phacomatosis):
Recurrent seizures are the most common and clinically important
manifestation of Sturge-Weber syndrome, also known as
encephalotrigeminal angiomatosis [28]. This syndrome consists of a
facial port-wine nevus in the trigeminal nerve distribution,
leptomeningeal angiomatosis, epilepsy, mental retardation, and
other neurologic deficits. The brain involvement is usually
unilateral with characteristic tram-tack gyriform calcification
appearing as linear low signal on MR. The involved hemisphere is
atrophic, often with overlying calvarial thickening.
Tuberous sclerosis is an inherited systemic disease with prominent
cutaneous and CNS manifestations with the classic triad of adenoma
sebaceum, epilepsy, and mental handicap. The diagnostic findings
include the presence of multiple sub-ependymal nodules and multiple
cortical tubers.
7) Miscellaneous Abnormalities:
The pathologic mechanisms for post-traumatic seizures include
deposition of tissue haemosiderin, which is a potent epileptogenic
agent, and cortical gliosis [29]. Gliosis usually appears as a
region of increased signal change on T2W images, often associated
with volume
Fig21) Caseating solid granuloma with enhancement.
Fig22) Caseating granulomas with central liquefaction showing rim
enhancement.
Fig23) MR spectroscopy reveals (SVS SE135) high lipid peak in
tuberculoma.
Fig24) Post traumatic gliosis in left frontal lobe. Patient has
history of recurrent seizures after injury five years ago
338 V Arora et al IJRI, 15:3, August 2005
339
loss (fig.24). Haemosiderin appears hypointense on T2W weighted
images and gradient images.
Beyond age 50, stroke is the most frequent cause of seizures [30].
Delayed - onset seizures after an acute stroke carry much greater
risk of developing into chronic epilepsy [22].
Eclampsia is defined as the development of convulsions in pregnant
women with hypertension and proteinuria. Studies of women with
eclampsia disclose multiple foci of cortical and subcortical white
matter edema, primarily in the occipital lobes. [31]
(Fig.25).
Perinatal / neonatal hypoxic-ischaemic insult is a common cause of
seizures in neonates (Fig. 26).
shown to be highly sensitive and specific in identifying the
underlying pathology in partial epilepsy. MRI may determine patient
selection for surgery and directly affects the presurgical
evaluation and operative strategy. Therefore MRI should be
performed early to avoid unnecessary medication in patients with
resectable intracranial mass lesions.
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