The purpose of the examination of the cranial nerves is to localize neurologic disease by looking for evidence of cranial nerve dysfunction that can occur due to disorders of the brain, brainstem, or cranial nerves themselves.

Assessment of the most diagnostically relevant cranial nerve functions should be performed on all patients as part of any standard neurologic examination. The examination elements that should be checked routinely, the elements that usually can be skipped, and the elements that need only be performed when specific clinical questions are being asked are explained in subsequent chapters of this section and summarized in Chapter 40, Performing a Complete Neurologic Examination.

All of the cranial nerves, with the exception of the first (olfactory) and second (optic) nerves, have nuclei within the brainstem and exit the brainstem to innervate their motor or sensory structures (or both) in the head. The first and second nerves are different than the others in that they are best thought of as extensions of the brain; the optic nerves actually are central nervous system structures.

The specifics of examining cranial nerve function are described in subsequent chapters of this section. These chapters are not categorized by individual cranial nerves; instead, each chapter describes a functional examination
element. For example, there is no chapter labeled “Examination of the Third Cranial Nerve.” Instead, the assessment of function of the ophthalmic nerve is imbedded within Chapter 14, Examination of Eye Movements, and Chapter 10, Examination of the Pupils, because these are the clinical tests that provide information about the function of this nerve.