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PERIVENTRICULAR LEUCOMALACIA (PVL) AS SEEN ON CT CAUSED DUE TO HYPOXIC ISCHAEMIC ENCEPHALOPATHY OR PYOGENIC MENINGITIS
ANAGHA JOSHI*, VAISHALI NIMBKAR**, MALINI SOMASHEKHAR***, SULEMAN MERCHANT****
*Associate Professor; **Resident; ***Lecturer; ****Professor and Head at Department of Radiology, LTMGH, Mumbai.
INTRODUCTION
PVL is softening of white matter around the ventricles. It was initially thought to be as a result of ischaemic insult. However postnatal meningitis may also be one of the mechanisms of damage as is seen in this case.
CASE REPORT
A four year old mentally retarded child, with cerebral palsy, was brought for computed tomography (CT) scan for investigation of generalized tonic-clonic convulsions which he was suffering from for the last three years.
He was a seven months premature baby, with delayed milestones, spastic diplegia and squint. He had an abnormal gait, posture and movements. A noteworthy past history was pyogenic meningitis at the age of six months. On clinical examination he was found to have a normal cranial and sensory examination but had decreased power and increased tone.
An axial plain and contrast enhanced CT scan of the brain was performed on a multislice CT scanner with 5 mm slice thickness and a collimation of 5 mm. The study revealed multiple non-enhancing hypodensities in the frontal and occipital periventricular white matter with a decrease in the volume of periventricular white matter (Figs. 1, 2 and 3). The body of the lateral ventricle was mildly dilated with irregularity of the margins noted on the right side. These CT features of multiple non-enhancing white matter hyodensities in the periventricular region along with reduction in white matter volume was indicative of periventricular leucomalacia.
in the periventricular region.
periventricular white matter. Decrease in volume of periventricular white matter .
white matter. Decrease in volume of periventricular white matter.
DISCUSSION
PVL is softening of the white matter around the ventricles and results from extensive necrosis of white matter adjacent to the lateral ventricles in the brain. The determining factor for different patterns of perinatal ischaemia in central nervous system (CNS) is the time of the anoxic insult. The deep periventricular white matter is thought to be at the highest risk for ischaemic insult before maturation of centrifugal arteries, since vascular supply to this area is tenuous, solely dependent on the medullary arteries arising from cortical surface. The periventricular white matter is particularly vulnerable in the perinatal period when metabolic activity and oxygen requirement is greatest. Impaired haemostasis results in decompensation and ischaemic damage and/or infarction. With progressive maturity, watershed areas move to the cortical and the subcortical zone of brain. Hence the term infant is more susceptible to cortical/subcortical ischaemic damage, especially in the watershed areas between the anterior, middle and posterior cerebral arteries. Hence an injury during the critical period of 26-34 weeks results in leucoencephalopathy in the form of PVL and encephalomalacia.
Significant risk factors are premature rupture of membranes, chorioamnionitis, hyperbilirubinaemia.[1] In case of preterm infants treated with steroids like dexamethasone, a two to three fold increase in the incidence of PVL was seen as compared to controls.[2]
Pathologically, most severely involved areas in PVL include the occipital radiation, the trigone of the lateral ventricle and the frontal periventricular white matter. In the acute stage, the white matter undergoes congestion and coagulative necrosis, which progresses to cavitation and cyst formation. End stage PVL is characterized by resolution of cysts, periventricular gliosis, demyelination and loss of the periventricular white matter. Haemorrhagic PVL is a variant of PVL that represents haemorrhagic white matter infarction.
The periventricular white matter includes the pyramidal tracts and the optic radiation. Hence the neurological consequences are as follows:
1. Motor symptoms - Spastic cerebral palsy, hemi, di or tetraplegia - 77% incidence.
2. Associated visual disorders like decreased visual activity and transient infantile upgaze holding insufficiency, which however disappears spontaneously by three years.
