neuroimaging advances in holoprosencephaly: reﬁning the spectrum of the midline malformation

American Journal of Medical Genetics Part C (Seminars in Medical Genetics) 154C:120–132 (2010)

A R T I C L E

Neuroimaging Advances in Holoprosencephaly:Refining the Spectrum of the Midline MalformationJIN S. HAHN* AND PATRICK D. BARNES

Holoprosencephaly (HPE) is a complex congenital brain malformation characterized by failure of the forebrain tobifurcate into two hemispheres, a process normally completed by the fifth week of gestation. Modern high-resolution brain magnetic resonance imaging (MRI) has allowed detailed analysis of the cortical, white matter, anddeep gray structural anomalies in HPE in living humans. This has led to better classification of types of HPE,identification of newer subtypes, and understanding of the pathogenesis. Currently, there are four generallyaccepted subtypes of HPE: alobar, semilobar, lobar, and middle interhemispheric variant. These subtypes aredefined primarily by the degree and region of neocortical nonseparation. Rather than there being four discretesubtypes of HPE, we believe that there is a continuum of midline neocortical nonseparation resulting in a spectrumdisorder. Many patients with HPE fall within the border zone between the neighboring subtypes. In addition, thereare patients with very mild HPE, where the nonseparation is restricted to the preoptic (suprachiasmic) area. Inaddition to the neocortex, other midline structures such as the thalami, hypothalamic nuclei, and basal ganglia areoften nonseparated in HPE. The cortical and subcortical involvements in HPE are thought to occur due to adisruption in the ventral patterning process during development. The severity of the abnormalities in thesestructures determines the severity of the neurodevelopmental outcome and associated sequelae.� 2010 Wiley-Liss, Inc.

KEY WORDS: holoprosencephaly; MRI; brain malformation; neuroimaging; development; midline; telencephalon; diencephalon;prosencephalon; preoptic area

How to cite this article: Hahn JS, Barnes PD. 2010. Neuroimaging advances in holoprosencephaly: Refiningthe spectrum of the midline malformation. Am J Med Genet Part C Semin Med Genet 154C:120–132.

INTRODUCTION

Holoprosencephaly (HPE) is a complex

congenital brain malformation charac-

terized by failure of the forebrain to

bifurcate into two hemispheres, a proc-

ess normally complete by the fifth week

of gestation [Golden, 1999]. Modern

high-resolution brain magnetic reso-

nance imaging (MRI) has allowed

detailed analysis of the cortical, white

matter, and deep gray structural anoma-

lies in HPE. This has led to better

classification of types of HPE, identi-

fication of newer subtypes, and under-

standing of the pathogenesis.

DEFINITION ANDCLASSIFICATION

The sine qua non feature of HPE is an

incomplete separation of the cerebral

hemispheres that results in lack of

cleavage, or nonseparation, of midline

structures. As the name HPE implies,

these nonseparated prosencephalic

structures affects parts of the telence-

phalon and diencephalon. Sometimes

The sine qua non feature

of HPE is an incomplete

separation of the cerebral

hemispheres that results in lack

of cleavage, or nonseparation,

of midline structures. As the

name HPE implies, these

nonseparated prosencephalic

structures affects parts

of the telencephalon

and diencephalon.

these abnormal midline structures are

described as being ‘‘fused,’’ but it should

be kept in mind that appearance of

fusion results from abnormal bifurcation

of those structures, rather than paired

Jin Hahn, M.D. Professor of Neurology and Pediatrics, Stanford University School of Medicineand Lucile Packard Children’s Hospital. Medical Director, Stanford Carter Center for Research inHoloprosencephaly and Related Brain Malformations. Research interests: brain development andmalformations, prenatal neurological consultation and fetal MRI, neonatal neurology.

Patrick Barnes, M.D. Professor of Radiology, Stanford University School of Medicine. Chief,Section of Pediatric Neuroradiology; Director, Pediatric MRI and CT, Lucile Packard Children’sHospital at Stanford Research interests: Advanced imaging, including magnetic resonanceimaging, of injury to the developing central nervous system; including fetal, neonatal, infant andyoung child; and, including nonaccidental injury (e.g., child abuse).

Grant sponsor: Carter Centers for Brain Research in Holoprosencephaly and RelatedMalformations; Grant sponsor: Don and Linda Carter Foundation; Grant sponsor: Crowley-Carter Foundation.

*Correspondence to: Jin S. Hahn, Department of Neurology, Stanford University MedicalCenter, 300 Pasteur Drive, Room A343, Stanford, CA 94305-5235, USA.E-mail: [email protected]

DOI 10.1002/ajmg.c.30238Published online 26 January 2010 in Wiley InterScience (www.interscience.wiley.com)

� 2010 Wiley-Liss, Inc.

structure actively merging. HPE has

traditionally been classified according

to the DeMyer’s three grades of severity:

alobar, semilobar, and lobar [DeMyer,

1987]. In addition to these classic forms,

a milder subtype of HPE, the middle

interhemispheric (MIH) variant or syn-

telencephaly has been characterized

[Lewis et al., 2002; Simon et al., 2002].

