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Paediatric neuro-imaging: diagnosis.

We congratulate Professor Savvas Andronikou of the Department of Radiology, University of Pretoria, for his spot-on diagnosis, for which he receives the award of R1 000 from the RSSA. Dr Misser elaborates below on the images and findings. Please refer to page 45 of the March 2013 issue of the SAJR (http://dx.doi.org/10.7196/SAJR.843) for the investigative images.

Diagnosis

Imaging and findings

A 2-year-old boy presented with microcephaly and developmental delay. The MRI images referred to above, were obtained.

The sagittal T1 (Fig. 1) sequence shows an abnormal appearance of the corpus callosum. The entire body of the corpus callosum is remarkably thin. Only the splenium and part of the genu is relatively spared. Along with the posterior frontal and parietal lobe cerebral fusion, note the absence of the septum pellucidum and fused frontal horns of the lateral ventricles (Figs 2 - 4). The sylvian fissures appear continuous across the hemispheres. MRA of the intracranial vessels (Fig. 5) demonstrates an azygous anterior cerebral artery. The combination of features is compatible with a syntelencephaly subtype of holoprosencephaly.

Discussion

Holoprosencephaly (HPE) is perhaps one of the more complex and captivating congenital brain malformations, with its striking imaging phenotype abnormalities and complex neurological impairment. It is characterised by failure of cleavage of the prosencephalon, a process that usually occurs between the 18th and 28th days of gestation. The overall incidence is much higher than expected, with up to 1 in 250 conceptions affected. With the immense improvement in neuro-imaging, particularly in magnetic resonance imaging, we are now able to delineate the specific areas of involvement including cerebral frontal and parietal lobes, basal ganglia, thalami, orbits and facial structures. There are four subtypes of this malformation. The spectrum includes the classic HPE subtypes in the De Myer classification (viz. alobar, semilobar and lobar HPE), in addition to the fourth subtype known as the middle interhemispheric variant of holoprosencephaly (MIH) or syntelencephaly (Table 1). Solitary median maxillary central incisor is one of several microforms of autosomal dominant HPE not usually included in the classification.
Table 1. Subtypes of holoprosencephaly and differentiating
features (1), (4), (5)

                      Alobar         Semilobar          Lobar

Degree of         Almost complete  Posterior       Rostral and
prosencephalon    non-separation   cerebral        ventral frontal
non-separation    with holosphere  hemispheres     lobe fusion.
                  of cerebral      near normal.    Rest of
                  parenchyma.      Fused anterior  cerebral
                                   aspect with     hemispheres
                                   undeveloped     separated.
                                   frontal
                                   lobes.

Falx cerebri and  Absent IH        Partial         Hypoplastic
interhemispheric  fissure. Dorsal  formation of    anterior falx.
(IH) fissure      cyst present.    shallow IH      Fissure is
                                   fissure and     present.
                                   falx
                                   posteriorly.
                                   [+ or -]
                                   Dorsal cyst.
                                   Falx absent
                                   anteriorly.

Corpus callosum   Agenesis of      Genu and        Genu mildly
abnormality       corpus           anterior body   dysplastic.
                  callosum.        absent.         Body and
                                   Splenium        splenium
                                   almost          well-formed.
                                   normal.

Severity,         Lethal subtype.  Intermediate    Mild subtype.
additional        Stillborn or     severity.       Present later
features and      short lifespan.  Dorsal cyst     with visual
prognosis         Severe facial    seen when       difficulties or
                  dysmorphism.     thalami are     developmental
                  Cyclopia.        fused. Frontal  delay. [+ or -]
                  Olfactory        horns absent.   Mild olfactory
                  aplasia.         Occipital       bulb and tract
                  Monoventricle.   horns normal.   hypogenesis.

                        MIH

Degree of         Posterior
prosencephalon    frontal and
non-separation    parietal lobe
                  fusion.
                  Caudate and
                  thalami often
                  incompletely
                  cleaved.

Falx cerebri and  Anterior falx
interhemispheric  is normal or
(IH) fissure      mildly
                  dysplastic.
                  Posterior
                  aspect of falx
                  is absent.
                  Azygous ACA.

Corpus callosum   Absent body of
abnormality       corpus
                  callosum with
                  normal
                  splenium and
                  genu.

Severity,         Mild cognitive
additional        and vision
features and      impairment.
prognosis         Facial
                  features
                  similar to
                  lobar subtype.
                  Best
                  prognosis.


MIH is a relatively milder clinical form of holoprosencephaly, first described in 1993 by Barkovich et al. (1) Although akin to other forms of holoprosencephaly, MIH is thought to develop on a somewhat different pattern of midline non-cleavage. The classic subtypes of HPE all involve induction of the embryonic floor plate, and SHH is the primary gene abnormality described among several others. In contrast, in the pathogenesis of MIH it is the embryonic roof plate that is involved. Mutations of the ZIC2 gene on chromosome 13, which plays a critical role in differentiation of the roof plate of the developing embryo, have been implicated in MIH. (2)

The affected children present with a variety of deficits including spasticity, dystonia and oromotor dysfunction affecting feeding and speech. Seizures as well as mild cognitive and visual impairment are noted in these children. Endocrinopathy is not a common feature owing to sparing of the hypothalamus and basal forebrain. When compared with other forms of HPE, the overall functional levels including mobility, hand/arm function and speech are better in patients with MIH.

On MR imaging, MIH is characterised by abnormal fusion of the posterior frontal and parietal regions of the cerebral hemispheres. The anterior portions of the frontal lobes, olfactory tracts and the occipital lobes are usually normal. There is frequent incomplete cleavage of the heads of the caudate nuclei and thalami, but the hypothalamus and lentiform nuclei are well-separated. Almost all patients with MIH have a single or azygous anterior cerebral artery. There is a coronal configuration of the sylvian fissures which are continuous across the cerebral vertex. Frequent association with heterotopias and malformations of cortical development have been reported. (3)

In the presence of typical MRI features, a reasonably confident diagnosis of syntelencephaly may be made even antenatally. (4) This radiological diagnosis may serve as a valuable tool in prognostication and in the counselling of parents.

S Afr J Rad 2013;17 (2):72-73. DOI:10.7196/SAJR.889

(1.) Barkovich AJ, Quint DJ. Middle interhemispheric fusion: An unusual variant of holoprosencephaly. AJNR 1993;14:431-440.

(2.) Brown L, Odent S, David V, et al. Holoprosencephaly due to mutations in ZIC2: Alanine tract expansion mutations may be caused by parental somatic recombination. Hum Mol Genet 2001;10:791-796.

(3.) Fujimoto S, Togari H, Banno T, Wada Y. Syntelencephaly associated with connected transhemispheric cleft of focal cortical dysplasia. Pediatr Neurol 1999;20:397-389.

(4.) Pulitzer SB, Simon EM, Crombleholme TM, Golden JA. Prenatal MR findings of the Middle Interhemispheric Variant of Holoprosencephaly. AJNR 2004;25:1034-1036.

(5.) Kanekar S, Shively A, Kaneda H. Malformations of ventral induction. Semin Ultrasound CT MRI 2011 32 (3):200-210.

S K Misser, MB ChB, FCRad (D) SA

Lake Smit and Partners, Durban, South Africa

Corresponding author: S K Misser (misser@lakesmit.co.za)
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Title Annotation:QUIZ CASE
Author:Misser, S.K.
Publication:South African Journal of Radiology
Article Type:Clinical report
Geographic Code:6SOUT
Date:Jun 1, 2013
Words:1033
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