American Journal of Neuroradiology 23:1424-1426, September 2002
© 2002 American Society of Neuroradiology
Case Report
PEDIATRICS
Metachromatic Leukodystrophy: Diffusion MR Imaging Findings
R. Nuri Senera
a Department of Radiology, Ege University Hospital, Bornova, Izmir, Turkey
Address reprint requests to R. Nuri Sener, MD, Professor of Radiology, Department of Radiology, Ege University Hospital, Bornova, Izmir, Turkey
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Abstract
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Summary: Herein the case of a 10-month-old boy with metachromatic
leukodystrophy is reported. Diffusion MR imaging performed with
an echo-planar trace sequence revealed a cytotoxic edema-like
pattern (high signal intensity on b = 1000 s/mm
2 images and
low apparent diffusion coefficient values) in the affected white
matter in the absence of an ischemic condition. This finding
was unchanged at a 6-month follow-up, as revealed by diffusion
MR imaging. A gradient-echo diffusion sequence, reverse fast
imaging in steady-state precession, revealed hyperintense changes
at the corresponding regions. It is likely that the cytotoxic
edematous pattern (restricted diffusion pattern) reflected restriction
of mobility of the water molecules within abnormal portions
of the myelin sheath, because impaired myelin breakdown and
reutilization are known features of metachromatic leukodystrophy.
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Introduction
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Metachromatic leukodystrophy is caused by a deficiency of arylsulfatase-A
resulting in impaired myelination (
1–
8). Diffusion MR
imaging features of the condition have not been reported previously.
We report diffusion MR imaging of metachromatic leukodystrophy
performed by using the echo-planar and gradient-echo sequences.
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Case Report
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The patient was a 10-month-old boy presenting with progressive
spasticity. An MR imaging examination was performed on a 1.5-T
MR unit (Magnetom Vision; Siemens, Erlangen, Germany) with a
gradient strength of 30mT/m. T2-weighted images revealed hyperintense
changes, mainly involving the deep white matter, and atrophy
(
Fig 1A). Diffusion MR imaging was performed by using the echo-planar
trace sequence (5700/139 [TR/TE]; acquisition time, 22 s) with
averaging of the three gradients. On b = 1000 mm
2/s images,
which were heavily diffusion-weighted images, hyperintense changes
were evident in the deep white matter and had a signal intensity
pattern similar to that of cytotoxic edema (
Fig 1B). On automatically
generated apparent diffusion coefficient (ADC) maps, the ADC
values obtained by region of interest evaluations were low at
the corresponding regions (eg, 0.55
x 10
-3 mm
2/s), compared
with the peripheral regions of the parenchyma (eg, 0.95
x 10
-3 mm
2/s) (
Fig 1C).
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FIG 1. Images obtained at initial examination, performed when the patient was 10 months old.
A, T2-weighted image reveals hyperintense changes in the deep white matter and atrophy.
B, A b = 1000 mm2/s (heavily diffusion-weighted) image from an echo-planar trace sequence reveals hyperintense changes similar to those of cytotoxic edema.
C, Corresponding ADC map reveals a low signal intensity and low ADC value (0.55 x 10-3 mm2/s) in the deep white matter compared with the peripheral regions of the parenchyma, including peripheral white matter and the cortex (0.95 x 10-3 mm2/s).
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A diagnosis of metachromatic leukodystrophy was confirmed by revealing decreased activity of arylsulfatase-A in leukocytes and increased urinary sulfatides. After 6 months, the patient was reexamined by diffusion MR imaging. The same echo-planar trace sequence was used, and hyperintense changes were again observed on b = 1000 mm2/s images (Fig 2A), as were low ADC values in the deep white matter. A gradient-echo diffusion sequence, the reverse fast imaging in steady-state precession sequence, was obtained at the same session. For the reverse fast imaging in steady-state precession sequence (21.6/5 [TR/TE]; acquisition time, 2 min 48 s), high pixel values were evident in the deep white matter, compared with the peripheral regions of the parenchyma (Fig 2B). (ADC value calculation is not possible with reverse fast imaging in steady-state precession.)
