it is caused by a deficiency of both hexosaminidase ( hex ) a and b , resulting in accumulation of glycosphingolipids and oligosaccharides in the brain . it has three clinical subtypes ( infantile , juvenile , and adult forms ) and represents around 7% of cases among all the lysosomal storage disorders . the infantile form presents in the first 618 months of age with regression of milestones , developmental delay , startle response , hypotonia , cherry red spots , and convulsions . we report this case as the infant presented with regression without hepatosplenomegaly and confirmed by gene testing . a 1-year - old second born male child , born to a third - degree consanguineously married couple with uneventful perinatal history , was brought with regression of milestones and seizures . the onset of clinical symptoms began at the age of 6 months with gradual loss of the milestones . the child had achieved response to sounds by 2 months ; head holding , social smile , and recognition of mother by 3 months of age . initially , he lost social smile and recognition of mother followed by control of neck . he lost all the milestones by 89 months of age . the child developed exacerbated startle response since 8 months of age and multiple episodes of right - sided focal seizures from 11 months of age . there was a history of sibling death at the age of 18 months with similar complaints . on examination , head circumference was 44 cm ( between 3 and 15 centile ) with coarse facies . the tone was increased in all the limbs with power of 3/5 ( medical research council grade ) . fundus examination showed cherry - red spot in the macula , and there was no hepatosplenomegaly . magnetic resonance imaging ( mri ) shows hyperintensity of the basal ganglia ( long white arrow ) and hypointensity of the ventral thalami ( open arrow ) on t2-weighted sequences [ figure 1 ] . based on above findings suspected to be a case of sandhoff disease , the enzyme assay in leukocytes for  hexosaminidase total ( a + b ) revealed 61 nmhol / h / mg ( normal , 9052878 ) . gene testing was positive for homogenous missense substitution p. cys534tyr , c1601g > a , chr5:74016560g > a in exon 13 of hexb gene . axial t2-weighted imaging showing hyperintensity of the basal ganglia ( long white arrow ) and hypointensity of the ventral thalami ( open arrow ) our patient had presented with regression of milestones , exaggerated startle response , decreased vision , and seizures . the child had coarse facies , cherry red spot , and normocephaly without hepatosplenomegaly . in a study conducted among 18 gm2 gangliosidosis patients by karimzadeh et al . , seven patients had macrocephaly , three patients had microcephaly , and eight patients had a normal head circumference . a distinguishing feature in standoff 's disease from other gm2 gangliosidosis is presence of hepatosplenomegaly and n - acetylglucosamine - containing oligosaccharides in urine likewise , karimzadeh et al . also noted , and only two patients of sandhoff disease out of their nine cases had hepatosplenomegaly . similar observation was made by ozkara et al . in their study of 18 cases affected by sandhoff disease , and hepatosplenomegaly was not found in 11 out of 18 infantile sandhoff disease patients , while the remaining seven had mild hepatosplenomegaly . closest differential diagnosis of sandhoff disease is tay  sachs disease , which is characterized by the absence of coarse facies , hepatosplenomegaly , skeletal deformities , and signs of peripheral nerve involvement . axial t2-weighted imaging on mri in our patient revealed hyperintensity of the basal ganglia and hypointensity of the ventral thalami . also reported as bilateral thalamic hyperdensity on computed tomography and hypointensity on t2-weighted mri images as the earliest diagnostic markers of sandhoff disease . the gold standard method for diagnosis of gm2 gangliosidosis is the measurement of -hex activity in plasma , serum , and/or fibroblasts . molecular characterization of the hexa and hexb mutations can also be performed for confirmation by direct sequencing of the entire coding region and intron / exon boundaries using genomic dna . the same mutation was reported previously in a japanese case which showed hepatosplenomegaly unlike our case . prenatal diagnosis advised for next pregnancy . sachs disease if any child presenting with regression of milestones and cherry red spot without hepatosplenomegaly .