Introduction

The spectrum of lissencephaly (LIS) corresponds to a group of serious brain malformations caused by a failure in neuronal migration. LIS, a term introduced by Owen Richard in 1868, comes from the Greek words “lissos” means smooth or soft and “enkephalos” means brain, it is a descriptive term. This term applied to malformations with abnormal formation of the cerebral convolutions, characterized by a smooth brain surface, intellectual disability, and seizures¹,².

The spectrum of LIS includes agyria (complete absence of cerebral convolutions), pachygyria (from the Greek “paqui” (παΧύς) means thick or broad, it is a malformation with wide cerebral gyri)³-⁵, and subcortical band heterotopia (SBH) (also called double cortex, consists of smooth layers of gray matter that frequently follow the curvature of the overlying cortex)⁶.

There are several forms of classification, but the most used is type I and type II LIS⁷. Type I or classic LIS in which it has four cell layers in the cortex, it can present in two forms such as isolated LIS and Miller-Dieker syndrome (MDS)⁸. Type II or cobblestone LIS in which the cerebral cortex is highly unstructured presents in three syndromes: Walker-Warburg syndrome, muscle-eye-brain disease, and Fukuyama congenital muscular dystrophy⁹,¹⁰.

Osborn et al.¹¹ estimated that the prevalence of LIS is approximately 1-4:100,000 newborns. Three genes are associated with classical LIS: LIS1, doublecortin (DCX), and TUBA1A¹². Likewise, some studies report association with viral infections.

Clinically, infants present marked psychomotor developmental delay, severe epileptic encephalopathy drug refractory, and some cases facial dysmorphias associated a specific genetic syndromes¹³. Magnetic resonance imaging (MRI) has become an important supplement evaluation and classification.

Although the overall incidence of LIS is rare, its comorbidities are serious, affecting neurodevelopment, causing a degree of disability, and impairing their quality of life and vital prognosis. The aim of our study was to describe clinical, neuroimaging, and neurophysiological features of pediatric patients with LIS type I attended at the National Institute of Pediatrics in Mexico City.

Methods

This was a single-center retrospective study of pediatric patients (< 18 years) with a diagnosis of LIS admitted to the National Pediatrics Institute from January 2009 to July 2019. We included patients with MRI showing structural alteration of the cerebral cortex compatible with the classic LIS included SBH and patients with sufficient clinical information and electroencephalograms (EEG) to be classified as LIS, patients with electroencephalograms (EEG) to be classified as LIS type I, and patients with sufficient clinical information. We excluded patients with another abnormal neuronal migration such as: cobblestone LIS, complex or congenital muscular dystrophy and schizencephaly, or with incomplete information in medical records. The local ethics committee approved this protocol.

We recorded demographic and clinical data as well imaging, genetic, and neurophysiological studies of the patients. Epileptic seizures were classified according to the International League Against Epilepsy 2017 guidelines. Drug-resistant epilepsy was defined as seizures occurring despite the use of two antiepileptic drugs (AEDs) at appropriate doses.

EEG patterns were classified according to the patterns described by Hakamada et al.¹⁴, recognized since 1979. This classification shows three unique EEG patterns: Grades I, II, and III.

The features observed in brain MRI were classified using the Grading System for Classical LIS and SBH, modified by Dobyns¹⁵. This system grades the neuroradiological appearance of LIS in 6-level based on the severity and anterior-posterior gradient of the abnormalities, from severe Grade 1 (complete agyria) to mild Grade 6 (SBH only) (Table 1).

We used descriptive statistics, frequencies and proportions were calculated for categorical data, and measures of central tendency (mean) were calculated for continuous data.

