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Primary microcephalies are incompletely understood malformations that are often associated with developmental brain anomalies, yet it is not understood if the associated anomalies result from the microcephaly itself or from associated developmental/genetic mishaps. This study reviewed and analyzed a large number of MRI scans of children with microcephaly to determine the frequency of associated morphological findings and to assess whether these findings were associated with the severity of the microcephaly.
MRIs of 119 patients with clinically diagnosed microcephaly were retrospectively reviewed, focusing on the degree of microcephaly, simplification of gyri, white matter volume, abnormalities of corpus callosum, size and structure of posterior fossa contents, and myelination. Associations among the findings were evaluated using the Spearman correlation coefficient and the Fisher exact test.
Among 7 patients with mild, 42 with moderate, and 70 with extreme microcephaly, a significant correlation was identified between a greater degree of microcephaly and both a greater degree of simplified gyration and decreased white matter volume. The severity of callosal anomaly showed a lower, but still significant, correlation with the severity of microcephaly. Degree of hypoplasia of posterior fossa structures, delay in myelination, and abnormality of basal ganglia did not correlate with the degree of microcephaly.
A strong correlation was found between the degree of microcephaly, the volume of white matter, and the presence of a simplified gyral pattern. These associations should be considered when attempting to use neuroimaging for segregation and classification of patients with microcephaly.
Congenital microcephaly (a very small head at birth) can be divided into two major categories: primary and secondary (acquired). Patients with primary congenital microcephaly have been described as having congenitally small but architecturally normal brains (thus distinguishing them from microcephaly associated with cerebral cortical malformations, such as holoprosencephaly or lissencephaly), typically associated with non-progressive mild mental retardation.1,2 The cause has been presumed to be genetic. In contrast, acquired microcephaly is presumed to result from brain injury, such as that associated with hypoxic-ischemic injury, intracranial infection or metabolic disease3; head size is initially normal, but decreases as a result of brain injury. It is known that some primary microcephalies show an abnormally simplified gyral pattern without thickening of the cerebral cortex; this group has been referred to as microcephaly with simplified gyral pattern (MSG).4-9 Patients with MSG often have various associated developmental brain anomalies, such as corpus callosal hypogenesis and hypoplasia, periventricular nodular heterotopia, and delayed myelination.2,3 Several reports of patients with MSG have been published; however, most were case reports or reviews of a small number of patients. Studies of larger numbers of patients are often useful to gain a better understanding of disorders and the frequency/significance of associated findings. Therefore, we undertook a retrospective study of MRI scans of 119 patients with clinically diagnosed primary microcephaly in an attempt to understand the significance of the morphological findings, including the gyral pattern.
A search was made for patients identified with microcephaly from the radiology teaching file of our institution, the personal teaching file of the senior author (acquired over 26 years), and patients ascertained in a study of the genetics of epilepsy. All patients were clinically diagnosed as microcephalic and referred for imaging after obtaining a head circumference greater than two standard deviations below the mean head circumference for age during the first 6 months of life. A total of 119 patients with microcephaly were identified. The age distribution of patients is summarized in the Table 1 (56 males and 57 females, ranging in age from 1 day to 30 years old -- only 4 patients were older than 10 years; no information about age or gender was available in 6 patients). Eighteen of the patients were related to other patients in the study, coming from a total of 9 pedigrees. Four patients were born to consanguineous parents (consanguineous: intermarriage among relatives of the third degree or closer). No overlap was found between pedigrees with several affected patients and consanguineous marriages. Evidence of brain damage, resulting from an ischemic or infectious event, metabolic disease or generalized cerebral malformation (such as diffuse polymicrogyria, lissencephaly or holoprosencephaly) was a cause for exclusion from this series. As thick cortex (lissencephaly) was a cause for exclusion from the study, the cortex was measured if there was any question of increased cortical thickness: cortex measuring 4mm thickness or more eliminated that patient from the study. The clinical and genetic data available on the subjects in the study were highly variable and, in many cases, extremely limited or even nonexistent (other than the diagnosis of microcephaly, which as the reason for examination), because the MR scans were obtained from many sources in many locations over many years. Therefore, this evaluation focuses exclusively upon imaging findings.
