Following test is most useful in diagnosing

The testing of evoked potentials is of the greatest utility in detecting subclinical spinal cord and optic nerve lesions. Up to two-thirds of persons with multiple sclerosis have neurologic deficits that are evident on visual or personal somatic evoked potentials but not on physical examination. A "second lesion" of this type frequently establishes the diagnosis of multiple sclerosis. Evoked potentials may be ab­normal in the other conditions listed in the question.