Results

A total of 22 patients with LIS type 1 were included. Fifteen were male (68%) and 7 females (32%). Their mean age was 9.3 years with a range of 3-16 years of age. Two patients died at the 4^th and 5^th years of age, respectively, due to pneumonia. Regarding genetic studies, 13 (59%) children had a normal karyotype of these only seven children were realized the test fluorescence in situ hybridization (FISH). Only one child was found with microdeletions of chromosome 17p13.3, this was diagnosed with MDS. Of the total of patients, only seven patients had toxoplasma, rubella, cytomegalovirus (CMV) and herpes simplex virus (TORCH) profile test and of these six were positive for CMV (Table 2).

Regarding clinical presentation symptoms in table 2, neurodevelopmental delay was present in all patients. Intellectual disability was reported in 19 (86%) children, this due to three patients was under 5 years of age and could not be classified that way. Intellectual disability was determined using the Wechsler Intelligence Scale in Children-IV 2003 for children of 6-16 years. Mild intellectual disability was found in 6 (27%), moderate in 3 (14%), severe in 8 (36%), and deep in 2 (9%) children. Facial dysmorphia was founded in 11 (50%) patients, among the findings were downward oblique palpebral fissures, epicanthus, long philtrum, hypertelorism, depressed nasal bridge, and high palate. Other findings not discussed in table 2 were clinodactyly, strabismus, cryptorchidism, scoliosis, clubfoot, and valgus foot. We founded that the four children with hypotonia in the 1^st year, later developed hypertonia with spasticity of the limbs.

Of the 22 patients in the study, 19 (86%) had electroclinical seizures. The most frequently reported epileptic syndrome was West syndrome in 6 (32%) children and of these 5 evolved to Lennox-Gastaut syndrome (Table 3).

The three EEG patterns of LIS described by Hakamada et al.¹⁴ are described as follows: Grade I, diffuse bi-hemispheric distribution of a mixture of high-amplitude alpha (8 Hz) and beta (14-16 Hz) activity 100-200 μV; Grade II, diffuse bi-hemispheric distribution of slow waves from 1.5 to 2.5 Hz with high-amplitude acute waves up to 300 μV, associated with short periods of attenuation of cortical activity lasting up to 3 s in duration; and Grade III generalized activity with acute 1-1.5 Hz high-amplitude waves of 400 μV. We found that the 19 children had at least one of the three EEG patterns. Other electroencephalographic findings were background patterns with generalized slow activity in 15 (78%) patients (Table 3). In all our patients, the amplitude during wakefulness ranged from 100 to 350 mV, with occasional high-amplitude discharges of up to 500 mV.

According to the classification system for LIS modified by Dobyns¹⁵, in brain MRI, the most frequent was Grade 4 correspond to pachygyria in 9 (41%) children. We founded predominance of anteroposterior gradient (more severe lesion anterior). Furthermore, we found 4 (18%) children in Grade 2, agyria with undulation of brain cortex; 3 (14%) children in Grade 1, diffuse agyria; 3 (14%) children in Grade 3, mixed agyria and pachygyria; 2 (9%) children in Grade 5, pachygyria and SBH; and only 1 (4.5%) child in Grade 6, SBH. The two patients who died had diffuse agyria this is the most severe form. Other findings in brain MRI were agenesis of the corpus callosum, polymicrogyria, and cisterna magna (Figs. 1 and 2).

Figure 1 Classification of the brain MRI findings of children with lissencephaly according to grading system for classic lissencephaly and SBH. SBH: subcortical band heterotopia. 

Figure 2 Brain MRI findings. A and B: agyria diffuse. C-F: agyria diffuse with few superficial undulations on frontal and temporal poles. G-H: agyria and pachygyria mixed, with frontal pachygyria and parieto-occipital agyria. I-J: partial pachygyria more severe anterior. K: more severe partial pachygyria posterior. L: heterotopia in subcortical band. 

Regarding other neuroimaging studies, we founded 12 patients who had a brain computed tomography (CT), 8 patients who had a prenatal ultrasound, and 2 patients with transfontanellar ultrasound. In the brain CT studies, reports of thick cortical gyrus were founded in five children, cortical atrophy in four children, and ventriculomegaly in two children. In the prenatal ultrasound studies, reports of intrauterine growth restriction were found in four children, ventricular dilation in a single child, and in three children, the report was normal. The two children who underwent transfontanellar ultrasound were both reported with ventriculomegaly.