The neuroimaging findings of these

types are provided below and summar-

ized in Table I. See Marcorelles and

Laquerriere [2010] for a review of HPE

neuropathology.

Alobar HPE

In the most severe form, alobar

HPE, there is complete or nearly

complete lack of separation of the

cerebral hemispheres with a single mid-

line forebrain ventricle (a crescent

shaped monoventricle), which often

communicates with a dorsal cyst

(Fig. 1). The cerebral holosphere usually

has the appearance of a pancake-like

mass of tissue in the rostral-most

portion of the calvarium. The posterior

aspect of the cerebrum is shaped like a

horseshoe with the posterior-dorsal rim

composed of a thin, cyst-like membrane

[Golden, 1999]. This membrane is

the posterior roof of the monoventr-

icle. When the dorsal cyst is smaller, the

posterior-dorsal rim is located more

posteriorly and the holosphere may

have a cup-like appearance (Fig. 2)

[DeMyer, 1987]. The structures of the

temporal axis are formed, but the

temporal limbs of the choroid fissures

are splayed open [Takahashi et al., 2004].

The interhemispheric fissure, falx cere-

bri, and corpus callosum are completely

absent. The basal ganglia and hypothala-

mic and thalamic nuclei often lack sepa-

ration resulting in absence of the third

ventricle [Simon and Barkovich, 2001].

At times, a mass of deep gray matter is

present with poorly differentiated stria-

tum and thalamus (Fig. 1B) and may in-

clude parts of the mesencephalon. This

mass may be attached to the holosphere

by a small anterior midline hinge of

tissue that lacks corticospinal, cortico-

thalamic, and thalamocortical tracts

[Muenke, 1995]. Olfactory bulbs and

tracts are absent [Yamada et al., 2004].

TABLE I. Neuroimaging Features of Various Types of HPE

Alobar Semilobar Lobar MIH

Cortical

nonseparation

Diffuse (holosphere) Frontal Basal frontal Posterior frontal and

parietal

Corpus callosum Absent Rostrum, genu, and

body absent. Splenium

present

Rostrum and genu

absent. Anterior body

variably present.

Splenium present

Body absent; genu

variably present.

Splenium present

IHF and Falx Completely absent

anteriorly and

posteriorly

Present posteriorly only Hypoplastic anteriorly

and present posteriorly

Absent in the posterior

frontal and parietal

region

Ventricles Monoventricle

communicating

widely with dorsal cyst

Anterior horns absent.

Posterior horns

present. Small third

ventricle

Rudimentary anterior

horns. Third ventricle

formed

Normal or hypoplastic

anterior horns. Third

ventricle formed

Dorsal cyst Usually present Variably present Absent Present in 1/4

Septum pellucidum Absent Absent Absent or dysplastic Absent

Thalamus Often fused Partial fusion Usually fully separated Fused in 1/3 to 1/2

Basal ganglia Often fused (may form

single mass with

thalami)

Partial fusion (especially

head of caudate)

Variable degree of fusion Separated

Hypothalamus Always fused to some

degree (100%)

Very often fused to some

degree (98%)

Often fused to some

degree (83%)

Separated

Sylvian fissure Often absent Anteriorly and medially

displaced (wide sylvian

fissure) with fused

frontal lobe

Anteriorly and medially

displaced (wide sylvian

fissure) with small

frontal lobes

Often abnormally

connected across the

midline over the

vertex

Cortical dysplasia

and heterotopic gray

matter

Frequent presence of

diffuse broad gyri with

too few sulci

Occasional broad gyri

with too few sulci

Rare midline subcortical

heterotopias in frontal

regions

Very common

Cerebral vasculature Rete of vessels branching

from the internal

cerebral arteries

Azygous anterior

cerebral artery

Azygous anterior

cerebral artery

Azygous anterior

cerebral artery

Based on Simon et al. [2000, 2002], Plawner et al. [2002].

ARTICLE AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS) 121

Affected children with alobar HPE

are typically abnormal from the neonatal

period and present with hypotonia and

seizures. Midline craniofacial mal-

formations including hypotelorism, pre-

maxillary hypogenesis and cleft, and

hypoplastic nose are often present. A

large dorsal cyst may result in hydro-

cephalus and macrocephaly. Otherwise,

the head is microcephalic.

Semilobar HPE

In semilobar HPE, the anterior hemi-

spheres fail to separate, while some

portions of the posterior hemispheres

show separation (Fig. 3). The non-

cleaved frontal lobes are usually small.

Figure 1. MRI of a 1-day-old neonate with alobar HPE. Sagittal T1-weighted image (A) shows absence of corpus callosum and amonoventricle (MV) that communicates with the dorsal cyst. Axial T1-weighted image at the level of the thalami (B) demonstrates largecentral gray mass that consists of thalamic nuclei and basal ganglia (arrowheads). At a more rostral level axial T1-weighted image (C) shows acrescentic shaped MV surrounded by a flattened holosphere and large dorsal cyst (DC) posteriorly.

