Evaluate for underlying cause based on clinical presentation
The use of optokinetic tape is highly sensitive for testing for<unk> 
MRI
Internuclear ophthalmoplegia
Multiple sclerosis 
Nystagmus
Stroke
Ocular palsy
Malignancy located in brainstem and 4th ventricle
Facial paralysis
CN III palsy would show additional deficit including weakness of elevation, ptosis, pupil dilation