Regarding comorbidities, the most common was gastroesophageal reflux disease in 15 (68%) patients and of these 10 patients underwent Nissen fundoplication and gastrostomy. Other comorbidities founded were recurrent pneumonia in 14 (63%) patients, heart disease (ventricular septal defect and ductus arteriosus) reported in two children, intestinal atresia in one, and hypothyroidism in another.

Discussion

There are no exact data on the prevalence of LIS, however, studies report the overall incidence is 1 in 100,000 live births or even higher, up to 1 in 13,000-20,000 live births¹⁶. The Royal Children’s Hospital in Melbourne reported 2-4 new patients with classic LIS per year, which is equivalent to an incidence of approximately 1:25,000 live births¹¹. In our study, we reviewed clinical records in the last 10 years and we found 22 patients with classic LIS, which represents approximately 2.2 cases per year.

The cause of LIS is unknown but probably heterogeneous⁵. In about 80% of cases of classical LIS, a genetic cause can be found, usually an abnormality of the LIS1 or DCX gene¹⁷. In our medium, the genetic study is limited to performing a karyogram and FISH for chromosome 17, probably due to the cost of genetic tests.

In our study, only one patient was diagnosed with MDS¹⁸ determined by a microdeletion spanning the gene LIS1 at chromosome 17p13.3. This patient was a male of 4 years old. He presented distinctive facial features (prominent forehead, bitemporal narrowing, midface hypoplasia, upturned nares, protuberant upper lip, low seat ears, and micrognathia), as well as heart disease (interatrial communication), gastroesophageal reflux disease, and epileptic spasms.

We were looking for a history of ingestion or exposure to drugs or toxins during pregnancy, but these data were not reported.

LIS has been associated with intrauterine viral infection, CMV is the most frequent. Early second-trimester CMV infection leads to LIS, while late second-trimester infection causes polymicrogyria¹⁹. Other pathogens involved are toxoplasmosis, rubella, herpes simplex, human immunodeficiency virus, and syphilis. In a review in Australia of 12 children with cerebral palsy and congenital CMV, the children had epilepsy, intellectual deficit and the brain malformations found were LIS, pachygyria, polymicrogyria, cerebellar hypoplasia, ventricular dilatation, and calcifications²⁰. In our study, more than half of the patients did not have a TORCH profile test and are unknown whether there is an association with these viruses.

We report a patient who had a diagnosis of prenatal Zika infection. The patient was a girl of 2 years old. She also had microcephaly²¹. Her diagnosis could be determined because her mother underwent the Zika test as a screening, her mother was from Chiapas State.

Our findings show the same clinical manifestations reported in the Dobyns⁵ study in 1990 and 2010 categorized as (1) neurological deficits, poor feeding, hypotonia, and opisthotonos; (2) delayed neurodevelopment; or (3) seizures¹³,²²,²³.

Studies report that the frequency of epilepsy in children with LIS could be 35-85% or up to 90% this number is like those found in our study⁵,²³. The onset of epilepsy is early usually between 3 and 12 months⁵, in our study, the average was 8 months. Children with LIS present several types of epilepsy. Epileptic spasms are the most frequent up to 80%, which often progress to West syndrome and Lennox-Gastaut syndrome²⁴. Focal motor seizures (myoclonic, tonic, and atonic), atypical absences, and generalized tonic-clonic seizure are also observed²⁴. Drug-resistant epilepsy can be an independent factor contributing to mental retardation, developmental delay, and eating problems. In classical LIS, studies in mutated mice have shown specific deficiencies in cortical interneurons that use g-aminobutyric acid (GABA) as neurotransmitter²⁵. Therefore, GABAergic AEDs could be used. However, there is currently no FAE recommended as the gold standard in treatment.²⁴,²⁵.

