Teaching Case

An 18-months old female child presented with macrocephaly and developmental delay. Patient was only child and had no siblings. Patient had normal birth history and had no history of previous hospital admission. On clinical examination macrocephaly was evident with delayed developmental mile stones. Patient was sent for MRI.

Magnetic Resonance Imaging (MRI) showed altered signal intensity areas which appeared hypointense on T1-weighted image, hyperintense on T2-weighted image and showed restricted diffusion on diffusion weighted image (DWI). Involved areas were subcortical and periventricular white matter, bilateral thalamus, bilateral globus pallidus, splenium of corpus callosum and dorsal part of midbrain and pons with sparing of bilateral caudate and putamen nucleus, bilateral internal capsule, ventral part of crus cerebri of midbrain and ventral part of pons. MR spectroscopy showed markedly elevated NAA and NAA-creatine ratio, which is pathognomic for canavan disease.

Canavan disease is a rare autosomal recessive leukodystrophy, caused by genetic mutation involving short arm of 17th chromosome. Which leads to deficiency of N-acetyl aspartoacylase, an enzyme required for myelin synthesis [1, 2]. In general population prevalence is 1:100000 [3]. Patients present with macrocephaly and neurological deficit usually during infancy and may have normal birth history. Neurological deficit ranges from developmental delay, lethargy and hypotonia to spasticity, blindness, seizures, decerebrate posturing and eventual leading to death. Sometimes in early stage macrocephaly could be the only finding evident [4]. Diagnosis is based on detection of increased N-acetyl aspartic acid (NAA) in urine, CSF and brain. CT and MRI are the initial investigation of choice [3]. In early stage CT shows megalencephaly with decreased attenuation of subcortical white matter without abnormal contrast enhancement. In late stage CT shows signs of cortical atrophy in the form of dilatation of ventricles and prominent sulci and basal cisterns [2]. MRI provide more details of white matter involvement due to excellent gray white matter resolution. T1 weighted images show low signal intensity whereas T2 weighted and FLAIR images show high signal intensity within subcortical white matter, globus pallidus and thalamus usually sparing corpus callosum, caudate and putamen nucleus and internal capsule. As the disease progresses, periventricular white matter is also involved with resultant dilatation of ventricles. Diffusion-weighted images show restricted diffusion within involved white matter [2-8]. MR spectroscopy shows increased NAA and NAA-creatine ratio, which is pathognomic for canavan disease [3]. Canavan disease is fatal condition; death occurs around 5 years of age. No definite treatment is available for the disease [2]. Generic therapy is being tried [3]. Take home message: Imaging with MRI and MR spectroscopy individually or in conjunction with each other leads to diagnosis of canavan disease in almost all instances. Written informed patient consent for publication has been obtained.