A myriad spectrum of seizures on magnetic resonance imaging – A pictorial essay
How to cite this article: Lingutla RK, Mahale A, Bhat AR, Ullal S. Myriad spectrum of seizures on magnetic resonance imaging – A pictorial essay. J Clin Imaging Sci 2022;12:3.
Patients with seizures represent a challenging clinical population both in pediatrics and adults. Accurate diagnosis of the cause of a seizure is important in choosing an effective treatment modality, surgical planning, predicting a prognosis, and follow-up. Magnetic resonance (MR) imaging using a dedicated epilepsy protocol plays a key role in the workup of these patients. Additional MR techniques such as T2 relaxometry and MR spectroscopy show a promising role to arrive at a final diagnosis. The spectrum of epileptogenic causes is broad. Radiologists and physicians need to be updated and require a patterned approach in light of clinical history and electroencephalogram findings to arrive at a reasonable differential diagnosis. This pictorial essay aims to review a few of the common and uncommon causes of seizures and their imaging features.
Magnetic resonance imaging
Dedicated magnetic resonance imaging protocol
Spectrum of causes
Mesial temporal sclerosis
Epilepsy is one of the most common diagnoses encountered by neurologists. The challenge in seizure evaluation is to identify the most likely pathology in each case taking age, clinical history, and type of seizure activity into consideration, to best direct the imaging resources and protocols. Accurate diagnosis of the cause of a seizure is important in choosing an effective treatment modality, surgical planning, predict prognosis, and follow-up.
Magnetic resonance imaging (MRI) with its excellent soft-tissue contrast resolution and multiplanar imaging capability is highly sensitive and specific in identifying the underlying structural pathology and demonstrating the precise anatomy with high detail. Additional MR techniques such as contrast-enhanced MR, T2 relaxometry, MR spectroscopy (MRS), and diffusion tensor imaging not only add value to the final diagnosis but also play an important part in pre-surgical evaluation and follow-up. A dedicated epilepsy protocol includes high-resolution fluid-attenuated inversion recovery (FLAIR) coronal oblique sequences obtained perpendicular to the long axis of the hippocampus [Figure 1], coronal oblique T1 inversion recovery sequence and T2 relaxometry in addition to the conventional sequences.[3-5] MRI has a maximum yield in the evaluation of patients with focal onset of seizures on history, clinical examination, or electroencephalogram (EEG), and refractory seizures.
This pictorial essay aims to illustrate the common and uncommon causes of seizures and their imaging features on MRI illustrated in Table 1[7,8] and a brief overview of the utility of T2 relaxometry in seizure evaluation.
DISORDERS OF CORTICAL MALFORMATION
|Disorders of cortical malformation||Gray matter heterotopia
Focal cortical dysplasia Hemimegalencephaly
|Mesial temporal sclerosis
|CNS tumors||Glioma, dysembryoplastic neuroepithelial tumor, ganglioglioma, pleomorphic xanthoastrocytoma, craniopharyngioma, dermoid cyst, and meningioma.|
|Vascular malformations||Cerebral arteriovenous malformation
Cerebral slow flow venous malformation
|Encephalomalacia and gliosis|
|Miscellaneous uncommon causes||Dyke-Davidoff-Masson syndrome
Posterior reversible encephalopathy syndrome
Arachnoid cyst causing significant mass effect
Skull base meningoencephalocele Megalencephalic leukoencephalopathy with subcortical cysts/Van der Knaap disease
GRAY MATTER HETEROTOPIA
It occurs due to arrest/disruption of normal neuronal migration anywhere between the periventricular germinal zone and cortex during fetal life. They can be subependymal/periventricular, subcortical, or band heterotopia. The heterotopic nodules show gray matter signal intensity on all sequences with no enhancement post-contrast [Figure 2]. Heterotopia can be unilateral or bilateral and can vary from a single small nodule to a coalescent clump of gray matter nodules. The major differential diagnosis for periventricular heterotopia is subependymal nodules of tuberous sclerosis which can be differentiated by the presence of calcification in the latter.
These are transmantle clefts in the brain parenchyma lined by dysplastic gray matter extending from the cortex to the ependymal lining of the lateral ventricle and occur due to abnormal post-migrational development. The two types include open and closed-lip schizencephaly. They are often associated with other forms of cortical migration abnormalities. A close differential to be wary, of is a porencephalic cyst which is lined by gliotic white matter.
FOCAL CORTICAL DYSPLASIA (FCD)
FCD is a common cause of medically refractory seizures in a pediatric population. On MR evaluation, the common findings include a focal area of cortical thickening/thinning with the blurring of gray-white matter margin [Figure 3] and adjacent T2W/FLAIR hyperintensity extending in a tapered fashion up to the ventricular margin called the transmantle sign, which is better appreciated in FCD Type IIb. FCD can occur together with mesial temporal sclerosis (MTS) and other epilepsy-associated tumors such as dysembryoplastic neuroepithelial tumor (DNET) and ganglioglioma.
