JCN Open Access REVIEW pISSN 1738-6586 / eISSN 2005-5013 / J Clin Neurol 2015;11(2):109-121 / http://dx.doi.org/10.3988/jcn.2015.11.2.109 Neuromuscular Ultrasound of Cranial Nerves Eman A. Tawfika b Ultrasound of cranial nerves is a novel subdomain of neuromuscular ultrasound (NMUS) Francis O. Walker b which may provide additional value in the assessment of cranial nerves in different neuro- Michael S. Cartwright muscular disorders. Whilst NMUS of peripheral nerves has been studied, NMUS of cranial a Department of Physical Medicine nerves is considered in its initial stage of research, thus, there is a need to summarize the re- and Rehabilitation, search results achieved to date. Detailed scanning protocols, which assist in mastery of the Faculty of Medicine, techniques, are briefly mentioned in the few reference textbooks available in the field. This re- Ain Shams University, view article focuses on ultrasound scanning techniques of the 4 accessible cranial nerves: op- Cairo, Egypt bDepartment of Neurology, tic, facial, vagus and spinal accessory nerves. The relevant literatures and potential future ap- Medical Center Boulevard, plications are discussed. Wake Forest University School of Medicine, Key Wordszz neuromuscular ultrasound, cranial nerve, optic, facial, vagus, spinal accessory. Winston-Salem, NC, USA INTRODUCTION Neuromuscular ultrasound (NMUS) refers to the use of high resolution ultrasound of nerve and muscle to assess primary neuromuscular disorders. Beginning with a few small studies in the 1980s, it has evolved into a growing subspecialty area of clinical and re- search investigation. Over the last decade, electrodiagnostic laboratories throughout the world have adopted the technique because of its value in peripheral entrapment and trau- matic neuropathies. Not yet fully explored, however, are the applications of NMUS for the cranial nerves. Although cranial nerves are commonly involved in neuromuscular disor- ders, they are not routinely evaluated due to technical limitations of electrodiagnostic techniques. Nerve conduction studies are typically restricted to the blink reflex, and re- petitive stimulation studies of the facial and spinal accessory nerves, while needle exami- nation is typically restricted to muscles innervated by facial, trigeminal, hypoglossal, and spinal accessory nerves. Given their sensitive locations, patients often find the studies painful and poorly tolerable, while few electromyographers have acquired a comfort level in performing such studies extensively. Magnetic resonance imaging (MRI) and comput- ed tomography (CT) can also be of diagnostic value in these disorders, but their use is Received September 18, 2014 limited by cost, radiation exposure, and limited access. Ultrasound as a safe, inexpensive, Revised October 27, 2014 rapid, and well-tolerated modality has the potential to complement electrodiagnostic Accepted October 28, 2014 studies in the evaluation of cranial nerves. Correspondence Four out of the 12 cranial nerves are accessible by ultrasound and are the focus of our Francis O. Walker, MD review article. These are the optic, facial, vagus, and spinal accessory nerves. Of possible Department of Neurology, Medical Center Boulevard, future interest, indirect evaluation of the occulomotor, trochlear, trigeminal, abducens, Wake Forest University and hypoglossal nerves is also possible by ultrasound through examination of the muscles School of Medicine, they innervate, but a discussion of muscle ultrasound is beyond the scope of this review. Winston-Salem, NC 27157-1078, USA This review article describes NMUS scanning techniques of four readily accessible cra- Tel +1-336-716-7794 cc This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Com- Fax +1-336-716-4101 mercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial E-mail [email protected] use, distribution, and reproduction in any medium, provided the original work is properly cited. Copyright © 2015 Korean Neurological Association 109 JCN Cranial Nerve Ultrasound nial nerves and discusses possible clinical applications. The ninges. The dura is continuous with sclera of the eye and the intent is to define what the current best evidence allows us subarachnoid space abuts against the posterior aspect of the to conclude and to help guide future investigations in this retina around the optic disc. Bulging of retina around optic new area. disc creates the sign of papilledema which is characteristic of increased intracranial pressure (ICP).1 OPTIC NERVE Optic nerve ultrasound (Table 1) Anatomy The optic nerve is most commonly assessed by ophthalmos- The optic nerve carries the sensory impulses generated by copy and MRI but recently ultrasound has emerged as a prom- stimulation of rods and cons in the retina. It passes posteri- ising additional assessment tool. Being filled with aqueous orly from the back of the eye through the orbit to enter the fluid, the eye is well suited to ultrasound imaging. Measure- optic canal. After exiting the optic canal, the