Figure 2. MRI of a 5-day-old newborn with alobar HPE. Sagittal T1-weight image (A) shows a holosphere that takes up more thanhalf of the cranial vault. Corpus callosum is not visualized. The ventricular system is composed of a single midline monoventricle (MV) thatcommunicates openly with the posteriorly positioned dorsal cyst (DC). Axial T2-weighted image (B) shows lack of separation of thehemispheres. The holosphere extends further posteriorly (compared to the patient in Fig. 1) forming a cup-like holosphere. The caudateand lentiform nuclei are not well differentiated and are fused in the midline (white arrow). Coronal T2-weighted image (C) shows acontinuity of gray matter in the midline without an interhemispheric fissure. The thalamic nuclei are partially fused (black arrow).

In semilobar HPE, the

anterior hemispheres fail

to separate, while some

portions of the

posterior hemispheres

show separation. The

noncleaved frontal lobes

are usually small.

122 AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS) ARTICLE

The frontal horns of the lateral ventricle

are absent, but posterior horns and

trigones are present. The septum pellu-

cidum is absent. The corpus callosum is

absent anteriorly, but the splenium of the

corpus callosum is present. The anterior

extent of the corpus callosum develop-

ment also correspond with the anterior

extent of the interhemispheric fissure

formation [Simon and Barkovich,

2001]. On imaging studies, some por-

tions of the posterior interhemispheric

fissure, falx cerebri, and splenium of the

corpus callosum can be identified. The

hippocampal formation in the temporal

lobes appears normal and the temporal

limbs of the choroid fissures are closed

[Takahashi et al., 2004]. The deep gray

nuclei are incompletely separated and

can usually be identified as discrete

structures, usually resulting in a small

third ventricle [Simon et al., 2000]. The

head of the caudate nuclei is often

nonseparated. Dorsal cysts are some-

times seen in semilobar HPE, especially

when there is nonseparation of the

thalamic nuclei. The olfactory bulbs

are either absent or hypoplastic.

Facial malformations are usually

mild or absent. The head is micro-

cephalic, unless a large dorsal cyst and

hydrocephalus are present. Patients often

have severe motor abnormalities in-

cluding dystonia, choreoathetosis, and

spasticity, as well as developmental

delay.

Lobar HPE

In lobar HPE, a milder phenotypic form,

the cerebral hemispheres are fairly well

developed and separated, while only the

most rostral/ventral aspects of the frontal

neocortex are nonseparated (Fig. 4).

Again, the corpus callosum is absent in

the region affected (usually rostrum and

genu). The posterior half of the corpus

callosum (including the splenium and

posterior body) is present. Rudimentary

formation of the frontal horns is usually

present. The third ventricle is fully

formed. The interhemispheric fissure

and falx cerebri are present anteriorly,

although these structures are hypoplastic

owing to the frontal lobe fusion [Simon

and Barkovich, 2001]. The thalamic

nuclei may be fully separated, although

an enlarged massa intermedia may be

present. A dorsal cyst is usually absent.

Olfactory bulbs and tracts may be

present, although they are usually hypo-

plastic. An azygous anterior cerebral

artery (ACA) is usually present in the

anterior IHF.

Middle Interhemispheric Variant

(Syntelencephaly)

MIH is a subtype of HPE that presents

clinically as a milder phenotype [Simon

et al., 2002]. The neuroimaging features

of MIH subtype are different from classic

HPE. Unlike classic HPE, where the

most severely nonseparated region of the

hemispheres is the basal forebrain, in

MIH the posterior frontal and parietal

lobes fail to separate (Fig. 5). The poles

of the frontal and occipital lobes are well

separated in MIH. The genu and

splenium of the corpus callosum appear

normally formed, but the callosal body is

absent. The hypothalamus and lentiform

nuclei appear normally separated in

MIH patients, but the caudate nuclei

and thalami are incompletely separated

in many patients [Simon et al., 2002].

The sylvian fissures in most patients are

oriented nearly vertically and were

abnormally connected across the mid-

line over the vertex of the brain [Simon

et al., 2002]. Approximately two-thirds

of the MIH patients have either sub-

cortical heterotopic gray matter or

cortical dysplasia. Abnormally thick

cortex lining the anterior interhemi-

spheric fissure was often present and was

contiguous across the midline. As in

other types of HPE, the anterior vascu-

Figure 3. MRI of a 7-month-old infant girl with semilobar HPE. Sagittal T1-weighted image (A) demonstrates absence of the genuand body of the corpus callosum, but presence of the splenium (white arrowheads). Axial T2-weighted image (B) shows absence ofinterhemispheric fissure anteriorly. The posterior hemispheres are well separated and the posterior horns of the lateral ventricles are wellformed. The head of the caudate are nonseparated and the thalami are partially separated. The third ventricle is abnormal and the frontalhorns are absent. Coronal T2-weighted image (C) at the level of the thalami shows a shallow interhemispheric fissure (black arrowheads) andfusion of cortical matter at the midline.


lature is abnormal with an azygous ACA

noted in all patients. Patients usually

have normal or large intraocular dis-

tances (hypertelorism).

Septo-Optic Dysplasia and HPE

Some of the patients with lobar HPE

may fall within the spectrum of septo-

optic dysplasia [Barkovich et al., 1989].