The gray-white matter junction and blurring of margins are often appreciated better on T1 inversion recovery sequences than on conventional T1W sequences. 3T MRI is proposed to have higher sensitivity in the detection of subtle FCDs which may be missed on 1.5 T MRI.[12-14] The main differential for FCD is cortical-based tumors like low-grade gliomas. Cortical thickening with the blurring of gray-white matter junction favors FCD while cortical expansion with mass effect on adjacent sulci favors tumor. Decrease in NAA and increase in Choline peak help differentiate low-grade gliomas from FCD on MRS.
MTS is a common structural abnormality seen in association with temporal lobe epilepsy. High-resolution coronal oblique T2W/FLAIR sequences obtained perpendicular to the long axis of the hippocampus are required for optimal evaluation of the hippocampus [Figure 1].
Classical features of MTS include increased signal intensity on the T2W/FLAIR sequence with hippocampal volume loss and loss of normal internal architecture and hippocampal indentations [Figure 4a]. The findings are presumed to be secondary to gliosis.
Secondary signs associated with hippocampal sclerosis are atrophy of the ipsilateral mammillary body, fornix and amygdala, dilatation of ipsilateral temporal horn and adjacent choroid fissure. A subtle indistinct gray-white interface in the adjacent anterior temporal lobe is also seen less frequently.[17,18]
MTS can be bilateral and often one side is more severely affected than the other. A closer look at the adjacent cortex is warranted as these lesions can also be associated with cortical dysplasia.
MR volumetric analysis of the hippocampus allows quantitative measurement of hippocampal volume and is obtained by tracing the entire length of the hippocampus. It can be used to detect the presence of hippocampal atrophy and enhance the standard visual analysis with subtle/borderline cases of MTS.
T2 relaxometry allows quantification of T2 relaxation times of the hippocampus which is markedly elevated secondary to neuronal loss [Figure 4b]. T2 relaxometry has the potential to detect early MTS, where subtle volume loss often escapes detection by volumetric or visual examination. T2 relaxometry was proved more efficient in non-invasive lateralization of epileptogenic focus and more sensitive than MR volumetry in detecting early MR-negative lesions. Abnormal relaxation time values are considered more than 2 standard deviations outside the mean value of control hippocampal T2 relaxation times. Placement of region of interest (ROI) for calculating the hippocampal relaxation time is of great importance as abnormal high values are obtained if the ROI includes CSF from the adjacent temporal horn.
Metabolically healthy neurons are marked by the presence of N-acetyl aspartic acid (NAA) on MRS. In hippocampal sclerosis, there is a loss of NAA resulting in decreased NAA/choline+creatinine ratio <0.71. MRS was found to have higher sensitivity and specificity than conventional MRI in the detection of MTS.
PERIVENTRICULAR LEUKOMALACIA (PVL)
PVL occurs as sequelae to perinatal hypoxic-ischemic encephalopathy and is characterized by the presence of patchy areas of gliosis with periventricular white matter volume loss. The posterior aspect of bilateral lateral ventricles is often dilated with irregular margins suggestive of colpocephaly [Figure 5].[24,25]
Tuberous sclerosis complex
Tuberous sclerosis complex is a neurocutaneous syndrome affecting multiple organ systems. Its CNS manifestations include multiple cortical tubers, radial migration lines and subependymal nodules with calcification [Figure 6]. Frequent follow-up is suggested for the development of subependymal giant cell astrocytoma in the region of foramen of Munro.
These patients should undergo further investigations to look for associated renal, cardiac, thoracic, musculoskeletal, and cutaneous abnormalities, thus, ensuring appropriate clinical management of patients and counseling of parents.
Sturge-Weber syndrome is a rare sporadic vascular neurocutaneous syndrome characterized by a dermal capillary venular malformation in the sensory distribution of the trigeminal nerve, retinal choroidal angioma, and a cerebral capillary venous leptomeningeal angioma. On MRI, there is hemicerebral atrophy with gyriform cortical/subcortical T2 hypointensity that blooms on SWI suggesting calcifications. FLAIR scans show serpentine sulcal hyperintensity called the ivy sign. Post-contrast images show enhancing pial angioma with enlarged ipsilateral choroid plexus and enlarged perimedullary veins.
The common CNS tumors associated with seizures include DNET [Figure 7], ganglioglioma, oligodendroglioma [Figure 8], pleomorphic xanthoastrocytoma, and low-grade glioma [Figure 9]. The epileptogenic feature among these would be the cortical base of the tumor. High-grade gliomas [Figure 10] can also frequently cause seizures due to their large size and extensive surrounding edema. Hypothalamic hamartoma is known to cause gelastic seizures [Figure 11].
