Cranial nerve assessment is basically an assessment of brainstem function because nuclei of 10 of the 12 cranial nerves are located in the brainstem. The proximity of these nuclei to the reticular activating system (arousal center) located in the midbrain is the anatomic rationale for assessing cranial nerves in conjunction with LOC. Important neurological functions and protective reflexes are mediated by the cranial nerves and many functions are dependent on more than one nerve. Some of the cranial nerves have both motor and sensory functions.
The two cranial nerves that do not arise in the brainstem are the olfactory nerve (CN I) and the optic nerve (CN II). CN I is located in the medial frontal lobe and is responsible for the sense of smell. This can be difficult to assess in the younger child, so is often omitted unless there is specific concern that there has been damage in that area. Taste may also be affected with injuries to CN I. CN II is assessed by determining a child’s visual acuity. This may be done more formally with visual screening or more generally by noting if the child’s vision appears normal in routine activities.
Pupil size and response to direct light are mediated by CN II and the oculomotor nerve (CN III) as well as the sympathetic nervous system. Many things can affect the pupillary response in a child, including damage to the eye or the cranial nerves, pressure on the upper brainstem, local and systemic effects of certain drugs, anoxia, and seizures. Pupillary size varies with age and is determined by the amount of sympathetic input, which dilates the pupil and is balanced by the parasympathetic input on CN III, which constricts the pupil. Pupillary response in the eye that is being checked with direct light as well as the other pupil (consensual response) are significant in that they can point to where damage to nerves exists and are an objective clinical sign that can be followed over time .
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| Diagram of the base of the brain showing entrance and exits of the cranial nerves |
The two cranial nerves that do not arise in the brainstem are the olfactory nerve (CN I) and the optic nerve (CN II). CN I is located in the medial frontal lobe and is responsible for the sense of smell. This can be difficult to assess in the younger child, so is often omitted unless there is specific concern that there has been damage in that area. Taste may also be affected with injuries to CN I. CN II is assessed by determining a child’s visual acuity. This may be done more formally with visual screening or more generally by noting if the child’s vision appears normal in routine activities.
Pupil size and response to direct light are mediated by CN II and the oculomotor nerve (CN III) as well as the sympathetic nervous system. Many things can affect the pupillary response in a child, including damage to the eye or the cranial nerves, pressure on the upper brainstem, local and systemic effects of certain drugs, anoxia, and seizures. Pupillary size varies with age and is determined by the amount of sympathetic input, which dilates the pupil and is balanced by the parasympathetic input on CN III, which constricts the pupil. Pupillary response in the eye that is being checked with direct light as well as the other pupil (consensual response) are significant in that they can point to where damage to nerves exists and are an objective clinical sign that can be followed over time .
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