3. Cognitive functions - There is evidence that severe bilateral white matter reduction in PVL is related to mental retardation, while unilateral reduction can be compensated. Mild cognitive dysfunctioning has not yet shown to be associated with periventricular brain lesions. However, there is a relationship between cerebellar lesions and the cognitive outcome in infants.[3]
Imaging modalities
Ultrasonography (USG), magnetic resonance imaging (MRI) and CT are the imaging modalities used to diagnose PVL.
On USG, PVL is recognized as heterogeneously ill-defined areas of increased reflectivity in the periventricular white matter. Small cystic foci develop over the following 2-7 days. If associated with haemorrhage, large cystic cavities evolve which eventually break down and coalesce with the adjacent ventricle. The lateral ventricle develops an irregular contour and may be enlarged due to loss of the adjacent brain substance. With moderate to severe involvement the damage is readily detected. However, USG is less sensitive in detecting minor degree of changes.
On CT, PVL manifests as bilateral but often asymmetric hypodensities in the periventricular white matter. CT is unable to identify small cysts in the acute stage but is useful to show ventriculomegaly with an irregular outline of the body and the trigone of the lateral ventricle, a decreased quality of periventricular white matter and a deep prominent sulci that abuts the lateral ventricle wall. Cysts located bilaterally or in the parieto-occipital region are associated with an increased risk of cerebral palsy. Solely anteriorly located cysts are associated with a normal outcome. An increased size of the cyst is associated with an increased risk of cerebral palsy. Solely anteriorly located cysts are associated with a normal outcome. An increased size of the cyst is associated with an increased risk of cerebral palsy with a cut off value of 1 cm; all infants with cysts more than 2 centimeters are likely to have cerebral palsy.
MRI in the acute phase shows pathological foci of T1 and T2 shortening in the white matter. The end stage demonstrates atrophy of the corpus callosum and the thalamus, loss of periatrial white matter, cerebral and cortical atrophy, decreased volume of the brainstem and porencephalic cysts.
Hence, USG is more informative in the acute phase, CT and MRI being more specific in the subacute and end phase. MRI is superior in detecting delayed myelination of the white matter.[4] A good correlation is seen between the severity of PVL and the severity of neurological disturbances on MRI. The sensitivity and specificity of MRI is very high in predicting neurological outcomes namely developmental delay, cerebral palsy and visual impairment. Group B streptococcal meningitis in the neonatal period causes white matter abnormalities in infants. The mechanism of white matter damage is probably mediated through cytokines. This however challenges the view that PVL is caused only as a result of ischaemic insult to preterm infants.[5]
Hence in this case, it is now difficult to say whether the periventricular white matter lesions were caused due to prenatal hypoxic-ischaemic encephalopathy or due to post-natal pyogenic meningitis.
REFERENCES
1.Resch B, Vollaard E, Maurer U, Haas J, Rosegger H, Muller W. Risk factors and determinants of neurodevelopmental outcome in cystic periventricular leucomalacia. Eur J Pediatr 2000; 159 (9) : 663-70.
2.Halliday H, Ehrenkranz RA. Early post natal (< 96 hours) corticosteroids for preventing chronic lung disease in preterm infants (Cochrane Review), The Cochrane Library, Oxford: Update Software. 2001; 4.
3.Kraegeloh-mann I, Tuebingen D. Consequences and compensation of periventricular brain lesions, 4th Congress of the EPNS, Baden-Baden Germany: IL6. 2001; 12-16.
4.Papadaki-Papandreou O, Hadjigeorgi C, Ioannides C, Manopoulou E, Athens GR. Periventricular leukomalacia in premature infants. Usefulness and limitations of ultrasonography, computed tomography and magnetic resonance imaging, 4th Congress of the EPNS, Baden-Baden Germany P34. 2001; 12-16.
5.Mclellan A, O'regan M, Gibson R, Brown JK. (Dundee, Glasgow, Edinburgh, UK), Neuro-imaging findings following neonatal group B streptococcal meningitis æ further evidence of periventricular leukomalacia occurring post-term, 4th Congress of the EPNS, Baden-Baden Germany:p26. 2001; 12-16.
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