Normal comparison data for this study was obtained by assessing craniofacial ratios and white matter volumes in 31 normocephalic infants and children imaged by MRI for indications that should not affect head size or white matter volumes. These included normal neonates ascertained for other studies and infants/children studied for headache, behavioral disorders, ophthalmologic disorders, suspected infection (only included if none was found), and febrile seizures; all were interpreted as normal studies by fellowship trained neuroradiologists and verified as normal by at least one of the authors. These patients ranged in age from 3 months to 5 years at the time of their MR scan and included 6 neonates, 4 aged 3-6 months, 3 aged 6-12 months, 4 aged 1-2 years, and 5 aged 2-3 years, and 8 aged 3-5 years. The presence of any brain abnormality on the MRI excluded that patient as a control.
As the MR imaging studies were performed over a period of 20 years at many different imaging centers and on many different types of MR scanners, the imaging techniques that were used differed considerably. All examinations in this study included at least one sagittal imaging sequence with section thickness of 5mm or less and one axial imaging sequence with sections of 5mm or less. All patients were studied with both T1- and T2-weighted sequences, and 84.0% had sequences performed in three orthogonal planes. The absence of volumetric data sets in the vast majority of studies precluded quantification of brain volumes.
Imaging assessment in the patient group was based on agreement between 2 neuroradiologists who reviewed the images. Each neuroradiologist made initial evaluations independently, and any disagreements regarding the final conclusion were resolved by consensus.
Degree of microcephaly was visually analyzed using midsagittal T1- weighted images to assess the craniofacial ratio (ratio of the area of the intracranial structures to the area of the face) in the midsagittal plane. The size of the facial area, from the bottom of the chin to the top of the orbit and from the front of the nose to the back of the orbit was visually assessed on the midline sagittal image and compared to the size of the remainder of the craniofacial area on that same image. The craniofacial ratio is known to be large at birth and slowly diminish with increasing age; a small craniofacial ratio is considered an indication of microcephaly.26 Because the precise head circumference at the time of imaging was not known in many patients, the craniofacial ratios in our subjects were compared to the age matched normocephalic children described above and these results were used to classify the severity of microcephaly. We defined mild microcephaly as having a craniofacial ratio reduced by up to 30% compared with age-matched normocephalic children, moderate microcephaly was reduced by 30 to 50%, and extreme microcephaly by more than 50%.
We defined a grading system for sulcation based upon the method of van der Knaap et al.11 In a normal pattern, gyri and sulci are branched appropriately for age according to published standards.12,13 The gyral pattern was considered mildly simplified if the width of the gyri was less than the depth of the sulci, moderately simplified if the width of the gyri was equal to the depth of the sulci, and severely simplified, the width of the gyri was greater than or equal to the depth of the sulci. If the severity of simplification varied in different parts of the cerebrum, we used the grade of the most severe area for evaluation. None of the patients whose scans we studied had myelination patterns compatible with premature birth and, therefore, it seems extremely unlikely that immature sulcation was mistaken for simplified sulcation.
We evaluated white matter volume separately from the degree of microcephaly. The severity of reduction of white matter volume was judged mainly by the proximity of the depths of the sulci to the lateral margins of the lateral ventricles. We defined mild loss of white matter volume as the distance from the depths of sulci to the ventricular wall being reduced by 10 to 30% when compared to age-matched normocephalic children, moderate loss if the distance was reduced by 30 to 70% and severe if the distance was reduced by more than 70% compared with age-matched controls. Quantitative volumetric analysis was precluded by the absence of volumetric imaging data in the vast majority of our cases.
The corpus callosum, posterior fossa structures, deep gray matter, degree of myelination and presence of any associated malformations were assessed on each MRI scan. The corpus callosum was subjectively assessed on mid-sagittal T1-weighted images and classified as normal, hypoplastic (fully formed but too thin), hypogenetic (partially formed), or completely absent. The posterior fossa structures (cerebellum, midbrain, pons and medulla) were visually assessed for morphology and size; the brain stem and cerebellar sizes were compared with the size of the cerebrum in each patient to determine whether they were proportional or disproportional (disproportionally large/small) to cerebrum. The size and configuration of thalami and basal ganglia were evaluated as normal, small, or dysmorphic. Myelination was assessed on T2-weighted images and compared with published standards12 and with age matched normal controls. Associated malformations (heterotopia, interhemispheric cyst, and PMG) were also visually assessed and recorded if present.
Our review was retrospective as the MR examinations were performed over many years at numerous institutions. Therefore, it was impossible to get informed consent from all patients or their family. The Committee on Human Research approved this retrospective study at our institution.
Correlations of the degree of microcephaly with other structures (gyral pattern, white matter volume, corpus callosum, posterior fossa structures, myelination and basal ganglia) were assessed by computing the Spearman correlation coefficient and Fisher test. P values below 0.05 were considerate to be significant.
