Gabriel L. Kalifa, MD #{149}Catherine Chiron, MD #{149}Nicolas Sellier, MD #{149}Philippe Demange, MD

S Gerard Ponsot, MD #{149}Guy Lalande, MD #{149}Olivier Robain, MD

Hemimegalencephaly:MR Imaging in Five Children’

29

Hemimegalencephaly is a rare brainmalformation characterized by cere-

bral asymmetry and cortical dyspla-sia. Infants with the condition

present with early seizures and Se-vere encephalopathy. Five patients

were studied with computed tomog-

raphy and magnetic resonance (MR)

imaging. MR imaging was the most

efficient diagnostic method for this

rare entity. It demonstrated brain

hemispheric hypertrophy with lat-

eral ventricle dilatation, abnormalgyral pattern, and a thick cortex on

the enlarged side. The images corre-late well with the known pathologic

data.

Index terms: Brain, enlarged, 10.144 #{149}Brain,

abnormalities, 10.144 #{149}Brain, diseases, 10.144

#{149}Brain, MR studies, 10.1214 #{149}Infants, central

nervous system, 10.144

Radiology 1987; 165:29-33

I I’rom the Departments of Radiology

(G.LK., N.S., PD., G.L.). Neuropediatrics (In-serm 429) (CC., Cr.). and Neuropathology

(OR.), Hospit.il Saint Vincent de Paul, 74 Ave.Denfert-Rochereau, 75674 Paris, France. 1:romthe 1986 RSNA annual meeting. Received Dc-comber 3, 1986; revision requested February 17,1987; revision received March 25; accepted Max’

6. Address reprint requests to G.L.K.CRSNA, 1987

H EMIMEGALENCEPHALY is a rare

malformation consisting of uni-

lateral hypertnophy of the brain. The

gyri are enlarged, and there is diffuse

enlargement of the cortex with disap-

pearance of horizontal layering of

the neurons. Glial abnormalities can

be observed. These features are ne-

stnicted to one hemisphere. Until

now, the diagnosis was usually made

only at autopsy. New imaging mo-

dalities, especially magnetic neso-

nance (MR) imaging, now allow nec-

ognition of this entity early in life. In

addition, similar, but more localized,

lesions have been discovered with

MR imaging in patients presenting

with the same symptoms.

We describe five patients with con-

genital unilateral brain hypertrophy

who presented with partial and very

early epilepsy. Focal neurologic signs

were present, and the circumference

of the head was larger than normal.

We discuss the diagnostic possibibi-

ties of MR imaging and consider the

classification of this malformation.

MATERIALS AND METHODS

Clinical data for our five patients are

summarized in Tab!e 1. All children were

admitted for early seizures. The convul-

sions were partial, motor, and intractab!e.

Clinical examination showed an enlarged

head circumference in five cases and cra-

nial asymmetry in three cases. Focal neu-

rologic signs were present in all cases.

Mental retardation was a constant early

finding. There was no evidence of somat-

ic limb or trunk asymmetry. No skin or

visceral lesion was noted, and there was

no fami!ia! history. In a!! cases electroen-

cephalography (EEC) showed abnormal

activity on the hypertrophic side.

All patients underwent computed

tomographic (CT) scanning and MR im-

aging. Sedation was always necessary for

MR imaging, for which a!imenazine (1

mg/kg) or sodium pentobarbital (5 mgI

kg) was used.

MR imaging was performed with a 1.5-

T magnet in three patients and 0.5-T mag-

net in the two others. Both short (partial

saturation) sequences (400 or 600/25)

(repetition time [TRI msec/echo time [TE]

msec) and long (spin-echo) sequences

(2,000/40 or 80) were used. First, sagittal

views were obtained with short se-

quences. Then coronal and axial views

were obtained with Ti- and T2-weighted

spin-echo sequences.

Examinations were reviewed separate!y

by three radiologists, and the results were

correlated with the clinical and EEC data.

All of our patients are alive, so no proof

of the pathologic condition is avai!ab!e,

but correlation was made with data on

similar cases found in the literature.

As indicated in Table 1, one chi!d (pa-

tient 3) has severe menta! retardation

with several seizures each day despite

therapy. Two patients (patients 1 and 2),

followed up for 9 and 7 years, respective-

ly, have mild retardation. They have oneor two epileptic crises each month. Two

other patients, followed up for only a few

months (patients 4 and 5), have moderate

retardation, but no recurrent seizures. A!l

patients have been treated with steroids

and several antiepileptic drugs.

RESULTS

In all cases CT scans disclosed hy-

pertrophy of a hemisphere responsi-

ble for a midline shift and dilatation

of the lateral ventricle on the hyper-

trophic side. These signs can exclude

a tumor. The ventricular dilatation

involved the entire hemisphere in

three cases and was limited to the oc-

cipital horn in two cases. The sulci

were poorly seen and the gyri ap-

peared less visible, mainly in the oc-

cipital area.

