Diplopia due to ocular motor cranial neuropathies.

PURPOSE OF REVIEW: Determining which cranial nerve(s) is (are) involved is a critical step in appropriately evaluating a patient with diplopia.

RECENT FINDINGS: New studies have looked at the various etiologies of cranial nerve palsies in the modern imaging era. The importance of the C-reactive protein test in evaluating the possibility of giant cell arteritis has recently been emphasized.

SUMMARY: Dysfunction of the oculomotor (third), trochlear (fourth), or abducens (sixth) cranial nerve will produce ocular misalignment and resultant binocular diplopia or binocular blur. A misalignment in the vertical plane of as small as 200 μm is enough to produce diplopia. Diagnosing diplopia from a cranial nerve abnormality requires an understanding of structure (the anatomy of the cranial nerves from nucleus to muscle), function (the movements controlled by the cranial nerves), possible etiologies, and exceptions to the rules.