We identified 7 mild, 42 moderate, and 70 extreme microcephaly cases. Results of the cases are summarized in Table 2.
In the 7 cases of mild microcephaly, 2 patients had normal sulcation, 4 patients had a mildly simplified gyral pattern, and 1 patient had a severely simplified gyral pattern. No patient was assessed as having a moderately simplified gyral pattern. The group of moderate microcephaly was composed of 42 patients. Nine of the patients had normal gyration, twenty-four had mildly simplified gyral patterns, eight had moderately simplified gyral patterns, and one had a severely simplified gyral pattern. The group with extreme microcephaly included 70 cases. Five of patients had normal gyration, eighteen had mildly simplified gyral patterns, twenty had of moderately simplified gyral patterns, and twenty-seven had severely simplified gyral patterns.
All 119 microcephaly patients had subjectively reduced white matter volume. Using the method defined above, we identified mildly reduced white matter volume in 16 patients, moderate in 52 patients and severe in 51 patients.
Of 16 patients with mild reduction of white matter volume, 2 patients had mild microcephaly, 10 had moderate microcephaly and 4 had extreme microcephaly. Normal gyration was identified in 6 of the 16 patients, 9 had mildly simplified gyral patterns and 1 had moderately simplified gyral pattern. No patients with severely simplified gyral pattern were identified in this group.
In 52 patients with moderately reduced white matter volume, 4 patients had mild microcephaly, 21 patients had moderate microcephaly and 27 patients had severe microcephaly. Normal sulcation was identified in 9 of the 52 patients, 25 had mildly simplified gyral patterns, 12 had moderately simplified gyral patterns and 6 patients had severely simplified gyral patterns.
In 51 patients with severely reduced white matter volume, one patient had mild microcephaly, 11 patients had moderate microcephaly and 39 patients had severe microcephaly. Normal sulcation was found in 1 of the 51 patients, 12 of the 51 patients had mildly simplified gyral patterns, 15 patients had moderately simplified gyral patterns and 23 patients had severely simplified gyral patterns.
The corpus callosum had subjectively normal shape and thickness in 28 patients. In 6 patients, corpus callosum could not be assessed because an adequate midsagittal image was not obtained. The remaining 85 patients had abnormalities of the corpus callosum; 59 of patients had callosal hypoplasia (thin, but all parts formed), 21 had callosal hypogenesis (not all callosal parts formed) and 5 had complete callosal agenesis.
Both the cerebellum and brain stem were proportional in size to the cerebrum in 45 patients. Fifty-four patients had a disproportionally large cerebellum; in all, the brain stem was proportional to cerebellum. In 19 patients, the cerebellum was disproportionally small; the brain stem was proportional to cerebellum in 10 of these 19 patients, and proportional to cerebrum in 9 of the 19 patients. One patient with a disproportionally small cerebellum had abnormal superior cerebellar peduncles that resembled a molar tooth malformation. In one patient, assessment of posterior fossa was difficult because we had limited MR images.
Small basal ganglia were observed in 20 patients; 10 of the 20 patients had small thalami. Small thalami with normal basal ganglia were revealed in 10 patients. All 30 patients with small basal ganglia and/or small thalami showed normal shape and gross anatomic relationships of the structures. Abnormal shape and gross anatomic relationship of basal ganglia and/or thalami were identified in 5 patients.
Seventy-seven patients had appropriate myelination for age, whereas thirty-two patients had delayed myelination. In 5 patients, myelination couldn't be assessed because of unknown age or extreme reduction of white matter volume.
Associated malformations included periventricular nodular heterotopia (n=7) and interhemispheric cyst (n=4). One patient had both heterotopia and interhemispheric cyst.
Assessment with the Spearman rank correlation test showed a significant correlation between the degree of microcephaly and the degree of simplified gyration (r=0.47, p<0.0001). Both white matter volume (r=0.36, P<0.001) and abnormality of the corpus callosum (r=0.26, P<0.05) showed a lower, but still significant, correlation with the degree of microcephaly. The relative size of cerebellum and brain stem to the cerebrum showed a negative correlation with the degree of microcephaly (r=−0.31, P<0.001). Myelination and abnormality of basal ganglia didn't show any correlation with the degree of microcephaly. Additionally, we assessed the correlation between volume reduction of white matter and degree of gyral simplification. The degree of volume reduction of the white matter showed a significant correlation with simplified gyral pattern (r=0.55pa, P<0.001)
The most interesting finding in this study is the strong correlation between the degree of microcephaly and the presence of a simplified gyral pattern. Although several previous communications have reported microcephalic patients with a simplified gyral pattern, most were case reports or reviews of a small number of patients.7-9 By their very nature, such reports lack perspective on the significance of their findings. In this study, MRI scans of 119 patients with clinically diagnosed microcephaly were retrospectively evaluated in an attempt to understand the significance and relationship, if any, of the associated morphological findings, including the gyral pattern.