right and left ment of optic nerve diameter using ultrasound correlates optic nerves join to form the optic chiasm and then the op- closely with MRI measurement in cadaver and vivo studies2,3 tic tracts. The optic nerve as part of brain is covered by me- with high intra- and inter-observer reliability4-6 and good re- Table 1. Scanning protocols of cranial nerves Optic Facial Vagus Accessory Trans-orbital to obtain Longitudinal (along the Technique Axial & longitudinal Axial & longitudinal axial image nerve) - Mode: B-mode - Frequency: 7–15 MHz - Mode: B-mode - Mode: B-mode - Mode: B-mode Machine - Linear probe - Frequency: >12 MHz - Frequency: >12 MHz - Frequency: >12 MHz adjustment - Mechanical index - Linear probe - Linear probe - Linear probe ≈0.2 (=power 30%) - Depth: 2–4 cm - Depth: 3–4 cm - Depth: 1–2 cm - Thermal index ≈0.0 Supine, eye closed, gaze Side-lying -head on a Side-lying/supine-head Sitting/supine-head rotated Patient position fixed to midline “closed pillow slightly extended to the opposite side eye technique” On temporal and superior Posterior triangle of the Transverse just under the Lateral neck at the level Probe position portion of closed neck behind ear lobule of thyroid cartilage eye-large pad of gel sternocleidomastoid Small oval hypoechoic lying Linear thin tubular-like Small round honey-comb/ Linear structure with on top of trapezius or Nerve sonographic structure with hypoechoic hypoechoic between hypoechoic center and levator scapulae and appearance center and hyperechoic common carotid artery hyperechoic outer sheath posterior to outer rim and internal jugular vein sternocleidomastoid - Optic nerve sheath diameter measured at - At thickest part of the distance 3 mm posterior nerve Cross sectional area inside Measurement to eye globe - Inclusion of outer rim Cross sectional area hyperechoic rim - Inclusion of the outer - Largest of 2 sheath measurements - Mean of 3 measurements - Manipulation of probe to - Keep probe just under align optic nerve with eye ear to avoid area of Release pressure by the globe confusion with other transducer to avoid Tips - Relaxed environment with structures Identify the muscles first obliteration of internal head fixation in children - Use color flow for any jugular vein - Least pressure by the suspected vascular probe structures 110 J Clin Neurol 2015;11(2):109-121 Tawfik EA et al. JCN producibility.6 ed. Based on anatomic and histological studies, optimal mea- To scan the optic nerve, the most important machine ad- sures of the diameter of the optic nerve sheath is taken at a justment is to reduce thermal index to 0.0 and mechanical distance 3 mm posterior to the posterior rim of the globe. index to 0.2 or lower (can be adjusted by reducing the power Maximal expansion of the optic nerve sheath predominately output) to avoid thermal and mechanical side effects. Note occurs in its anterior part and specifically at a distance 3 mm that color flow and power Doppler imaging significantly in- behind the globe, whereas the posterior part shows less dila- crease the insonation energy of ultrasound. The eye is vul- tation with increased ICP.10,11 Also, it has been found that nerable to the heat generated by the sound waves, and ul- measuring the diameter at 3 mm is more reliable and repro- trasound, if done without proper precautions can injure the ducible than measuring it at 5 mm.6 Three measurements are retina and cause cataract formation in the lens. The earliest usually taken and averaged for accuracy. optic nerve studies7,8 used A-mode imaging (amplitude An alternative technique involves a lateral approach to ob- mode); however, B-mode has since become standard. A lin- tain a coronal image, avoiding the shadowing artifact some- ear transducer of frequency 7–10 MHz is usually sufficient. times encountered with the axial approach.9 However, fur- The optic nerve sheath can be visualized using transor- ther trials are needed to validate pathologic values in patients bital technique to obtain an axial image. The patient should with elevated ICP before this approach can be more widely lie in supine relaxed position and for children, head fixation used. may be needed. Closed eye technique (Fig. 1A) must be ad- Reference data for optic nerve sheath diameter are not yet hered to, in which the patient is instructed to keep both eyes standardized, although most literature considers diameter closed and the probe is placed on temporal and superior por- >5.7 mm abnormal. The determined cut-off value is proposed tion of the eye with large pad of gel and minimal pressure. mainly for detection of increased ICP and doesn’t necessarily Some9 prefer to apply non-adhesive clear dressing on the eye apply for diagnosis of other conditions, such as optic neuritis. to avoid irritation by gel. The subject should be instructed to Moreover, different mean optic nerve sheath diameters have maintain his/her gaze at midline to align the optic nerve been reported.5,9 Therefore, as with other reference values, along the probe.