The fourth cranial nerve is the only cranial nerve which arises from the opposite side of the brain. It supplies the superior oblique muscle with innervation. As it progresses through the brain it crosses in the roof of the fourth ventricle and exits on the other side. The function of the fourth nerve is to turn the eye down and in towards the nose as well as intorts the eye. Many fourth nerve palsies are incomplete and as a result the patient tries to compensate by positioning their head. In order to compensate for a deficiency of ocular depression the patient lowers his head. In order to compensate for decreased intorsion the head is tilted to the opposite side and puts the effected eye in extorsion. In order to compensate for decreased adduction the face is turned to the opposite side so the eye moves more into abduction. These three parts of head positioning are applicable to any of the cyclovertical muscles. On rare occasions the patient positions his head opposite to the above rules. This separates the images all the more. If they are very close initially it is hard for the patient to ignore one of the images. He therefore positions his head to make the fourth nerve more ineffective and separate the images more so one of the images can be ignored. Sometimes the head position is subtle and can be missed during your examination producing an inaccurate assessment of the ocular motility. As mentioned above, the vertical function of a cyclovertical muscle is in one particular position for each cyclovertical muscle. In the case of the fourth nerve it is when the eye is adducted towards the nose. If the patient has total third nerve palsy the eye is positioned down and out. Therefore, the vertical component of the fourth nerve cannot be tested to ascertain if it is functioning. However, if you ask the patient to move the unaffected eye down and out quickly, the affected eye will intort and confirm that the fourth nerve is functioning. Isolated four nerve palsies are almost impossible to diagnose from radiologic studies. The cranial nerve company they keep best identifies them. The fourth nerve passes between the posterior cerebral and superior cerebellar artery with the third nerve. Long tract signs usually accompany this. If the fourth nerve is affected in the cavernous sinus then some or all of these nerves (3,5,6,and sympathetic) are affected. An unusual combination is a fourth nerve on one side and a Horner's syndrome on the other side. The initial impression is that this is two different lesions. However, it does not have to be. This fourth nerve can be affected on the same side as the Horner's before it crosses over in the roof of the fourth ventricle. This position in the roof of the fourth ventricle is a common place for bilateral fourth nerve palsies secondary to hypertension. A rare presentation is myokymia of the fourth nerve. The eye constantly or intermittently intorts causing rotary diplopia. There is no particular work up and although it is very annoying there is no known cause or very effective treatment. An alternative cause of this can be due to severe blepharospasm that moves the eye and is not a primary abnormality of the fourth nerve. Myasthenia also needs to be considered when the cause is not obvious but is very rare in the absence of a ptosis. An isolated fourth nerve palsy is rarely caused by multiple sclerosis. Treatment after a reasonable waiting period is either prisms or surgery or a combination of both. Thomas J. Walsh, M.D. Editor's Note: This excellent review of fourth nerve palsy represents the experience of an expert neuro-ophthalmologist. It also reflects the clinical experience typical to this area of ophthalmology. In contrast, the fourth nerve palsy patients who present to the strabismologist represent a slightly different clinical picture. This is described in the Strabismus Minute.