1 Introduction to Cost-Effective Evaluation of Cranial Neuropathy

10.1055/b-0040-174400

1 Introduction to Cost-Effective Evaluation of Cranial Neuropathy

Seilesh C. Babu and Neal M. Jackson

1.1 Introduction

Cranial neuropathy may be an indication of a devastating, life-threatening condition or it may be a simple benign disease for which spontaneous, complete recovery is expected. In the evaluation of cranial neuropathy, the clinician must make decisions in order to determine the significance and permanency of the diagnosis. While there are numerous testing options available, cost containment and necessity of these tests need to be taken into account. Because of this, the authors of this book recruited a team of experts from various disciplines to provide their expert interpretations of available literature combined with their informed opinions to discuss a cost-effective evaluation and management of cranial neuropathy.

1.2 Cranial Nerves

A cranial nerve is defined as a nerve that leaves the brain and innervates an organ, muscle, gland, or sensory receptor. There are 12 paired cranial nerves which are presented in ▶Table 1.1.

**Table 1.1** Names and functions of cranial nerves
Nerve number	Name	Fiber types	Function
I	Olfactory	Purely sensory	Transmits smell sense from the nasal cavity
II	Optic	Sensory	Transmits vision information from the retina to the brain
III	Oculomotor	Mainly motor	Innervates most eye muscles (levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique) and muscles of the ciliary body and sphincter pupillae
IV	Trochlear	Motor	Innervates the superior oblique muscle, which depresses, rotates laterally, and intorts the eyeball
V	Trigeminal	Both sensory and motor	Receives sensation from the face and innervates the muscles of mastication
VI	Abducens	Mainly motor	Innervates the lateral rectus muscle, which abducts the eye
VII	Facial	Both sensory and motor	Provides motor innervation to the muscles of facial expression, posterior belly of the digastric muscle, stylohyoid muscle, and stapedius muscle. Also receives the special sense of taste from the anterior two-thirds of the tongue and carries secretomotor fibers to most salivary glands (not the parotid) and the lacrimal gland
VIII	Vestibulocochlear	Mostly sensory	Transmits sensation of sound and head movement
IX	Glossopharyngeal	Both sensory and motor	Carries taste sensation from the posterior one-third of the tongue, provides secretomotor innervation to the parotid gland, provides motor innervation to the stylopharyngeus, and contributes to pharyngeal plexus
X	Vagus	Both sensory and motor	Supplies sensory and motor innervation to most laryngeal and pharyngeal muscles and provides parasympathetic fibers to nearly all thoracic and abdominal viscera down to the splenic flexure
XI	Accessory	Mainly motor	Innervates the sternocleidomastoid and trapezius muscles
XII	Hypoglossal	Mainly motor	Provides motor innervation to most of the muscles of the tongue

Cranial nerve functions are vital to human existence, from airway protection to communication and sensing the environment. All of our life functions occur via the cranial nerves. Actions such as tasting, smelling, swallowing, speaking, visualizing, and hearing are critical to our survival as well as enjoyment.

Although the 12 pairs of cranial nerves are grouped as such, they are heterogenous in their nerve fiber types. Some cranial nerves are purely motor (e.g., CN IV, CN VI, and CN XII) and some are purely sensory (e.g., CN II, and CN VIII). Many nerves perform mixed functions (e.g., CN III carries motor efferents and parasympathetics, while CN V has sensory and motor branches).

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