Primary microcephaly was originally described as a congenitally small but architecturally normal brain, with affected patients having non-progressive mild mental retardation.1,2 As the interest in primary microcephaly and its genetic causes has grown, many such patients have been studied with MRI and found to have abnormalities of the cerebral cortex characterized by a reduced number of gyri and shallow sulci. This group is referred to as microcephaly with simplified gyral pattern (MSG).3 Although it is unclear whether primary microcephaly (with normal sulcation) and MSG are part of a phenotypic continuum, it is clear that microcephaly has heterogeneous clinical, radiological and genetic features.5-7 Previous reports have suggested that MSG patients may have more severe mental retardation, neurodevelopmental delay, and associated CNS malformations than primary microcephaly.5,7,9 Although MSG was initially subdivided into six groups based upon the neonatal clinical features and neuroimaging findings,5,14 the authors of those papers had few patients in each group and commented that the classification may change as more patients were described. We believe that the larger number of imaging studies evaluated in this study have allowed a somewhat better perspective on the significance of the simplified gyral pattern, which is important for management of affected patients and counseling their relatives (although, ultimately, counseling may be based more on genetic analyses).
Malformations of cortical development are classified based on the stage of development at which the developmental process was likely disturbed: cell proliferation/apoptosis, neuronal migration, and cortical organization.15 Microcephaly is classified as a disorder of cell proliferation15, suggesting that either the number of cycles of cell proliferation is reduced or, perhaps, apoptotic cell death is increased. Recent studies show that many of the proteins that are mutated in primary microcephalies localize to centrosomes for at least part of the cell cycle; it was hypothesized that the centrosome is a final integration point for many regulatory pathways affecting prenatal neurogenesis in mammals,16 supporting the concept of primary microcephaly as a disorder of neuronal proliferation. However, the frequency and significance of simplified gyral pattern among primary microcephalies is not discussed in any classifications that were found by the authors. In this study, a simplified gyral pattern was detected in 103 of 119 cases of primary microcephaly of various severities. Although these cases presumably have many different genetic causes, an association was found between the severity of microcephaly and the degree of simplification of the gyral pattern. These results suggest a potential cause/effect relationship between the microcephaly and the simplified gyral pattern, possibly as an effect of reduced cell proliferation. A reduced number of neurons and axons could cause both microcephaly and the simplified gyral pattern. Several hypotheses have been put forward on the processes that underlie the normal gyration of the fetal brain.17-19 Although the mechanisms of sulcal formation are not well understood, a respected hypothesis by Van Essen suggests that cortical folding is related to tension exerted on the developing cortex by axonal connections.17 Possible factors influencing the tension are the number of subcortical and corticocortical axonal projections, as well as white matter and intracortical myelination.19 We postulate that decreased neuronal proliferation results in fewer cortical neurons and, thus, fewer axons exiting from the cortex; this might result in decreased tension arising from axons and, consequently, fewer and more shallow sulci. Moreover, the degree of volume reduction of cerebral white matter also showed a correlation with simplified gyral pattern. This result supports the concept that gyral simplification reflects reduced tension on the cortex due to the production of fewer axons. A more recent study, by Toro et al, analyzed the dynamics of cortical folding using a finite-element implementation of the morphogenetic model 20; this study suggested that cortical growth also plays a fundamental role in cortical folding. As cortical growth is directly related to the number of neurons produced, less cortical growth would be expected to result in less sulcation. This consideration further supports our hypothesis that the simplified gyral pattern is, at least partially, a consequence of the degree of microcephaly (more specifically the amount of cortical growth and axonal production) and not a separate, unrelated feature of certain types of primary microcephaly.
