Cranial Nerve Pathways and Functions

Anatomy and Basic Function

The American College of Radiology notes that cranial nerves (CN) are 12 pairs that provide specialized sensory and motor innervation to the head and neck region 1.
Functionally, cranial nerves are divided into sensory (afferent) and motor (efferent) components, including sensory visceral, general, and special, as well as motor somatic, branchial, and visceral 2.
The cranial nerves emerge in an orderly fashion from the rostral portion of the developing neural tube, forming the brain and brainstem 3.

The American College of Radiology states that cranial neuropathy can result from pathologies affecting nerve fibers at any point from the CN nucleus to the final organ 1.
Key concepts for approaching cranial neuropathy include the complex and lengthy routes of cranial nerves, requiring detailed anatomical knowledge for clinical localization of potential lesions 2.
Individual nerve fibers, such as autonomic nerves, can travel with multiple cranial nerves, so loss of a specific function may indicate involvement of more than one cranial nerve 4.
Due to the proximity of many CN nuclei, segments, and exit sites, some lesions can result in multiple cranial neuropathies 5.

The American College of Radiology recommends magnetic resonance (MR) imaging as the standard modality for imaging cranial nerves, with 3.0T preferred over 1.5T for its better signal-to-noise ratio, gradient strength, and spatial resolution 1, 5.
Fundamental techniques include T1-weighted, T2-weighted, and contrast-enhanced T1-weighted sequences, with intravenous contrast imperative for evaluating cranial neuropathy with MR 5, 6.
A complete evaluation should include the entire course of the nerve from the brainstem nuclei to the "final organs" 6.

The American College of Radiology notes that CN VII (Facial) lesions can also be located in the parotid gland 6.
CN IX (Glossopharyngeal), X (Vagus), XI (Accessory), and XII (Hypoglossal) lesions can extend to the neck 6.

The diagnostic approach should include a detailed neurological evaluation to localize potential lesions, MR imaging with contrast to evaluate nerves from their nuclei to final organs, and analysis of cerebrospinal fluid (CSF) and serological studies in patients with multiple cranial neuropathies 1, 6.
Evaluation of otalgia may require assessment of CN V, VII, IX, X, and upper cervical nerves C2 and C3, as any of these nerves can be the source of pain 6.