Introduction
Achieving adequate velopharyngeal closure for optimal feeding and speech development is a main objective in cleft palate closure. Unfortunately, 20–30 % of the cleft palate closures result in velopharyngeal insufficiency [
1,
2]. Numerous surgical techniques for palate closure have been described [
3,
4]. Most studies focus on the anatomical repair of the musculature of the cleft palate. Surgical techniques do not mention the possible nerve damage that could result from the surgical dissection. Several studies describing the palatal musculature have been described [
5,
6]. However, for an optimal functional muscular repair of the soft palate, thorough understanding of the motor innervation of these soft palate muscles is crucial. This may prevent nerve damage during surgical dissection and therefore may result in a better functional outcome and less complications in patients with cleft palate or velopharyngeal insufficiency. Two major anatomical textbooks mention that both palatopharyngeus muscle (PP) and the levator veli palatini muscle (LVP) are innervated by the cranial part of the accessory nerve (CN XI) via the pharyngeal plexus and the tensor veli palatini muscle (TVP) is innervated by the mandibular nerve [
7,
8]. Nevertheless, anatomical uncertainties remain such as the possible involvement of the facial nerve (CN VII) and the exact neural route via the pharyngeal plexus to the soft palate [
7]. This review provides an update of our current understanding of the origin, course and ramification patterns of the nerves that supply the three most important soft palate muscles. The muscles discussed are the palatopharyngeus (PP), the levator veli palatini (LVP) and the tensor veli palatini (TVP) muscles. This information could subsequently aid cleft surgeons during the cleft palate repair.
Methods
An extensive literature search was conducted using Embase and Medline (April 2015) and performed by RJH Logjes. First the terms “soft palate AND (innervation OR nerve)” and “velum AND (innervation OR nerve)” were used. This resulted in respectively
n = 551 and
n = 109 results. Secondly, the names of the three muscles were used as a term resulting in “tensor veli palatini” (
n = 226), “levator veli palatini” (
n = 283) and “palatopharyngeus” (
n = 105). After selecting the relevant articles by reading title and abstracts, 12 articles were used in this review. Five articles described the course of the nerves to the soft palate in human cadavers [
9‐
13] and one article described an electromyography (EMG) study [
14]. Another article did not describe material and methods [
15].
A total of five studies on the innervation of the soft palate muscles in animals were also used in this review [
16‐
20].
Discussion
Only few studies investigated the innervation of the soft palate muscles in humans. The innervation of the TVP by the mandibular nerve is universally accepted [
9‐
11,
15]. However, knowledge about the innervation of the LVP and PP remains controversial. All authors mentioned the contribution of the pharyngeal plexus but the details of their descriptions vary. According to two major anatomical textbooks, the pharyngeal plexus receives its motor fibres from the cranial part of the accessory nerve [
7,
8].
The studies by Broomhead were on small series of human heads [
9,
10]. The same applies to the study by Doménech-Ratto who only investigated 10 embryos [
11]. Studies by Shimokawa et al. were far more extensive and resulted in more robust conclusions about the course of the nerves and ramification patterns towards the soft palate muscles [
12,
13]. Shimokawa et al. in 2004, who mainly focussed on the LVP and the superior constrictor, did not report any contribution of the lesser palatine nerve to the supply of LVP [
12]. In another manuscript, Shimokawa et al. concluded in 2005 that the LVP and PP are innervated by the lesser palatine nerve and the pharyngeal plexus. Subsequently, in this second manuscript, nerve staining was performed by Shimokawa et al., which could explain the finding of the small lesser palatine nerve innervating part of the soft palate [
13].
Furthermore, Shimokawa et al. assumes that the hypothesis of Nishio et al. and Ibuki et al. that motor fibres of the facial nerve run inside the lesser palatine nerve in animals is also applicable to humans [
13,
16,
17]. Shimokawa et al. dissected the lesser palatine nerve but did not perform a functional characterization of the nerve fibres [
13]. Gray’s Anatomy states that every branch of the trigeminal nerve contains afferent fibres, including the maxillary nerve from which the lesser palatine nerve is derived [
8]. There is a possibility that lesser palatine nerve fibres which run to the LVP and PP contain sensory fibres only, namely for propriocepsis, pain and temperature information. There are examples in human anatomy where motor and sensory supplies of muscles go via different nerves. For instance, the trapezius muscle receives its motor supply from the spinal root of the accessory nerve and plexus cervicalis, whereas only the second, third and fourth cervical spinal nerves carry proprioceptive fibres from it [
21]. Another example are the facial muscles, which are efferently innervated by the facial nerve while their afferent fibres are part of the trigeminal nerve and end up in the mesencephalic nucleus [
8].
It would be useful to investigate the presence of motor fibres in the lesser palatine nerve by specific staining techniques. Sedlácková et al. did the only human study that assumes the involvement of the facial nerve innervating the LVP together with the pharyngeal plexus [
14].
Studies on the motor nerves to the soft palate muscles in animals had very conflicting results and seem to be less useful as a model for the human situation.
This review demonstrates crucial information for the cleft surgeon that innervation of the superior-extravelar part of the LVP and the PP enters the muscle form the lateral side. Subsequently, the lesser palatine nerve enters from the lateral side of the inferior-velar nasal part of the LVP. Although this anatomy is applicable to the normal soft palate, it will likely be applicable to the cleft palate. During cleft surgery, intravelar velar reposition is performed when the LVP is released from PP and retropostioned ventrally [
4]. This analysis suggests that during surgical dissection caution should be taken to dissect the dorsal/lateral aspect of the LVP from the PP because that is the area where the lesser palatine nerve enters the LVP. This theory is applicable to the von Langenbeck, two flap palatoplasty and also to the Furlow plasty. During the von Langenbeck procedure, the LVP should be adequately released from the nasal mucosa (and a thin layer of PP) and care should be taken not to perform a rigorous dissection on the lateral side of the LVP.
Conclusion
This review of the literature demonstrates the lack of accurate information about the innervation of the levator veli palatini and palatopharyngeus muscle. Most likely, the lesser palatine nerve and the pharyngeal plexus dually innervate these two muscles. However, since the type of nerve fibres of the lesser palatine nerve is unclear, the role of the facial nerve in motor-innervating the soft palate is uncertain. The pharyngeal plexus plays a major role in innervating the levator veli palatini and palatopharyngeus muscle and receive its motor fibres from the accessory nerve. The tensor veli palatini is innervated by the mandibular nerve. This information should aid the surgeon during repair of the cleft palate.
Acknowledgments
I Janssen, illustrator at the University Medical Centre Utrecht, illustrated the three figures used in this manuscript.
Compliance with Ethical Standards
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