Neuromuscular ultrasound of cranial nerves

Abstract

Ultrasound of cranial nerves is a novel subdomain of neuromuscular ultrasound (NMUS) which may provide additional value in the assessment of cranial nerves in different neuromuscular disorders. Whilst NMUS of peripheral nerves has been studied, NMUS of cranial nerves is considered in its initial stage of research, thus, there is a need to summarize the research results achieved to date. Detailed scanning protocols, which assist in mastery of the techniques, are briefly mentioned in the few reference textbooks available in the field. This review article focuses on ultrasound scanning techniques of the 4 accessible cranial nerves: optic, facial, vagus and spinal accessory nerves. The relevant literatures and potential future applications are discussed.

Keywords: cranial nerve; facial; neuromuscular ultrasound; optic; spinal accessory; vagus.

Fig. 1. A: Trans-orbital technique to obtain axial image of optic nerve. The subject in supine position, probe is placed on temporal and superior portion of the closed eye using large pad of gel with the subject's gaze at midline. B: Axial image of optic nerve showing the nerve emerging from posterior aspect of eye globe. The nerve aquires hypoechoic center and hyperechoic rim representing the sheath. Diameter is measured 3 mm posterior to eye globe with inclusion of outer sheath.

Fig. 2. A: Facial nerve scanning technique. The subject is in lateral decubitus position with the head resting on a pillow. The probe is placed transversly just below the ear lobule along the longitutdenal course of facial nerve. B: Longitutdinal image of facial nerve. The nerve (arrow) appears as linear tubular-like hypoechoic structure with hyperechoic rim inside the homogenous relatively hyperechoic parotid gland. C: Enlarged image of facial nerve showing diameter measurment at the nerve's thickest part with inlcusion of its hyeprechoic border.

Fig. 3. A: Vagus nerve scanning technique. The subject in lateral decubitus position with head slightly extended. The probe is placed on lateral neck at level of thyroid cartialge. B: Axial image of vagus nerve. The nerve (arrow) appears as hypoechoic oval structure wedged posteriorly between common carotid artery (CA) and internal jugular vein (IJV).

Fig. 4. A: Spinal accessory nerve scanning technique. The subject is in lateral decubitus position with head rotated to the side opposite the scanned side. Probe is placed on posterior triangle of the neck just behind the sternocleidomastoid muscle. B: Axial image of spinal accessory nerve. The nerve appears as small rounded hypoechoic structure (arrow) superficial to trapezius muscle and deep to sternocleidomastoid muscle.