The term simplified gyral pattern is most often used to describe a reduced number of gyri and shallow sulci with normal cortical thickness. However, this term can be confusing because other terms are sometimes used to describe malformations with too few gyri and too shallow sulci: oligogyria, lissencephaly, pachygyria, and microlissencephaly.3,14 It is useful to define these conditions and their causes, as these malformations result from different causes and developmental periods. A simplified gyral pattern is considered to result from reduced cell proliferation or increased apoptosis18; it is synonymous with oligogyria. When seen in a mature brain (term equivalent neonate or older), it is almost always associated with microcephaly and reduced white matter volume; the cortex is of normal thickness. The term lissencephaly is used to describe a cortical malformation caused by disturbed neuronal migration. The affected cortex is agyric and, as a result of the disturbed migration, it is abnormally thick, usually at least 1 cm. Pachygyria (also called incomplete lissencephaly) refers to a lesser degree of disturbed migration of neurons. A few shallow sulci are present and the gyri are broad. This cortex is thick, but less so than in lissencephaly: usually 6-9 mm. The few pathological studies of simplified gyral pattern that have been published have revealed a normal cerebral cortical thickness and normal lamination or subtle alterations of cortical lamination.3, 14, 21 “Microlissencephaly” is a less well defined term; indeed, it is defined differently in different publications.5, 6, 14 However, it should be characterized by a thickened cortex and thereby be distinguished from MSG. We have avoided using the term microlissencephaly in this study because this term may be confusing and misleading. We suggest that this term be avoided and that microcephaly should be considered a characteristic sometimes associated with lissencephaly.
A few previous reviews based on the genetics and etiology of microcephaly have been published1, 22, 23 and the presence of a simplified gyral pattern in association with microcephaly has been discussed by many authors.24-25 However, to our knowledge, no previous reports have demonstrated the significant relationship among the severity of microcephaly, the reduced white matter volume, and diminished sulcation.
Our study also revealed a correlation between the degree of microcephaly and the severity of an associated callosal anomaly. In theory, this may be because related to the generation of fewer cortical neurons, resulting in production of fewer callosal axons. This relationship will be investigated in a future study. Microcephaly is extremely heterogeneous disorder and is also seen in association with various genetic syndromes, including some with small posterior fossa structures.8,9 Although our study included 19 patients with disproportionally small posterior fossa structures, statistical analysis showed no correlation between the degree of microcephaly and the size of posterior fossa structures. This result suggests that the cell proliferation of the cerebrum is independent from that of the cerebellum and brain stem in many types of microcephaly.
One potential weakness of this study is that studies of teaching collections, whether personal or institutional, might be biased by a greater tendency to select cases that have obvious abnormalities, such as abnormal sulcation or callosal anomalies, as compared to those with normal or nearly normal proportion of structures. A prospective study would be superior. However, this potential bias would be much more likely in patients with mild microcephaly, rather than the moderate or severe microcephaly that was present in the vast majority of these infants. Therefore, the authors do not believe that this was a significant factor, but cannot completely exclude it. While balancing the numbers by adding more MRIs from mildly microcephalic patients might, in some ways, “balance” the study, mildly microcephalic patients may be, paradoxically, more difficult to find, as patients are rarely referred for imaging because of mild microcephaly unless they have associated encephalopathy or focal neurologic deficits. Thus, even if included in a teaching file, the files would be unlikely to have the information about their head size. Practically speaking, therefore, it would be difficult to prospectively find significant numbers of mildly microcephalic patients from a retrospective study such as this. Thus, although we agree that a prospective study of imaging of microcephalic patients would be interesting and informative, we believe that our results are valid.
In summary, our review of MR imaging studies of a large number of patients with primary microcephaly confirms that microcephaly is a heterogeneous disorder. However, our data suggest that simplification of the gyral pattern may be a consequence of the severity of the microcephaly rather than exclusively a consequence of the underlying cause. Our data also suggest, although less convincingly, that callosal hypogenesis is related to, and may be a consequence of, the severity of microcephaly. Therefore, these characteristics may not be very useful as characteristics for classification of patients with microcephaly. A large, prospective study of microcephalic patients with known causes of primary microcephaly would be useful to evaluate other features seen in this disorder, and to determine whether they are useful to differentiate microcephalies of different etiologies.
MRI scans of 119 patients with clinically diagnosed microcephaly were reviewed. Strong correlation between the degree of microcephaly, the volume of white matter, and the presence of a simplified gyral pattern was revealed. An association of severity of microcephaly with the presence of callosal anomaly was also demonstrated, although less convincingly. These associations should be considered when attempting to use neuroimaging for segregation and classification of patients with microcephaly.
The authors wish to thank the Microcephaly Collaborative for the contribution of patient data that made this study possible and, specifically, thanks to Dr. Grace Yoon (Toronto), Dr. Mustafa Salih (Riyadh) and Dr. Ann Poduri (Boston) for contributing figures to the manuscript. C. A. W. and A. J. B. are supported by the NINDS, 2R01NS035129. C.A.W. is an Investigator of the Howard Hughes Medical Institute.