The initial diagnostic assessment for brain cancers is based on family history, symptoms and physical examination. After acquiring this information, a magnetic resonance imaging scan (MRI) of the brain and spine is usually carried out. A CT scan of the brain is insufficient in diagnosing gliomatosis. The MRI offers anatomical and metabolic information on the nervous system, using various sequences acquired before and after injecting contrast and further sequences gained using methods such as diffusion MRI and magnetic resonance spectroscopy (MRS). The MRI displays the characteristic infiltration pattern of the disease. In some cases, it is necessary to study the tumour using PET (positron emission tomography).

However, definitive diagnosis is based on examination of the lesion, obtained through biopsy, under the microscope. Under the microscope, malignant cells can be observed, distributed amongst the brain tissue.

Diferential diagnosis with other diseases

The neurological symptoms of gliomatosis are also seen in other types of brain tumours, as well as some non-tumour diseases of the nervous system, such as acute disseminated encephalomyelitis (ADEM), and demyelinating diseases. ADEM, in particular, can cause an inflammation of the cerebral lobes, visible in the MRI, which can cause confusion. Therefore, gliomatosis cerebri must appear in its differential diagnosis. Some children are incorrectly diagnosed with ADEM, including Izas.