Also called CN IV or trochlear nerve palsy
Trochlea innervates superior oblique muscle
Intorts, depresses and abducts the globe
Trochlear nerve palsy causes an inability to move the eye in inward rotation, downward, and laterally. 
Most common cause of vertical diplopia 
Intorts, depresses and abducts the globe
Head trauma 
Mechanisms that increase intraocular pressure
Practically never caused by aneurysm
Herpes zoster ophthalmicus
Meningitis
Diabetic neuropathy
Lupus/Sjögren syndrome
Practically never caused by aneurysm
Fourth nerve palsy
Increased intracranial pressure 
Third nerve palsy
Vascular:  Stroke, subarachnoid hemorrhage, aneurysm 
Sixth nerve palsy
Neoplastic: Tumor in the subarachnoid space
Ulnar neuropathy
Degenerative/deficiency: Vitamin B12 deficiency, Wernicke-Korsakoff syndrome