In these patients, there is complete

absence of the septum pellucidum

and hypoplasia of the optic nerves

and chiasm. These patients have visual

Some of the patients with

lobar HPE may fall

within the spectrum of

septo-optic dysplasia.

In these patients, there

is complete absence of the

septum pellucidum and

hypoplasia of the optic

nerves and chiasm.

impairment and hypothalamic–pitui-

tary axis abnormality resulting in endo-

crinopathies. Careful examination of the

MRI images will show anterior callosal

dysgenesis and hypothalamic or pre-

optic area dysgenesis or fusion (Fig. 6).

Figure 4. MRI of a 19-month-old boy with lobar HPE. Sagittal T1-weighted image (A) demonstrates the presence of posterior bodyand splenium of the corpus callosum (arrowheads), but the genu is not developed and the anterior body is very hypoplastic. Axial T2-weighted image (B) shows that the anterior and posterior interhemispheric fissures (IHFs) are present (arrows). The cerebral hemispheresthat are fairly well separated both anteriorly and posteriorly, but there is abnormal gray matter continuous across the midline anteriorly(curved arrow). An azygous anterior cerebral artery flow void is present in the anterior IHF. The fontal horns are rudimentary. CoronalSPGR image (C) shows failure of complete separations of the frontal lobes across the interhemispheric fissure (curved arrow) and continuousgray matter in the basal frontal regions (arrow).

Figure 5. MRI of a child with an initial diagnosis of septo-optic dysplasia and diabetes insipidus taken at 1 month of age. Sagittal T1-weight image (A) shows absence of rostrum and genu of the corpus callosum (arrowheads). Axial T2-weighted image (B) shows absence ofthe septum pellucidum and hypoplastic frontal horns. Coronal T1-weighted image (C) shows absence of the septum pellucidum and theanterior corpus callosum. The hypothalamus and basal structures appear to be fused and the third ventricle is not well formed. The opticchiasm was noted to be hypoplastic and was given diagnosis of septo-optic dysplasia. She had optic nerve hypoplasia, visual impairment,multiple endocrine deficiencies, and diabetes insipidus. At 2.5 years of age lobar HPE was diagnoses based on this MRI.


Without evidence of nonseparation in

these regions, the patient should be

classified as having septo-optic dysplasia

only, not HPE.

Spectrum of Hemispheric

Nonseparation in HPE

The classification of classic HPE based

on the degree of hemispheric non-

separation falls within a spectrum.

DeMyer [1987] brought attention to

this spectrum when he stated, ‘‘from the

holospheric brain with no hint of an

interhemispheric fissure, the spectrum

of the malformation extends in unbro-

ken continuity through intermediate

and minimal stages.’’ Because of this

continuum, neuroradiologists have

found categorizing an individual patient

into one of the three classic forms to

be challenging at times [Simon and

Barkovich, 2001], particularly in milder

subtypes. We also believe that in

HPE there is a continuum of abnormal-

ities in the degree of hemispheric

nonseparation.

Continuum between semilobar and

lobar HPE

For example, the precise distinction

between lobar and semilobar HPE is

difficult in some patients, as there is a

continuum of nonseparation of the

frontal lobes and development of the

anterior interhemispheric fissure and

anterior falx. Generally, the patient is

classified as semilobar if the frontal lobes

are greater than 50% fused and lobar if

less than 50% fused. However, this is a

somewhat arbitrary criterion, one that is

often difficult to quantitate. Presence of

a fully developed third ventricle, some

frontal horns, and posterior half of the

corpus callosum (posterior body and

splenium) would favor classification as

lobar HPE.

Continuum between MIH and lobar

At times there may be features of both

MIH and lobar HPE. The anterior and

posterior poles of the hemisphere are

separated but there is nonseparation of

the perirolandic cortex (as seen in MIH),

as well as, the basal frontal lobes (as seen

in lobar) (Fig. 7). In MIH, the corpus

callosum is dysplastic most often in the

region of the body, whereas in lobar

HPE, the genu and rostrum are most

often affected. In patients who fall in the

borderline, the corpus callosum dys-

genesis may involve all of these callosal

areas (excluding the splenium).

Minimal Forms of HPE

We have identified several patients

with a minimal form of HPE with

abnormal midline fusion limited to the

preoptic area (involving the suprachias-

mic region and anterior hypothalamus)

or the septal region (subcallosal region)

(Fig. 8). These patients have no

or minimal fusion of the frontal neo-

cortex. The fornices are often thickened

and dysplastic and the anterior commis-

sure may also be maldeveloped. An

azygous ACA is often present. These

patients are often brought to imaging

because of subtle craniofacial malforma-

tion such as single median maxillary

central incisor (SMMCI) and congeni-

tal nasal pyriform aperture stenosis

(CNPAS), endocrinopathies, or mild

developmental delays.

