Bottom of sulcus focal cortical dysplasia, or simply bottom of sulcus dysplasia, refers to a highly epileptogenic and localized focal cortical dysplasia that is anatomically restricted to, and maximal at, the bottom of a cortical sulcus.
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Epidemiology
The true incidence is not known given bottom of sulcus focal cortical dysplasia is difficult to detect radiographically, but it is thought to be rare.
Clinical presentation
The typical clinical presentation is with drug-resistant focal epilepsy, with the exact semiology depends on the location of the bottom of sulcus focal cortical dysplasia 1,2. Most reported cases are of frontal lobe epilepsy, including some cases autosomal dominant sleep-related hypermotor (hyperkinetic) epilepsy (previously nocturnal frontal lobe epilepsy) 5.
In addition to epilepsy, affected patients may also have varying degrees of cognitive impairment, sometimes only apparent following dedicated neuropsychological assessment 1.
A characteristic neurophysiological feature of these lesions is highly localized interictal rhythmic spiking as demonstrated on intracranial electroencephalography or electrocorticography 1-3.
Pathology
Location
The most common locations for diagnosed (and therefore, symptomatic) bottom of sulcus focal cortical dysplasia are in the sulci of the frontal lobe (~60% in one study 1), including 2:
Sulci in other lobes of the brain are less commonly affected, with the sulci of the temporal and occipital lobes being very rarely affected 1.
Microscopic appearance
Histologically, bottom of sulcus focal cortical dysplasia are usually classified as ILAE (Blumcke) type IIb, or less commonly, type IIa 1-3.
Genetics
Approximately 60% of patients with bottom of sulcus focal cortical dysplasia have either somatic (in sporadic cases) or germline (in familial cases) mutations in genes affecting the mTOR pathway (e.g. DEPDC5, NPRL3) 1,3. Thus, bottom of sulcus focal cortical dysplasia may be considered an mTORopathy in most instances 1,3.
There is an important but unsurprising overlap between germline mutations causing bottom of sulcus dysplasia (e.g. DEPDC5, NPRL3) and those that cause autosomal dominant sleep-related hypermotor (hyperkinetic) epilepsy (previously nocturnal frontal lobe epilepsy) 7.
Radiographic features
MRI
MRI brain is the imaging modality of choice to detect bottom of sulcus focal cortical dysplasia, ideally performed with an epilepsy protocol 1,4. These lesions may be subtle, and may be missed on an initial MRI brain performed for work-up of epilepsy 1,5.
Generally, bottom of sulcus focal cortical dysplasia demonstrates 1,2,4:
focal blurring of the grey-white matter junction, maximal at the depth of the affected sulcus, present in nearly all patients
focal and abnormal cortical thickening, maximal at the depth of the affected sulcus, present in nearly all patients
focal increased T2/FLAIR signal at the grey-white matter junction, present in most patients (~60%)
a transmantle sign (a funnel-shaped T2/FLAIR band of high signal extending from the affected focal region of cortex subcortically to the ependymal surface) is present in most patients (~60%)
Importantly, the gyral crown and adjacent sulci are unaffected and are normal 2.
Nuclear medicine
PET-MRI
FDG-PET coregistered with MRI brain demonstrates focal (most commonly) or regional hypometabolism at the location of the bottom of sulcus focal cortical dysplasia 1,2,6.
Treatment and prognosis
Management is usually with antiseizure medications in the first instance, often with sodium channel blocking agents 1,5. Should patients develop drug-resistant epilepsy, then surgery (e.g. corticectomy) may be considered 2,5. Surgery is highly efficacious at curing the epilepsy, with over 80% of patients remaining seizure-free upon long-term follow-up 2,5.
Differential diagnosis
transmantle veins 4
subcortical white matter lesions (often non-specific in etiology) 4
tumors (e.g. astrocytoma, oligodendroglioma, DNET, ganglioglioma etc...)
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