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FIG 2. Images obtained during follow-up, when the patient was 16 months old.
A, A b = 1000 mm2/s (heavily diffusion-weighted) image from echo-planar trace sequence reveals same hyperintense lesions.
B, Gradient-echo diffusion sequence (reverse fast imaging in steady-state precession) reveals high signal intensity and high pixel values (165) in the deep white matter, compared with the peripheral regions of the parenchyma (99).
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Discussion
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It has been reported that in cases of metachromatic leukodystrophy,
the decreased activity of arylsulfatase-A accounts for failure
of myelin breakdown and reutilization: hence, dysmyelination
(
1–
8). On the diffusion MR images in the current case
of metachromatic leukodystrophy, a cytotoxic, edematous pattern
(restricted diffusion pattern) was evident in the deep white
matter in the absence of an ischemic condition. The distribution
of the lesions (
Fig 1B) and their unchanging pattern on 6-month
follow-up diffusion MR images (
Fig 2A) strongly suggested that
the lesions were directly related to the disease process. Restricted
diffusion was manifested by hyperintense changes on heavily
diffusion-weighted (b = 1000 s/mm
2) images (
Fig 1B) and low
ADC values (eg, 0.55
x10
-3 mm
2/s) on ADC maps (
Fig 1C) of the
echo-planar trace sequence. In addition, on the gradient-echo
diffusion-weighted images, obtained using reverse fast imaging
in steady-state precession, high signal intensity and high pixel
values were evident in the deep white matter (
Fig 2B) and were
similar to those seen on b = 1000 s/mm
2 images, which again
indicated definite abnormality in these regions. However, absolute
ADC value calculation is currently not possible with the reverse
fast imaging in steady-state precession sequence.
A similar diffusion MR imaging pattern on echo-planar diffusion images of patients with phenylketonuria was recently reported by Phillips et al (9). In that study, patients with phenylketonuria had highly restricted diffusion of water molecules with high signal intensity of the white matter on b = 1000 s/mm2 images in association with low ADC values on ADC maps (between 0.56 and 0.63 x 10-3 mm2/s (9). The authors concluded that this restricted diffusion pattern in cases of phenylketonuria reflected impaired myelination, and protons within partially that destroyed portions of the myelin sheath may not be as mobile as those in normal parenchyma (9). On the other hand, in a recent experiment, Branco (10) showed that transition of water from the solid to the gel state contributes to the signal intensity on diffusion MR images, and in the gel state, high signal intensity is observed on b = 1000 s/mm2 images (this should result in low ADC values). Considering the observations by Phillips et al (9) and the experiment by Branco (10), in the current case of metachromatic leukodystrophy, the restricted diffusion pattern probably was consistent with some disintegration of the white matter secondary to impaired myelination, and restriction of mobility of the water molecules within abnormal portions of myelin sheath resulted in a diffusion MR imaging pattern similar to that of cytotoxic edema.
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Conclusion
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In a case of metachromatic leukodystrophy, an echo-planar trace
sequence revealed a cytotoxic edematous pattern (restricted
diffusion pattern) in the deep white matter in the absence of
an ischemic condition. This pattern, manifested by high signal
intensity on b = 1000 s/mm
2 images and low ADC values, probably
reflected restriction of mobility of the water molecules within
abnormal portions of the myelin sheath, considering that impaired
myelin breakdown and reutilization are known features of metachromatic
leukodystrophy. Hence, dysmyelination is a feature of metachromatic
leukodystrophy. A gradient-echo diffusion sequence, reverse
fast imaging in steady-state precession, revealed hyperintense
changes at the corresponding regions similar to those seen on
b = 1000 s/mm
2 images. Further diffusion MR imaging studies
of dysmyelinating disorders could contribute to our understanding
of the imaging properties of these diseases.
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Received December 5, 2001;
accepted after revision May 1, 2002.
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Abstract
Introduction