Hemimegalencephaly: A rare cause of hemihypoperfusion on 99m technetium-ethyl cysteinate dimer brain perfusion single-photon emission computed tomography

INTRODUCTION

Hemimegalencephaly is a rare congenital neuronal migration disorder characterized by unilateral hypertrophy of a cerebral hemisphere and ipsilateral ventriculomegaly.[12] It can be an isolated abnormality, or it can be associated with neurocutaneous syndromes.[1] Clinical manifestations include intractable motor seizures, impaired psychomotor development, hemiparesis, and hemianopsia.[1] Patients having hemimegalencephaly usually have a very poor prognosis. These patients require early surgical interventions to prevent progression to intractable epilepsy.[3] Hemispherectomy is the treatment of choice.[1] The definition of the dysfunctional areas of dysplastic cortex may affect surgical management of these anomalies.[4] Evaluation of regional cerebral blood flow using technetium-99m ethyl cysteinate dimer (Tc-99m ECD) single-photon emission computed tomography (SPECT) provides useful information in the patient of hemimegalencephaly[56] because it clearly delineates the border of the dysplastic cortex by showing altered blood flow to the dysfunctional areas.[4]

CASE REPORT

An 11-year-old male child was referred to a tertiary care hospital with history of gradually increasing seizures frequency since infancy. The child had delayed milestones and right hemiparesis. Seizures were mainly right sided, complex partial in nature and refractory to medical treatment over the past decade. The child was born out of non-consanguineous marriage, at term with a normal vaginal delivery. There was no family history of neurologic diseases. Non-contrast computed tomography (CT) of head showed diffuse enlargement of left cerebral hemisphere with midline shift towards right while the right cerebral hemisphere was normal [Figure 1a]. T2W magnetic resonance imaging (MRI) images showed diffuse gyral thickening involving the left cerebral cortex causing midline shift and scalloping of inner table of left calvarium [Figure 1b]. White matter in both cerebral hemispheres show normal myelination. Linear hyperintensities involving the subcortical U fibers of left fronto-parietal region were seen on the fluid attenuation inversion recovery image [Figure 1c].

Post-gadolinium T1W image in axial plane showed no abnormal enhancement [Figure 1d]. Tc-99m ECD brain perfusion SPECT revealed diffusely decreased tracer uptake in the left cerebral hemisphere, with normal uptake in the right hemisphere suggestive of left cerebral hemihypoperfusion [Figure 2a-c].

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Figure 1 Non-contrast computed tomography of head (a) Axial shows diffuse enlargement of left cerebral hemisphere (*) with midline shift towards right while the right hemisphere is normal. Non-contrast magnetic resonance imaging T2W (b) Images (axial) show diffuse gyral thickening (thin arrow) in the left cerebrum causing midline shift and scalloping of inner table (arrow head) of left calvarium. White matter in both cerebral hemispheres show normal myelination. Linear hyperintensities involving the subcortical U fibers of left fronto-parietal region seen on fluid attenuation inversion recovery image (c) Post-gadolinium T1W image in axial plane (d) shows no abnormal enhancement

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Figure 2 99m technetium-ethyl cysteinate dimer brain perfusion single-photon emission computed tomography transaxial, coronal and sagittal images reveal diffusely decreased tracer uptake in the left cerebral hemisphere (arrow), with normal uptake in the right hemisphere. Findings are suggestive of left cerebral hemihypoperfusion

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DISCUSSION

Tc-99m ECD is a perfusion agent similar to Tc-99m hexamethylpropyleneamine oxime (HMPAO). Regional differences in perfusion within the affected hemisphere with relative interictal hypoperfusion and ictal hyperperfusion of the seizure focus in a neonate with hemimegalencephaly has been reported on Tc-99m HMPAO brain SPECT imaging.[7] Ictal SPECT using Tc-99m HMPAO is also useful in demonstrating the site of epileptogenesis and its spread.[2] Sometimes it has been found that brain SPECT shows increased radiotracer uptake in the pathological hemisphere in the early part of life, which gets reversed in the later stage with decreased radiotracer uptake in the pathological hemisphere. These serial changes of radiotracer uptake on SPECT probably reflect either changes in epileptic activity or maturational changes in cerebral perfusion in hemimegalencephaly.[8] Brain SPECT helps in excluding seizure foci or other perfusion abnormalities in the contralateral side before surgery.[1]

Considering recent recommendations for hemispherectomy in these patients, Tc-99m ECD/HMPAO SPECT scan should be used to complement electroencephalogram (EEG) as a method to define the extent of abnormality, which may be more relevant to long-term prognosis than EEG alone.[9] Also, while interpreting a brain SPECT, hemimegalencephaly must be considered as a differential in hemihypoperfusion, for which CT/MRI correlation should be sought before reporting. The other differentials for hemispheric hypoperfusion are Rasmussen's encephalitis,[10] Sturge Weber syndrome,[11] Aspergers syndrome,[12] exanthema subitum[13] and gliomatosis cerebri,[14] which can even be mistaken for hemimegalencephaly.