Increased density of the white mat-

ter was noted in only one case. The

opposite hemisphere, the brain stem,

and the cerebellum were considered

normal (Fig. la).

MR imaging not only showed the

same lesions well but it also provided

further information. The hem ispher-

ic asymmetry was well seen on cor-

onal views. The entire hemisphere

was enlarged in three cases. The hy-

pertrophy was limited to the occipi-

.:.

d. e.

30 . Radiology October 1987

Figure 1. Patient 2. Images in a patient with focal right neurologic signs who first experienced seizures at 9 months. (a) On CT scan, left

hemispheric hypertrophy with left lateral ventricle dilatation is evident. Sulci are poorly depicted. (b) MR image (600/25 sequence) depicts

moderate left hypertrophy with left ventricle dilatation. Some compression of left side of cerebellum can be seen, and the gyri are wider and

roughly defined. (c) On MR image (600/25 sequence), a thick cortical ribbon on the left side, wide gyri, and asymmetric enlargement of

white matter can be seen.

Figure 2. Patient 3. MR images in patient

with clinically evident left hemispheric

asymmetry who experienced seizures the 1st

day of life. (a-c) Coronal views (600/25 se-

quence) depict the main signs, including

huge left hemispheric hypertrophy and en-

largement of white matter, thickened cortex,

abnormal ventricle midline shift to right,

and loss of norma! gyral pattern, with no

sulcus seen. (d, e) 12-weighted (2,000/40-80

sequence) transverse images show abnormalwhite-matter signal. The volume of the

white matter is increased, there are gyral ab-

normalities, and the cortex is thickened.

topanietab areas in two cases. The hy-

pentrophy bed to midline shift in all

cases and cerebellar compression in

four cases (Fig. 2a, 2b). Otherwise,

the opposite hemisphere and the cer-

ebelbum were apparently normal

(Fig. 2c).

b.

Figure 3. Patient 4. (a, b) 12-weighted (2,000/40-80 sequence) images depict hypertrophylocalized to the posterior segment of the left hemisphere. Enlarged left occipital horn, wid-

ened gyri in left occipital lobe, and thickened cortex are well seen on first-echo image

(a). Second-echo image (b) reveals a high-intensity signal in periventricular white matter of

the left posterior horn.

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VoIumel65 Numberl Radiology . 31

Unilateral ventricular dilatation

was clearly demonstrated with MR

imaging (Fig. 2d). Gyral pattern ab-

normalities were obvious with this

technique; the sulci were shallow

and poorly defined and the gyni ex-

cessively wide. The cortical ribbon

was thicker on the hypentrophic side;

in comparison with the opposite side,

the limit between cortex and subcor-

tex was poorly seen (Figs. ic, 2b).

In all our cases, a very intense sig-

nab was noted in the white matter on

T2-weighted sequences. The signal

was significantly more intense than

that of the contralateral white matter,

and the extent of this signal was

roughly parallel to that of the hyper-

trophy (Figs. 2d, 3). MR imaging did

not reveal focal areas of hetenotopia.

DISCUSSION

Literature Review

Fifteen well-documented cases of

hemimegalencephaly have been re-

ported in the literature (1-12). The

anatomic features of the condition

are distinct and fairly homogeneous.

In all cases, one can note a hemi-

spheric hypertrophy, a firm consis-

tency, and a modified gynal pattern

on the enlarged side. The sulci are

present but shallower than normal,

and the gyri are excessively wide.

The cortex is thicker than on the op-

posite side, and its inner contour is

not well delineated. The white mat-

ter is thicker than normal and the lat-

eral ventricle is dilated (Fig. 4).

Microscopic examination shows

complete disorganization of the cyto-

architecture. All lamination into hon-

izontal layers is lost. A large number

of heterotopic subcortical neurons

are seen. The most singular feature is

the presence of giant neurons dif-

fuseby scattered in the cortex. Finally,glial abnormalities are less constant

(fibnibbary or protoplasmic gliosis).

All of these abnormalities are clearly

unilateral.

Radiologic Patterns

Very few nadiologic data exist con-

cerning hemimegalencephaly. On CT

scans, hypertrophy of one hemi-

sphere with dilatation of the lateral

ventricle on the same side is ob-

served.

No description of this finding on

MR images has been previously re-

ported, to our knowledge. All the im-

ages obtained in our patients seem to

correlate well with the pathologic

data. The brain hypentrophy and the

ventricular enlargement are well

demonstrated with MR imaging, es-

pecially on coronal images. The gynal

modifications, suspected with CT, are

well seen with MR imaging. The con-

tical ribbon appears thicken on patho-

logic and MR imaging examinations,

but was not seen on CT scans. Its

poorly defined inner contour may be

explained by the existence of numer-

ous heterotopic neurons. The high

signal intensity of the white matter

on T2-weighted sequences may be re-

lated to the glial abnormalities. Such

I

Figure 4. Pathologic specimen shows typi-

cal findings: hypertrophic right hemi-

sphere, enlarged white matter, thickened

cortex with loss of normal layering, and ab-

normal neuronal migration in the white

matter.

