Cranial nerves are involved in multiple sensory functions (such as seeing, hearing, and tasting) and motor functions (such as the control of facial muscles). There are twelve pairs of cranial nerves in total. The nerves are paired in order to serve both the left and right sides of the body.
Each cranial nerve is referred to with both a name and a Roman numeral – for example, the fifth cranial nerve is known as the Trigeminal nerve (V).
Disorders in the cranial nerves
Disorders in the cranial nerves are often caused by another underlying medical condition. Some examples of conditions that might affect cranial nerves include diabetes mellitus, multiple sclerosis, sarcoidosis (an inflammatory condition), vasculitis, systemic lupus erythematosus (Lupus), meningitis, and syphilis. A cranial nerve disorder may cause the disruption of a single cranial nerve, or it may affect the connection between cranial nerve centers within the brain.
Examples of the former include oculomotor palsy, trigeminal neuralgia, Bell palsy, and hemifacial spasm; an example of the latter is internuclear ophthalmoplegia.
As each nerve is responsible for a different function, symptoms of cranial nerve disorders vary depending on which nerves are damaged. The damage can be on just one side of the body, or on both sides. Symptoms can vary from decreased visual acuity and lack of taste to decreased muscle contractions. The affected cranial nerve can be identified based on the types of symptoms displayed by the patient.
For example, impaired taste and smell would suggest a lesion in the olfactory nerve. The location of the lesion can also be determined by the cranial nerve affected.
If multiple nerves are affected, they are likely to be next to each other or have complementary functions. For example, the cranial nerves III, IV, and VI all communicate with the eye, and sometimes lesions are found on all three of these nerves.
Each physician uses their own protocols to test for cranial nerve lesions. Systematic testing of motor and sensory functions of the cranial nerves will reveal where the damage might be. Some of the most common techniques include vision tests, taste tests, and manual palpation of muscles. Because many of these tests are for sensory functions, the only information available to the doctor is what the patient can tell them.
Therefore, it is important for patients to describe symptoms and sensations as accurately as possible, as well as report any changes. A common clinical finding is a decrease in sensation in a specific area. Tests of motor functions are somewhat easier for patients as the doctor does not require them to interpret the information.