Classifying these minimal patients as

HPE may be controversial. Many who

study HPE consider the presence of

fornix, septum, and anterior commis-

sure to essentially exclude the diagnosis

of HPE, especially when the two

cerebral hemispheres are completely

Figure 6. MRI of a 2-year-old girl with MIH. Midsagittal T2-weighted image (A)shows the presence of genu and splenium of the corpus callosum (white arrowheads). Thebody of the corpus callosum is absent in the region of nonseparated hemispheres (whitecurved arrow). Coronal inversion-recovery image (B) at level of lentiform nucleidemonstrate continuity of the gray matter (black arrowheads) and absent septumpellucidum. Heterotopic gray matter is present in midline at the roof the lateral ventricle.Axial FLAIR image (C) shows continuity of gray matter anterior to the genu of corpuscallosum and absent septum pellucidum. At a higher level, axial FLAIR image (D) showsanteriorly displaced sylvian fissures with abnormal gray matter crossing the midline.


separated (i.e., absence of neocortical

fusion). Nevertheless, both the preoptic

area and the septal region are tele-

ncephalic structures (with the former

being closely related in structure to the

hypothalamus, a diencephalic structure).

Therefore, the nonseparation of these

midline structures would be consistent

with the ventral patterning defect seen in

classic HPE. We believe that these

patients represent the mildest end of

the spectrum of HPE. These patients are

not microforms of HPE, which by

definition exclude brain involvement.

OTHER BRAINSTRUCTURES INVOLVEDIN HPE

In addition to the hemispheric non-

separation, attention should also be paid

to other midline structures of the brain.

Several neuroimaging studies of a large

cohort of HPE patients [Simon et al.,

2000, 2001, 2002; Barkovich et al.,

2002a,b] have provided a grading system

for various involved brain structures of

HPE. This has allowed the correlation of

imaging findings and clinical character-

istics [Lewis et al., 2002; Plawner et al.,

2002] and led to a better understanding

of the embryological derangements that

lead to HPE.

Figure 7. MRI of 20-month-old girl with features of lobar HPE and MIH. Axial T2-weighted image (A) shows smallunderdeveloped frontal horns and absence of septum pellucidum. Gray matter band and azygous anterior cerebral artery are noted in midlinein the shallow anterior interhemispheric fissure. Coronal T2-weighted image (B) shows large midline fusion of the frontal lobes anteriorly(arrowheads). Coronal T2-weighted image (C) further posteriorly at level of anterior thalami shows midline seam of gray matter(arrowheads).

Figure 8. MRI of a 10-year-old boy with learning disabilities, single median maxillary central incisor, congenital nasal pyriform aperturestenosis, and endocrinopathies. T1-weighted midsagittal image (A) shows hypoplasia of the rostrum of corpus callosum and a rectangular areaof abnormality in the subcallosal region, anterior to the hypothalamic region (white arrowheads). The dysplastic appearing fornix is anterior tothis region (white arrows). T2-weighted axial image (B) shows well-developed anterior and posterior interhemispheric fissures and an azygousanterior cerebral artery flow void in the interhemispheric fissure. There is an area of midline fusion just anterior to the anterior commissure,which appear as a dark bow-like band (white arrowheads). Further anterior to this region is the dysplastic fornix (white arrows). CoronalSPGR image (C) slight anterior to the anterior commissure shows the dysplastic fornix (white arrows) traveling below the septum pellucidumand inferior to that the area of midline fusion (white arrowheads).


Deep Gray Nuclei

The deep gray nuclear structures and

diencephalon are often profoundly

affected in many patients with HPE. A

neuroimaging study of 57 classic HPE

patients (43 MRI studies and 14 high-

quality CT studies) revealed that the

hypothalamus and caudate nuclei were

the most commonly nonseparated deep

gray structures in HPE [Simon et al.,

2000]. Nearly all patients (99%) with

classic HPE had some degree of hypo-

thalamic nonseparation (Fig. 9). The

caudate nuclei were not fully separated

in 96% of the patients. The thalami were

Nearly all patients (99%) with

classic HPE had some degree of

hypothalamic nonseparation.

The caudate nuclei were not

fully separated in 96%

of the patients.

the least frequently involved of the deep

gray nuclei, showing nonseparation in

67%. Abnormal orientation of the long

axis of the thalamus (outside of 308–458)was seen in 71% of the patients.

In 27% of the patients, the mesen-

cephalon showed some degree of non-

separation, implying that in a large

proportion of patients the rhombence-

phalon is also disturbed during develop-

ment. The midbrain involvement may

be seen pathologically a failure of two

distinct paired superior and inferior

colliculi to form, continuity of the

oculomotor nuclei across the midline,

and aqueductal atresia or stenosis [Vogel

et al., 1990; Sarnat and Flores-Sarnat,

2001]. In 11% of the HPE patients, a

single deep gray nuclear mass without

discrete basal ganglia, thalami, and

mesencephalon was noted (Fig. 1B).

The pattern of deep gray nuclei

abnormalities, in particular the universal

involvement of the hypothalamus, sup-

ports the theory that a lack of induction

of the most rostral aspects of the

embryonic floor plate is the cause of

classic HPE. The finding of mesence-

phalic abnormalities implies that in the

abnormal development in HPE, the

rostral-caudal gradient at times extends

more caudally beyond the prosencepha-

lon.

