A 50-year-old male patient, previously undiagnosed HIV infection, presenting with a 3-month history of progressive gait and swallowing difficulty. Mild nystagmus on examination. No motor weakness or sensory/autonomic complaints.
Magnetic resonance imaging (MRI) brain showed symmetrical bilateral cerebellar atrophy with T2 / FLAIR hyperintensities in bilateral cerebellar hemispheres (Fig. 1), in bilateral middle cerebellar peduncles (Fig. 2), entire brainstem predominantly in pons with peripheral dominance R>L (Fig. 3). No supratentorial hyperintensity or diffusion restriction or Gradient Recalled Echo (GRE) blooming was seen (Fig. 4).
John Cunningham Virus Granule Cell Neuronopathy (JC-GCN) is one of the CNS manifestations caused by the reactivation of JC VIRUS in immunocompromised patients. Other more common manifestations being Progressive multifocal Leukoencephalopathy, encephalitis, and meningitis. 
JC-virus is usually acquired in childhood through the respiratory or faecal-oral route and remains latent in the kidney, bone marrow, and spleen. In immunocompromised states, the virus spreads to CNS and infects, lyse the glial cells. However, a mutation in the VP1 capsid gene of the virus makes it more tropic to granule cells thereby selectively lysing the granule cell in the cerebellum and spares the glial cells. [2-5]
Hence it predominantly shows infratentorial lesions in such cases and causes isolated cerebellar symptoms such as ataxia and nystagmus.
It is seen in immunocompromised states such as Human immunodeficiency virus (HIV), lymphoproliferative diseases, with the use of immunosuppressive drugs (natalizumab, rituximab), and rarely in sarcoidosis.
MRI is the imaging modality of choice that characteristically shows bilateral cerebellar atrophy and confluent T2/ FLAIR hyperintensities due to neuronal loss. marked involvement of bilateral middle cerebellar peduncles was also seen. The brainstem, particularly pons is also involved and shows similar hyperintensities. No contrast enhancement or diffusion restriction or GRE blooming is seen in the lesion. Basal ganglia and thalamus are spared. 
Close differential diagnosis includes Multisystem Atrophy (MSA-C) and Spinocerebellar ataxia which typically involves pontocerebellar tracts leading to cross-shaped hyperintensities -Hot cross bun(HCB) sign. However, this case shows patchy involvement of pons without any brainstem atrophy or HCB sign.
Currently, no disease-specific treatment is available, so the treatment is mainly based on the quick reversibility of underlying immunosuppressive conditions. Anti- Retroviral Treatment (ART) initiation, discontinuation of immunosuppressive agents is the main cornerstone of the treatment.
Cerebro-Spinal Fluid (CSF) Polymerase Chain Reaction (PCR) for JC VIRUS was advised however the patient refused. So in the context of HIV, clinical, and MRI findings, the radiological possibility of JC-GCN appears most likely.
in immunocompromised patients presenting with isolated cerebellar symptoms and predominant infratentorial affection on MRI, GCN should be strongly considered. With the increased use of monoclonal antibodies, clinicians and radiologists should be well versed with the features and findings of JC-GCN.
‘Written informed patient consent for publication has been obtained.’
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