As previously mentioned, Hakamada et al.¹⁴ determined three specific EEG patterns in LIS. Ferrier et al.²⁶ suggested that these EEG changes may be due to the lack of sulci and denervation super sensitivity of the abnormal neurons, picked up by scalp electrodes. Gastaut et al.²⁷ proposed that the rhythmicity and increased amplitude were related to the abnormal organization and orientation of these cortical layers.

Pattern I EEG of high-amplitude fast activity of alpha frequency intermixed with beta is most characteristic of LIS²⁸. Jauhari et al.²⁹ reported that EEG pattern recognition aids in diagnosis of LIS. EEG pattern III is associated with severe developmental delay and drug-resistant epilepsy. In this study with 28 children, the EEG pattern I was the most common 14 (50%)²⁹. In our study, the EEG pattern I also was the most common in 8 (42%) children.

Many children with LIS have bulbar difficulties that result in difficulties with feeding and respiratory function²³, as reported in our research, the most frequent comorbidities were gastroesophageal reflux and recurrent pneumonia. Furthermore, there are reports of various non-neurological abnormalities such as congenital heart disease, cataracts, duodenal atresia, renal agenesis, polydactyly or syndactyly, and cryptorchidism³⁰.

Prenatal diagnosis of LIS, although difficult, can be suspected from 23 weeks of gestation. The abnormal features in prenatal US images at 23 weeks are ventriculomegaly and shallow Sylvian fissure³¹. In older gestational ages, other findings are widespread agyria, abnormal insula, corpus callosum, microcephaly, intrauterine growth restriction, and hydramnios³².

Dobyns et al.¹⁵ observed two types of patterns or gradient of severity according to brain MRI imaging findings. The first pattern is an anteroposterior gradient (a > p) with greater involvement in the anterior cerebral cortex (frontal lobe). The second pattern is a posteroanterior gradient (p > a) with greater involvement in the posterior cerebral cortex (parietal or occipital lobe)⁵,¹⁵. In the most severe form of LIS (complete agyria), can be difficult to differentiate the type of pattern. Di Donato et al.²² reported that the most common form of LIS was partial agyria-pachygyria that was most severe posteriorly (posteroanterior gradient) or Grade 3a (3p > a). In our study, only three children have this grade. The next most common patterns were pachygyria²². In our review, pachygyria (grade 4) was the most common.

Treatment is generally symptomatic and supportive³³. The most patients used antiepileptic polytherapy that was ineffective. As previously mentioned, a large percentage of patient required gastrostomy, several had multiple hospitalizations for status epilepticus and pneumonia. Children with heart disease received cardiology treatment and some children required orthopedic management and surgery.

The prognosis depends on the severity of the malformations, some forms of LIS have a severe neurological phenotype with a markedly reduced life expectancy and many dies before the age of 10 years³³. For classic LIS, the mortality rate is > 50% at 10 years and few children live more than 20 years⁵,³⁴. Respiratory diseases are the most common causes of comorbidities and death³⁵. In our study, the two patients died due pneumonia and both had diffuse agyria.

Conclusions

LIS includes pathologies with a poor prognosis, manifested predominantly in the 1^st year of life. Neurodevelopmental cognitive and motor alterations are frequent and significant. All patients with LIS have neurodevelopmental delay, drug-resistant epilepsy and are associated with different comorbidities. The pattern most frequent in EEG is Grade I. In brain MRI, pachygyria is the malformation most frequent. An early diagnosis can be made with an adequate prenatal ultrasound. Our suggestions are: to recognize EEG patterns in LIS and clinical findings to facilitate the diagnosis; To consider the types of patterns or gradient of severity according to brain MRI imaging findings as a prognostic factor; To research etiology with genetic studies and testing for viral infections, (and in addition, offering genetic counseling); Diagnosing and initiating symptomatic treatment in early stages to avoid comorbidities. Finally, more studies are required to better understand and improve the treatment and outcome of this pathology.