Midline Dorsal Cyst

Dorsal cysts are more often present in

alobar HPE (92%), compared to semi-

lobar HPE (28%) and lobar HPE (9%)

[Plawner et al., 2002] (Figs. 1 and 2). The

presence of dorsal cysts strongly corre-

lates with the degree nonseparation of

the thalami [Simon et al., 2001; Plawner

Dorsal cysts are more often

present in alobar HPE (92%),

compared to semilobar

HPE (28%) and lobar

HPE (9%). The presence

of dorsal cysts strongly

correlates with the

degree nonseparation

of the thalami

et al., 2002]. It is hypothesized that the

nonseparated thalamus physically blocks

egress of CSF from the third ventricle.

The egress of the CSF through the path

of least resistance, which is the thin

posterior wall of the third ventricle in

the suprapineal recess, results in expan-

sion of the posterodorsal portion of the

ventricle to form the dorsal cyst. Sup-

porting this theory, hydrocephalus is

often, but not always, noted in associa-

tion with dorsal cysts [Simon and

Barkovich, 2001; Plawner et al., 2002].

Two-fifth of HPE patients with dorsal

cyst require CSF shunting procedure

[Plawner et al., 2002]. Additional abnor-

malities of the cerebral aqueduct of

Sylvius, such as atresia or stenosis that

have been found in HPE on neuro-

pathologic examination [Vogel et al.,

1990] may also contribute to the

obstruction of CSF flow and require-

ment for shunting.

The gross morphologic descrip-

tions of the holosphere as pancake, cup,

or ball shape is a reflection of the size of

the dorsal cyst. Smaller dorsal cysts may

remain stable without causing hydro-

cephalus, or disappear after a CSF

shunting procedure. Occasionally, the

dorsal cyst herniates through the ante-

rior fontanelle to form a vertex ence-

phalocele that is unique in HPE [Sarnat

and Flores-Sarnat, 2001].

The dorsal cyst of HPE is similar in

appearance to the interhemispheric cyst

Figure 9. Hypothalamic involvement in HPE. Coronal T2-weighted image (A) in16-year-old girl with lobar HPE demonstrates complete separate of the hypothalamuswith fully formed third ventricle. Coronal T2-weighted FSE image (B) in a neonate withsemilobar HPE reveals complete nonseparation of the hypothalamus and absent thirdventricle.


associated with agenesis of the corpus

callosum [Young et al., 1992]. The latter

is frequently misdiagnosed as HPE, but is

distinguished by normal cleavage of the

basal forebrain structures. Differentiat-

ing these can be especially difficult when

the abnormal brain anatomy is further

distorted by hydrocephalus. Definitive

diagnosis in these patients often requires

a repeat MRI after decompression.

Cortical Gyral Abnormalities

In a neuroimaging study of 96 patients

with classic HPE, the cortical thick-

ness was normal in all patients, and

gyral/sulcal sizes were normal in 83%

[Barkovich et al., 2002a]. Diffuse dys-

plastic cortex and broad gyri with too

few sulci may be present is some patients

with HPE. Although the cortex may

appear too thick, as in pachygyria, the

measured cortical thickness is normal.

This gyral pattern is more common in

alobar (seen in 8/96) but can also be seen

in semilobar (Fig. 10).

Subcortical heterotopia occur in a

small percentage of patients with classic

HPE (only 4 of 96 patients in study by

Barkovich et al. [2002a]), and often

consist of large masses that crossed the

midline in the noncleaved regions

(Fig. 11A,B). Large midline mass of

subcortical dysplastic cortex have been

noted in rare patients (Fig. 11C) which

some authors refer to as ‘‘brain in brain

malformation’’ [Widjaja et al., 2007].

These patients often have lobar HPE or

MIH, and develop severe localization-

related epilepsy.

In the most severe patient with

alobar HPE, no sylvian fissure can be

identified. In less severe forms the

sylvian fissures are displaced more ante-

riorly and medially as HPE reflecting the

degree of frontal lobe development

[Barkovich et al., 2002a]. The degree

of the displacement of the sylvian fissures

(as measured by the sylvian angle)

correlated strongly with the severity

of classic HPE (alobar> semilobar>lobar) and with the degree of abnormal

frontal lobe development.

White Matter Abnormalities

MRI studies of the white matter in

HPE have focused on callosal abnormal-

ities [Simon and Barkovich, 2001;

Simon et al., 2002]. The degree of

callosal dysgenesis correlates with the

severity of the midline hemispheric

fusion. In addition in classic HPE, there

is a delay in myelination of the white

matter maturity [Barkovich et al.,

2002b]. Patient with MIH had normal

myelination development.

Diffusion tensor imaging (DTI)

and tractography techniques have been

applied to analyze white matter tracts

abnormalities in HPE [Albayram et al.,

2002; Rollins, 2005]. In patients with

alobar HPE, the cortico-ponto-spinal

tracts were absent bilaterally, confirming

the findings from prior neuropatholog-

ical studies [Kobori et al., 1987]. In most

patients with less severe types of HPE,

the corticospinal tracts were present

bilaterally. HPE type and neurodevelop-

mental score correlated strongly with

cortico-ponto-spinal tracts and middle

cerebellar peduncle dimensions. Thick-

ened dysplastic fornices have been noted

with DTI tractography in a patient with

semilobar HPE [Rollins, 2005]. These

findings demonstrate that analysis of

white matter tracts in HPE using DTI

adds complementary information to

traditional MRI analysis.

