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primary cerebellar agenesis is rare as only eleven living cases have been reported till date to the best of our knowledge .
our case is unique in that the child presented with isolated cognitive impairment with no motor impairment .
mild to profound motor impairment , mental and behavioral abnormalities have been described in these patients .
this case is among the few cases which shows that it is possible to have normal motor function even in the absence of the cerebellum and that the cerebellum has an important role to play in nonmotor functions .
a 5-year - old male child was brought to the pediatric outpatient department on account of parental concern for impaired academic performance at school .
he was referred for magnetic resonance imaging ( mri ) study of the brain in view of learning disability and high degree of parental anxiety .
t1-weighted images in the axial and sagittal plane , t2-weighted images in the axial and coronal plane , and axial fluid - attenuated inversion recovery images were obtained .
images revealed a normal - sized posterior fossa which was empty and showed cerebrospinal fluid ( csf ) signal intensity on all sequences [ figures 1 and 2 ] .
the brainstem ( pons and medulla ) was hypoplastic as seen on sagittal t1-weighted images [ figure 3 ] .
small remnants of the cerebellum were seen anteriorly adjacent to the pons and likely represented residua of the anterior quadrangular lobes [ figures 1 and 2 ] .
axial t1-weighted magnetic resonance image shows an empty , normal sized posterior fossa ( large white arrow ) filled with hypointense cerebrospinal fluid .
remnants of the quadrangular lobes are seen anteriorly ( small white arrows ) axial t2-weighted magnetic resonance image shows an empty , normal sized posterior fossa ( large white arrow ) filled with hyperintense cerebrospinal fluid .
remnants of the quadrangular lobes are seen anteriorly ( small white arrows ) sagittal t1-weighted magnetic resonance image shows hypoplastic brainstem ( white arrow ) with empty posterior fossa ( black arrow )
sometimes , small remnants of the cerebellum may be present on account of which the term subtotal cerebellar agenesis has also been used in the literature .
the first case of primary cerebellar agenesis was described by combettes in 1831 . to the best of our knowledge
this may point toward neuronal plasticity which allows the supratentorial brain to compensate for cerebellar function .
a case of cerebellar agenesis has been reported to show improvement in motor function over the years , which lends support to the proposed theory that the remaining brain may compensate for cerebellar function over a period of time . moreover , small remnants of the palaeocerebellum may be present , which could compensate for the motor function .
a review of imaging in a case of cerebellar agenesis reported earlier in a man who died in 1951 without any motor impairment revealed a small residual cerebellum .
remnants of cerebellar tissue were also seen in a case of cerebellar agenesis in a 22-year - old male by velioglu et al .
our patient had remnants of the anterior quadrangular lobes which may account for the lack of motor manifestations .
recent studies have revealed the role of cerebellum in important nonmotor functions including attention , language , working memory , and learning , which explains cognitive and behavioral abnormalities in cerebellar agenesis .
the pons and medulla are hypoplastic due to the lack of afferent input from the cerebellum .
primary cerebellar agenesis needs to be differentiated from secondary degeneration of the cerebellum ( vanishing cerebellum ) in chiari ii malformation .
chiari ii is characterized by a small posterior fossa unlike the normal sized fossa in primary cerebellar agenesis .
the other differential diagnosis for primary cerebellar agenesis is dandy walker syndrome and its variants . in that case
, the posterior fossa is enlarged with superior displacement of the tentorium , straight sinus , and torcular herophili .
primary cerebellar agenesis is a rare entity . to the best of our knowledge 
 , eleven living cases have been reported till date . 
 most of these were associated with some degree of motor impairment . 
 we present a case of cerebellar agenesis in a child who presented with cognitive abnormalities leading to poor performance at school . 
 no motor impairment was seen . among the eleven cases reported earlier , only one case showed lack of motor impairment .