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Biology Articles » Anatomy & Physiology » Anatomy, Human » Anatomy of the Spinal Accessory Nerve Plexus: Relevance to Head and Neck Cancer and Atherosclerosis » The Spinal Accessory Nerve Plexus in the Neck With Further Anatomic, Operative and Theoretic Considerations of Its Innervation of the Trapezius and Sternocleidomastoid Muscles

The Spinal Accessory Nerve Plexus in the Neck With Further Anatomic, Operative and Theoretic Considerations of Its Innervation of the Trapezius and Sternocleidomastoid Muscles
- Anatomy of the Spinal Accessory Nerve Plexus: Relevance to Head and Neck Cancer and Atherosclerosis

 

This part of the review will discuss not only anatomic issues, but also methodology, outcome, and possible improvements in cancer operations involving these structures. Many texts also address operative issues with divergent opinions (70, 71, 89).

Animal work as to motor innervation of the trapezius muscle has yielded divided opinions, perhaps due in some measure to species differences. The studies of Ueyama et al. (90) in the Japanese monkey, Macaca fuscata, and investigations in the cat by Reighard and Jennings (91) and later by Rose et al. (92) all indicate that only motoneurons of the spinal accessory nerve innervate the trapezius muscle. Contrary to this, Gurushanthiah’s (93) observations would indicate motor innervation both from the spinal accessory and cervical nerves in the Bonnet monkey, Macaca radiata.

In humans, the two muscles innervated by the spinal accessory nerve have different embryologic origin. The sternocleidomastoid muscle is branchiomeric, i.e., of branchial arch origin, whereas the trapezius muscle is telomeric, i.e., of somatic origin. A teleological theory states that innervation to the sternocleidomastoid muscle is cranial, and almost always by the spinal accessory nerve. The sternocleidomastoid functions to turn the head, essential to being alerted to danger not coming from directly ahead. The trapezius, on the other hand, does not serve this essential function so that it may be innervated occasionally by a cranial nerve alone (i.e., the spinal accessory nerve), or more commonly by a spectrum of combinations of cranial and cervical nerves, or rarely, by cervical nerves alone. It appears most reasonable anatomically and clinically that this explanation is the correct view of innervation of those two muscles (1, 94).

The anterior rami of the second through fourth cervical nerves of the cervical plexus, in addition to frequently connecting to the branches or trunk of the spinal accessory nerve, may also interconnect with: 1.) Peripheral branches of the trigeminal, facial, glossopharyngeal and hypoglossal nerves. 2.) The brachial plexus; and sensory nerves of the scalp as the great auricular and the greater and lesser occipital nerves. The sensory connections may carry over from this plexus beyond the mid-line of the face and head and also may interconnect with all other cervical nerves. A listing of many of these peripheral connections observed in the anatomy laboratory has been published (1). Cruvielhier in 1871 described a spinal accessory nerve plexus within the trapezius muscle with connections to the posterior divisions of the second through fourth cervical nerves (95). Possibly, fibers from the posterior divisions, which are largely motor, also randomly supply motor fibers to the trapezius muscle, which might be a further anatomic reason for different motor and sensory responses in patients having the same cervical nerves cut in surgical neck dissections.

Identification of the spinal accessory nerve, nevertheless, is not always easy (1, 39). Confirming this view, the late Dr. Narakas, with an extensive experience in reconstruction of brachial plexus injuries (96), told the writer anecdotally that the spinal accessory nerve, even in the correct anatomical position, was not always a reliable source for use as motor reinnervation for the damaged brachial plexus. Regarding this statement, the position of the spinal accessory nerve as it left the posterior border of nine sternocleidomastoid muscles has also been recorded (1). The nerve exited at approximately the mid-point in five, but between the junction of the upper and middle third in four. Absence of the spinal accessory nerve was reported in three subjects dissected in Piersol’s experience at the University of Pennsylvania (97). Relatively more commonly in the author’s experience as previously stated, the sternocleidomastoid muscle nearly always has solely spinal accessory innervation, but the trapezius muscle may have a wide spectrum from almost all to very little innervation from that nerve (1). Likewise, Soo et al. (98) in 1990 wrote that their anatomical, clinical and electrophysiological studies 1–156 months after neck dissections suggested a motor input from the cervical plexus by way of the spinal accessory nerve. In view of these lines of evidence, it is difficult to accept the thesis that motor fibers from the cervical plexus to the trapezius muscle are purely proprioceptive (99). Miyata et al. (100) in 1997 again reviewed the controversy in the literature as to whether cervical plexus innervation of the trapezius muscle is purely proprioceptive.

More recently Terzis et al. (101) have reported the results of surgical reconstruction of 204 patients with brachial plexus injury and paralysis with overall good to excellent results in 75% using their evaluation system. Sixty-nine of 204 patients had the spinal accessory nerve used as a motor source, so that it was undoubtedly successful in some patients; but the exact number of successful outcomes for this specific nerve is not stated (101). Samardzic et al. (102), one year later, reported their use of the spinal accessory nerve to reinnervate the musculocutaneous nerve in 20 patients with brachial plexus avulsion injuries. With their evaluation scale, 13 of 20 patients (65%) had a good functional recovery (102). In this light, one might theorize that failure of a spinal accessory nerve repair after neck dissection injury occurred in some instances because the wrong nerve, a sensory nerve, was repaired; or that more than one injured motor nerve in the spinal accessory nerve plexus should have been repaired.

In using the spinal accessory nerve for reinnervation procedures, some investigators believe that the upper part of the trapezius muscle function may be preserved. To accomplish this end, they recommend severing the spinal accessory nerve distal to where branches innervating the upper trapezius muscle are given off (103). This writer would urge caution in trusting anatomic observation alone in using this technique for preserving upper trapezius muscle function at operation. Intraoperative use of nerve and muscle stimulators has been suggested to help to identify nerves to avoid impairment (71, 104). Even these measurements have not always given results, which are in agreement with each other (100, 104), possibly due to differences in methodology. In support of this latter statement are the electromyographic (EMG) studies of Wiedenbauer and Mortensen done 50 years ago (105). Their studies over separate regions of the trapezius muscle during voluntary movements showed even minor changes in experimental conditions to be associated with considerable variation of activity patterns of individual subjects (105).

Finally, many more factors are known to affect operative outcome and its predictability, only three of which will be cited here. 1.) A recent review of head and neck surgery points out that comorbidity is an important factor in cancer severity staging (73). This information is useful in both planning an operation and in predicting its outcome. 2.) The same review points out that the immune system, with T lymphocyte and monocyte function in particular, is similarly useful (73). 3.) Medical imaging is becoming more and more important in improving operative outcome. For example, by showing precisely where neck masses are located, these studies help to prevent iatrogenic injury to the spinal accessory nerve plexus at operation. These imaging methods include computerized tomography (CT), positron emission tomography with fluorodeoxyglucose (EDG-PET), magnetic resonance imaging (MRI), and ultrasonography. Color flow duplex ultrasonography might be added to this list (73). In some instances, only one imaging method may be used successfully. An example is a report that a 3 x 3 cm paraganglioma at the left carotid artery bifurcation could only be successfully delineated by "color-coded Doppler ultrasonography with power Doppler" (106).


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