Hemimegalencephaly: Evolution From an Atypical Focal Early Appearance on Fetal MRI to More Conventional MR Findings

Abstract

Hemimegalencephaly, or unilateral megalencephaly, is a sporadic congenital brain malformation characterized by enlargement of a cerebral hemisphere due to an abnormal proliferation of neurons or glial cells. Hemimegalencephaly is part of a spectrum of disorders, increasingly referred to as mTORopathies, which arise as a result of dysregulation or hyperactivation of the mammalian target of rapamycin (mTOR)signaling cascade resulting in less restricted cell growth and survival. The resultant cortical disorganization and enhanced neuronal excitability often manifest clinically in the form of seizures. Ultrasound and magnetic resonance imaging (MRI) are often used to characterize hemimegalencephaly. Typical imaging findings seen include diffuse unilateral enlargement of a cerebral hemisphere with overlying cortical malformation and ipsilateral dilation of the lateral ventricle. This paper will review an unusual case of focal hemimegalencephaly diagnosed on prenatal imaging. Initial in utero MRI revealed a mass-like lesion in the frontal lobe without associated perilesional cerebral edema. Keying in on abnormalities within the overlying cortex was crucial in suggesting focal hemimegalencephaly as a leading diagnosis and distinguishing it from alternative diagnoses such as a neoplasm. Follow-up fetal MRI demonstrated the evolution of the cerebral abnormality and confirmed the diagnosis. Early diagnosis facilitated appropriate counseling of the parents and guided postnatal imaging and management.

Neuroimaging Findings in Fetal Hemimegalencephaly: Case Study and Review

“…Among these reports, 18 (78%) were based on a single-case study including ours [10-16, 18, 20, 21, 23, 25-30], two (9%) involving 2 cases [17,22,31], one (4%) included four cases [19] (one of which had been previously reported [14]), and one (4%) outlined fetal MRI findings in 9 cases [24]. Notably, six articles (27%) focused primarily on fetal MRI findings, often lacking detailed prenatal ultrasound description [12,18,23,24,27,29]. One report describing a prenatal case of bilateral megalencephaly [32] and one series detailing genetic studies in postmortem samples from 11 HME cases in which no clear description between prenatal ultrasound and MRI findings were available [33] were excluded from analysis.…”

“…Only 1 case (3%) was referred to confirm the suspicion of HME [24]. The rest of cases were referred for routine scans (15%) [13,15,18,28,29] or for the evaluation of extra-CNS anomaly [22].…”

“…To systematically review prenatal ultrasound and fetal MRI features of HME, a comprehensive search of the Medline/PubMed database was conducted using the keywords “hemimegalencephaly,” “unilateral megalencephaly,” “fetus,” “fetal,” and “prenatal diagnosis.” The search yielded 23 reports in the English literature, including ours, totaling 36 cases detailing prenatal ultrasound and/or fetal MRI features of HME [10‒31]. Among these reports, 18 (78%) were based on a single-case study including ours [10–16, 18, 20, 21, 23, 25–30], two (9%) involving 2 cases [17, 22, 31], one (4%) included four cases [19] (one of which had been previously reported [14]), and one (4%) outlined fetal MRI findings in 9 cases [24].…”

Vertical Hemispherotomy: Contribution of Advanced Three-Dimensional Modeling for Presurgical Planning and Training

“…We considered a hemimegalencephaly case and post-ischemic parenchymal damage (Figures 5 and 6). These diseases are frequently characterized by a distortion of the usual anatomical landmarks, including midline deviation, alteration of expected vessel course, and narrower corridors, that can compromise the quality of disconnection towards both incomplete disconnection or damage to the normal hemisphere with consequent worsening impact on epilepsy and neurological outcomes [20,21,47,48].…”

“…On the other side, neuronavigation accuracy may often be reduced intraoperatively due to brain shift or deformation of the cerebral parenchyma, potentially compromising the completeness of disconnection. Moreover, some etiologies are more frequently characterized by a consistent distortion of brain anatomy, making the recognition of crucial structures and midline preservation more difficult [20,21].…”