Other Cerebral Anomalies

On rare occasions, other cerebral

anomalies may be associated with HPE

including schizencephaly, Dandy–

Walker complex, Chiari malformations,

and various encephaloceles. Other pos-

terior fossa abnormalities such as rhom-

bencephalosynapsis, in which there is

abnormal nonseparation of the cerebel-

lar hemispheres, are also occasionally

seen in HPE. A rare patient of diffuse

polymicrogyria in a patient with MIH

has been reported [Takanashi et al.,

2003].

Vascular Anomalies

The anterior circulation vasculature is

often abnormal in HPE. In more severe

types (alobar and semilobar) of HPE,

there is a lack of formation of normal

middle and anterior cerebral arteries,

being replaced by a rete of vessels arising

from the internal carotid and basilar

arteries. In less severe patients, including

MIH, the arterial system is nearly

normal but an azygous, or unpaired,

ACA is nearly always noted [Simon and

Barkovich, 2001]. We also noted an

azygous ACA in our patients with

the minimal form of HPE involving

the preoptic/septal regions.

Figure 10. MRI of a 3-day-old female infant with semilobar HPE. Axial (A) andcoronal (B) T2-weighted images shows very simplified gyral pattern with broad gyri andtoo few sulci. The images also show fusion of the thalamic nuclei and large monoventriclewithout a clear dorsal cyst.


Neuroimaging Techniques and

Systematic Method of Review

To properly interpret the neuroimaging

study of a patient suspected of having

HPE, a high-resolution MRI scans that

include thin-section image sequences in

three orthogonal planes (axial, sagittal,

and coronal) are preferred. T2-weight

axial and coronal images are preferred.

To properly interpret the

neuroimaging study of a patient

suspected of having HPE, a

high-resolution MRI scans that

include thin-section image

sequences in three orthogonal

planes (axial, sagittal, and

coronal) are preferred.

T2-weight axial and coronal

images are preferred.

The study should also include a

volumetric dataset (three-dimensional

spoiled gradient-echo sequences),

which provides good gray-white matter

differentiation and permits reformatting

in other planes and volumetric analyses

[Simon and Barkovich, 2001].

To determine the type of HPE,

careful assessment of the telencephalon is

performed. Close attention is paid to the

presence of anterior and posterior inter-

hemispheric fissures and the extent of

nonseparation of the two hemispheres.

In addition, the basal ganglia, thalamic

nuclei, hypothalamus, pituitary gland,

and mesencephalon are analyzed for

degree of nonseparation or dysgenesis.

Other structures that are scrutinized

include the morphology of the ventric-

ular system and dorsal cyst, cortical

malformations, subcortical heterotopia,

sylvian fissure development, optic

nerves and chiasm, and olfactory bulbs

and tracts. The posterior fossa should

also be examined carefully for abnor-

malities, including aqueductal dysgene-

sis, Dandy–Walker complex, Chiari

malformations, cerebellar abnormalities,

and other brainstem abnormalities.

Extracerebral structures that alsowarrant

review are the presence of an SMMCI,

CNPAS, premaxillary clefts, other mid-

line clefts, and distance between the eyes

(for hypo- or hypertelorism). When

these extracerebral anomalies are found,

careful attention should be paid atten-

tion to the preoptic area and septal

region for minimal forms of HPE.

Neuroimaging evaluation of the

brain in HPE may be challenging in

infants because of the inherent small

brain size and immature myelination.

Follow-up imaging after a period of

brain growth may be required. Difficul-

ties in assessment also occur when

hydrocephalus distorts underlying brain

structures [Simon and Barkovich, 2001].

Definitive diagnosis in these patients

may require repeat MRI after ventricu-

loperitoneal shunting.

Ideally, a pediatric neuroradiologist

with experience in brain malformations

should review the imaging studies.

Approximately 1/5 to 1/3 of the imag-

ing studies referred to our centers for

HPE fail to meet the HPE neuroimaging

criteria [Stashinko et al., 2004; Hahn

et al., 2008]. The ultimate diagnoses

given to these studies include septo-

optic dysplasia, absent septum pelludi-

cum with schizencephaly, agenesis of

corpus callosum, or callosal agenesis

with interhemispheric cyst (CAIHC).

Figure 11. A,B: MRI of a 2-year-old girl with lobar holoprosencephaly andsubcortical heterotopia. Axial T2-weighted (A) and oblique coronal STIR (B) imagesshow large subcortical gray matter heterotopia (white arrowheads) located in the rightfrontal lobe white matter in addition to midline gray matter fusion in the anteriorinterhemispheric fissure. This patient also had refractory epilepsy. C: T2-weightedcoronal MRI of a 1.5-year-old patient with MIH and subcortical heterotopia consistingof large infolding dysplastic cortex (black arrowheads). This child also had severelocalization-related epilepsy. D: Coronal spoiled gradient-recalled image in a patientwith semilobar HPE shows heterotopia gray at the roof of the monoventricle (whitearrowheads).


