Mechanism of production of gait unsteadiness by tumours in the posterior fossa.

The hypothesis is put forward that the disturbance of gait seen with tumours of the posterior fossa is largely due to subacute dilatation of the ventricular system and not to the involvement of midline cerebellar structures concerned with balance or the coordination of truncal muscles, as is widely taught. This hypothesis is examined in the light of the clinical evidence provided by the symptomatology and treatment of 19 consecutive cases of intracranial mass lesions exhibiting truncal ataxia.