Extra-axial tumors such as craniopharyngioma [Figure 12], meningioma [Figure 13], and dermoid cyst [Figure 14] can also present with seizures due to mass effect and vasogenic edema in the adjacent neuroparenchyma. Intracranial dermoid cysts may remain asymptomatic until they rupture and cause chemical meningitis.
Acute infections causing meningitis [Figure 16], viral encephalitis [Figure 17] and chronic infective etiologies such as fungal/tubercular granulomas [Figure 18], pyogenic abscess, and parasitic infections such as neurocysticercosis [Figure 19] are frequently associated with seizures.
Most frequently involved as a cause of seizure are cerebral arteriovenous malformation (AVM) and cerebral slow flow venous malformations. Seizures can occur de novo given their supratentorial location and involvement of the cortex but also secondary to intracerebral hemorrhage.
AVMs are characterized by an abnormal tangle of flow voids representing a nidus, with multiple prominent arterial feeders and dilated early draining veins [Figure 20].
Cerebral slow flow venous malformations, previously called cavernous malformations [Figure 21], are composed of blood-filled vascular spaces with blood in different stages of maturation. They range from Type I to Type IV according to the Zabramski classification of cavernous malformation based on the stage of hemorrhage. They are commonly associated with developmental venous anomalies as a part of mixed vascular malformations.
ENCEPHALOMALACIA AND GLIOSIS
It occurs as a result of a variety of CNS injury including infarction, infection, trauma, and hemorrhage. It can act as a potential epileptogenic focus and is one of the common findings on MR evaluation of patients with focal seizures.
It is characterized by the presence of focal or diffuse volume loss representing encephalomalacic changes with associated T2W/FLAIR hyperintense signal which suggests gliosis [Figure 22]. The margin may be lined by hemosiderin due to a break in the blood-brain barrier better seen on susceptibility-weighted imaging.
A rare entity, characterized by hemicerebral atrophy secondary to primary CNS insult in fetal or early infantile. It is frequently associated with compensatory hyperostosis of adjacent calvarium and hyperpneumatization of ipsilateral paranasal sinus [Figure 23]. This needs to be differentiated from Rasmussen encephalitis which is another rare cause of seizures and hemicerebral atrophy by the absence of calvarial and paranasal sinus changes [Figure 24]. A probable immunological cause has been proposed to be the etiology of Rasmussen encephalitis.
Fahr disease is characterized by the presence of extensive bilateral symmetrical intracerebral calcifications predominantly involving the deep gray nuclei [Figure 25]. The usual presentation of Fahr disease is that of extrapyramidal and behavioral symptoms but can rarely present with an epileptic seizure.
POSTERIOR REVERSIBLE ENCEPHALOPATHY SYNDROME (PRES)
PRES can be associated with a variety of conditions such as hypertension, eclampsia, toxin exposure, and autoimmune diseases and can also have a range of presentations such as headache, altered sensorium, visual disturbances, and rarely seizures. It is manifested by the presence of vasogenic edema in bilateral parieto-occipital lobes and T2 shine through in diffusion-weighted imaging [Figure 26].
SKULL BASE MENINGOENCEPHALOCELES
It can be spontaneous or traumatic. Spontaneous skull case meningoencephaloceles can be associated with idiopathic intracranial hypertension and central obesity which impair cerebral venous return to the heart which, in turn, increases the intracranial CSF pressure. They usually present with CSF rhinorrhea, otorrhea, or can remain asymptomatic and can be incidentally detected. They can rarely present as temporal lobe epilepsy due to herniation and traction of the involved cortex.[37-39]
MEGALENCEPHALIC LEUKOENCEPHALOPATHY WITH SUBCORTICAL CYSTS
Previously called Van der Knaap disease is another rare slowly progressive neurodegenerative disorder characterized by infantile macrocephaly, mild neurological symptoms, seizures and cerebral leukoencephalopathy. MRI shows early confluent involvement of subcortical and deep white matter with subcortical cysts predominantly in anterior temporal poles [Figure 27].
Both the radiologist and the referring clinician must be updated and keep refreshing their current knowledge given the myriad causes of seizures encountered in clinical practice. Employing dedicated imaging protocols and systematically reviewing the images, taking the relevant clinical history and EEG findings into consideration, radiologist plays a significant role in the evaluation of patients presenting with seizures. A dedicated coronal oblique T2W and FLAIR sequences planned along the axis of the hippocampus, T2 relaxometry, and MR volumetry are ideal in the diagnosis of early MTS. Moreover, the T1 inversion recovery sequence aids in the diagnosis of malformations of cortical development by a better depiction of gray-white matter junction.
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