Table 2

Figure 5. T2-weighted (300/28 sequence

performed on model 5000 scanner, Magni-

scan, [CGR, Paris]) images in 6-year-old girl

with agyria-pachygyria with typical history.

Note thickened cortex, few sulci, widenedSylvian fissure, asymmetry of brain, and

normal T2-weighted signal intensity.

Differential Features of Hemimegalencephaly and Similar Conditions

Hemi- Corticalmegal- Focal Agyria- Tuberous

Feature encephaly Dysplasia Pachygyria Sclerosis

Clinical presentationSeizures + + + +Mental retardation + - + +

MacropathologyThick cortex + + + -

UnilateralityEnlarged white matter

MicropathologyHorizontal layersRadiate dispositionGliosis

++

-

++

+-

-

+±

-

-

-

+-

-

-

+++

Giantneurons + ± - +

Note.-+ = typically found, - typically not found, ± sometimes found and sometimes notfound.

32 . Radiology October 1987

mass effect, the absence of modifica-

tion at follow-up, and the association

of features such as ventricular dibata-

tion. No association with congenital

hemihypertnophy, such as Beckwith-

Wiedemann syndrome, has been

found.

Contralaterab atrophy can be ruled

out for several reasons: (a) increased

head circumference, (b) abnormalities

seen on the enlarged side in MR im-

ages, (c) focal clinical signs come-

sponding to the enlarged hemi-

sphere, and (d) the normal pattern

seen on the “small” side.

Agynia-pachygynia (13) must be

considered in the differential diagno-

sis. In both entities, the cortex is

thickened, the gyrab pattern is abnor-

mab, and the clinical presentation is

similar. There is evidence in both

diseases of abnormal neunonal migra-

tion in the first 4 months of fetal life

(14). But in agyria-pachygynia lesionsare bilateral, frequently one can note

cerebral atrophy instead of hypertno-

phy, and no giant neuron is present

(Fig. 5).

Giant neurons are found in two

other diseases, tuberous sclerosis and

focal cortical dysplasia. Tuberous

sclerosis (15) can easily be ruled out

by its specific clinical and radiobogic

signs. Focal cortical dysplasia (16, 17)

exhibits some clinical and radiobogic

similarities with hemimegabencepha-

by. More experience is needed to con-

firm or exclude the possibility that

focal dysplasia is a minor expression

of the same disorder. In both focal

cortical dysplasia and hemimegalen-

cephaby the cause of the condition me-

mains unknown.

CONCLUSION

imaging features have been observed

in our experience in lesions accompa-

nied by gbiosis. MR imaging proved

to be very efficient in aiding the di-

agnosis of the different lesions, giv-

ing more precise details than CT did.

For all these reasons, the diagnosis of

hemimegalencephaly seems likely in

our patients, even without histologic

proof.

Presentation and Outcome

In our cases, as in the literature,

the clinical presentation is fairly ho-

mogeneous. The patients present

with early epilepsy, severe encepha-

bopathy, and focal neunobogic signs.

However, the prognosis may be dif-

fenent from one patient to another: In

some patients this disorder is lethal

in the newborn period, whereas oth-

en patients survive with greater or

lessen intellectual deficiency. The oc-

cunrence of seizures before 1 month

of age carries a poor prognosis, but

the severity also seems related to the

extent of the lesions. For instance,

our two patients with localized mab-

formations (patients 4 and 5) expeni-

enced a less-severe evolution than

the early symptoms would have sug-

gested. The child who presented

with the most severe encephabopathy

(patient 3) had the most important

cortical thickening. However the

beast mental retardation was seen in a

patient (patient 2) with a moderately

thickened cortex.

No other cases have been reported

in the families of patients with hemi-

megabencephaly.

Differential Diagnosis

In the differential diagnosis (Table

2) an infiltrative tumor can be easily

ruled out, considering the absence of

Hemimegalencephaly is a rare

brain malformation revealed clinical-

by by early onset of seizures. The di-

agnosis now can be made easily with

MR imaging, which demonstrates in

one hemisphere a largely diffuse hy-

pentrophy, lateral ventricular dilata-

tion, abnormal gynab pattern, thick-

ened cortex, and high-intensity

signal in the white matter. The ex-

tent of the lesion seen on MR imag-

ing examination seems to correlate

with the severity of the prognosis.

The cause of hemimegalencephaly

remains unknown. There is no famil-

jab incidence, but relation to focal

cortical dysplasia may be confirmed

in the future. U

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