CAIHC type 1b is frequently mis-

diagnosed as HPE, but is distinguished

by normal cleavage of the neocortical

structures. The dorsal cyst of HPE

CAIHC type 1b is frequently

misdiagnosed as HPE,

but is distinguished by normal

cleavage of the neocortical

structures.

is similar in appearance to the in-

terhemispheric cyst associated with

CAIHC type 1b (Fig. 12) [Young et al.,

1992; Barkovich et al., 2001]. The

thalamic nuclei may be nonseparated in

some patients with CAIHC, thus creat-

ing a blockage of CSF egress. Some

authors refer to this malformation as

‘‘holodiencephaly,’’ meaning that the

diencephalon, but not the telencepha-

lon, is abnormally nonseparated. In

absence of some degree of neocortical

or preoptic area nonseparation, these

patients should not be diagnosed with

HPE. Nevertheless, patients with thala-

mic fusion may share similar pathoge-

netic mechanisms involved in HPE, and

further studies are needed to understand

the boundaries and continuities of mid-

line malformations.

Three-dimensional MRI with

reconstruction may provide comple-

mentary anatomic information that is

not apparent from conventional MRI

[Takahashi et al., 2003; Takahashi et al.,

2004]. In semilobar HPE three-dimen-

sional reconstructions reveal a rostro-

caudally aligned midline gray matter

‘‘seam’’ that extends from the supra-

chiasmatic hypothalamus to a caudally

positioned diminutive body and sple-

nium of the corpus callosum [Takahashi

et al., 2003]. In the rostral to caudal

direction, the interhemispheric fissure

also transitions from being absent, to a

shallow but deepening zone, and finally

to a zone where the fissure is at full

depth. At this full depth zone, the

midline gray matter is continuous with

the neocortex of the cerebral surface.

Caudal to the seam, the telencephalic

structures are normally separated

(between the right and left hemispheres)

and the seam is replaced by the posterior

corpus callosum. DTI has also been

studied in patients with other types of

HPE and is discussed in the ‘‘White

Matter Abnormality’’ section above.

Fetal Neuroimaging

Prenatal ultrasounds have been used

to detect the CNS and facial abnormal-

ities of severe HPE as early as the first

trimester [Filly et al., 1984; Nyberg

et al., 1987; Tongsong et al., 1999].

Failure to identify the characteristics of

the developing choroid plexuses (‘‘but-

terfly sign’’) during the first trimester

may be a sensitive indicator of HPE

[Sepulveda et al., 2004]. In alobar and

semilobar HPE, prenatal diagnosis can

readily be made by ultrasound [Peebles,

1998]. The sensitivity of ultrasonogra-

phy for detection of milder forms of

HPE (lobar and MIH) may be low, since

in these forms the anterior and posterior

interhemispheric fissures are present and

the characteristic dorsal cyst of HPE is

often absent. In our experience, prenatal

ultrasonography had low sensitivity.

Although prenatal ultrasound was per-

formed in 93% of 104 HPE patients

(weighted toward less severe type),

prenatal diagnosis was made in only

22% [Stashinko et al., 2004].

Fetal MRI will provide better

characterization of the malformations

[Sonigo et al., 1998]. Modern ultrafast

MRI techniques reduce movement

artifacts significantly and are ideal for

fetal imaging (Fig. 13). Fetal MRI has

been used to diagnose various forms of

HPE including alobar, semilobar, lobar

[Wong et al., 2005], and MIH variant

[Pulitzer et al., 2004; Picone et al.,

2006]. Other midline anomalies, such

as agenesis of corpus callosum, CAIHC,

Figure 12. MRI in a 15-month-old female infant with thalamic fusion (holodiencephaly) and interhemispheric cyst. T2-weightedaxial (A) and coronal (B) images show thalamic fusion (black arrows) and a very large interhemispheric cyst (IHC). Anteriorinterhemispheric fissure is complete between the frontal lobes (white arrow) and contains an azygous anterior cerebral artery. Sagittal T2-weighted image (C) shows lack of a visible cerebral aqueduct. On axial image of the midbrain (not shown), the aqueduct was not visible andthe tectum appeared dysplastic with nonseparation of the colliculi.


absent septum pellucidum, and hydro-

cephalus with communication of the

lateral ventricles, are sometimes mis-

diagnosed prenatally as HPE [Malinger

et al., 2005]. The detection of craniofa-

cial malformations associated with HPE

on prenatal imaging often aid in the

diagnosis of HPE. See Mercier et al.

[2010] for information on ‘‘molecular’’

prenatal diagnosis.

ACKNOWLEDGMENTS

This research was supported by the

Carter Centers for Brain Research

in Holoprosencephaly and Related Mal-

formations, the Don and Linda Carter

Foundation, and the Crowley-Carter

Foundation. We thank Nancy Clegg

and Elaine Stashinko from the Carter

Centers for providing the source images.

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neuroimaging advances in holoprosencephaly: reﬁning the spectrum of the midline malformation

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