Injury to the Superior Laryngeal Nerve (SLN) is often overlooked and also not well understood. In the past, damage to SLN has been hypothesized to have only limitations in vocal pitch range and vocal projection (Orestes & Chhetri 2015), but recent information suggests other factors might also be affected. Because symptoms can be so subtle, it is difficult as Speech Language Pathologists to clinically characterize SLN damage as the cause of our patients' voice problems.
Orestes & Chhetri's 2015 study suggests that the SLN is one of the least understood nerves, and usually finds a backseat to research about another portion of the Vagus Nerve, the Recurrent Laryngeal Nerve (RLN). If the Vagus were on the market for a favorite, you might say he likes the RLN a bit more. This might be because SLN damage prevalence (at least in one study after anterior cervical surgery) is from 0-1.25%. (Tempel et al 2017) That is not that often.
What is it?
A quick anatomy breakdown will infom us that the SLN's internal branch goes through the thyroid membrane providing the supraglottic larynx with sensory innervation. The external branch goes through the cricothyroid muscle, on the anterior portion of the larynx, providing motor innervation.
How to Diagnose?
Several case studies show that phase asymmetry can result from SLN paralysis, but since that can surface in RLN paresis and paralysis, it is not a solid diagnosing factor. Also, visualization via laryngoscopy alone will not diagnose SLN damage. Our current gold standard is the Laryngeal EMG test because it can tell if there is denervation, but interpretation of findings is subjective. Like vocal fold paralysis, there is no way to determine if SLN damage will improve or not.
Should we treat as SLP's?
Since SLN damage is so difficult to diagnose, we have limited evidence to support that voice therapy is helpful. Some small case studies have looked at surgical treatment, including cricothyroid approximation and medialization, but more research needs to be completed. While voice therapy is the most commonly recommended intervention, focusing on building cricothyroid muscle strength, effectiveness is variable. Orestes and Chhetri state that "There are no studies adequately evaluating the effects of voice therapy on SLN paralysis."
Because diagnosis of SLN damage is so difficult, it can often be overlooked or discounted as a possibility of dysphonia when cases are multifactorial. Since decreased pitch range is not the only symptom of SLN damage, I feel it is important as clinicians that we are considering that this could possibly be an underlying issue in voice patients who have subtle voice changes and seemingly have no blatantly obvious laryngeal pathology.
Kristie Knickerbocker, MS, CCC-SLP, is a speech-language pathologist and singing voice specialist in Fort Worth, Texas. She rehabilitates voice and swallowing at her private practice, a tempo Voice Center, and lectures on vocal health to area choirs and students. She also owns and runs a mobile videostroboscopy and FEES company, Voice Diagnostix. She is an affiliate of ASHA Special Interest Group 3, Voice and Voice Disorders, and a member of the National Association of Teachers of Singing and the Pan-American Vocology Association. Knickerbocker blogs on her website at www.atempovoicecenter.com. She has developed a line of kid and adult-friendly therapy materials specifically for voice on TPT or her website. Follow her on Pinterest, on Twitter and Instagram or like her on Facebook.
-Superior laryngeal nerve injury: effects, clinical findings, prognosis, and management options (2014) Orestes M & Chhetri DK. Current Opinion Otolaryngology Head Neck Surgery. doi: 10.1097/MOO.0000000000000097
- A Multicenter Review of Superior Laryngeal Nerve Injury Following Anterior Cervial Spine Surgery (2017) Zachary J. Tempel, MD,1 Justin S. Smith, MD, PhD,2 Christopher Shaffrey, MD,2 Paul M. Arnold, MD, FACS,3Michael G. Fehlings, MD, PhD,4 Thomas E. Mroz, MD,5 K. Daniel Riew, MD,6,7 and Adam S. Kanter, MD1