Structural MRI Changes in Atonic Seizures
Key Structural Findings
MRI brain abnormalities in atonic seizures frequently reveal frontal lobe lesions, with approximately half of patients showing structural pathology that is often frontal in location. 1
Common Structural Abnormalities
Frontal lobe lesions are the most frequently identified structural abnormality, with the presumed epileptogenic zone being frontal in 43% of cases with atonic seizures 1
Developmental abnormalities including cortical dysplasias may be present, particularly focal cortical dysplasias that can range from easily identifiable lesions to subtle areas of discrete cortical thickening and blurring of the gray-white matter interface 2
Hippocampal involvement can occur in partial epilepsy manifesting as atonic seizures, though this is more commonly associated with partial simple and complex seizures 3, 4
Parietal and posterior temporal/occipital lesions are less common but documented causes, accounting for approximately 9% of cases 1
Specific MRI Characteristics in Documented Cases
In frontal lobe epilepsy presenting with atonic seizures, hypometabolic regions on FDG-PET correspond to structural abnormalities visible on MRI in the frontocentral areas 3
In parietal lobe epilepsy with atonic features, structural lesions in the central/parietal regions can be identified 3
Approximately 48% of cases have unclear or multifocal epileptogenic zones, making precise structural localization challenging even with advanced imaging 1
Important Clinical Context
Atonic seizures from partial epilepsy demonstrate slower falls (2-5 seconds) compared to generalized epilepsy, which can help distinguish the underlying structural etiology 3. This slower progression suggests involvement of specific cortical areas such as the negative motor area or supplementary negative motor area 3, 5.
Patient Population Characteristics
Approximately 50% of patients with atonic seizures have developmental delay and learning difficulties, suggesting underlying structural brain abnormalities 1
Structural lesions are identified on MRI in about half of patients with epileptic drop attacks and atonic seizures 1
Imaging Recommendations
MRI brain without contrast using a dedicated epilepsy protocol is the imaging modality of choice, as it provides superior gray-white matter differentiation and multiplanar imaging capability with 70-80% sensitivity for detecting epileptogenic lesions 6, 7
Optimal MRI Protocol Components
Coronal T1-weighted imaging (3mm) perpendicular to the hippocampal long axis 6
High-resolution 3D T1-weighted gradient echo with 1mm isotropic voxels 6
Coronal T2-weighted sequences and FLAIR imaging (coronal and axial or 3D) 6
3T MRI is preferred over 1.5T when available for improved lesion detection 6
When Standard MRI Appears Normal
In 20-40% of patients with atonic seizures, no clear lesion is visible on standard MRI despite having epileptogenic foci 7. In these cases:
FDG-PET/CT can identify hypometabolic regions corresponding to epileptogenic zones, with sensitivity of 87-90% for temporal lobe and 38-55% for extra-temporal epilepsy 6, 3
Advanced imaging techniques including diffusion tensor imaging and magnetic resonance spectroscopy may reveal subtle cortical abnormalities not visible on conventional sequences 2
Critical Pitfalls to Avoid
Do not assume a normal CT excludes structural abnormality, as CT has only 30% sensitivity compared to MRI's 70-80% for epileptogenic lesions 6, 8
Focal cortical dysplasias are frequently missed on routine MRI protocols, requiring dedicated epilepsy sequences for detection 2
Seizure-induced transient MRI changes can mimic structural lesions, showing high T2 signal, restricted diffusion, and variable reversibility between 15-150 days; follow-up imaging is essential to distinguish these from permanent structural abnormalities 4