Abra-chemesthis……Carbonation ta-da! Reviewing the effects of carbonation on swallowing

Title: Effects of Carbonation on Swallowing: Systematic Review and Meta-Analysis
Authors: Ayano Nagano, Keisuke Maeda, Akio Shimizu, Kenta Murotani, & Naoharu Mori
Journal: The Laryngoscope
Year of Publication: 2022
Design Type: Systematic Review/Meta-Analysis
Purpose: “This systematic review aimed to evaluate the effectiveness of carbonation on improving swallowing function in adult subjects”
Population: Adult participants >20, irrespective of sex, medical history, clinical settings (of selected studies); literature published before March 2021
Inclusion criteria: Randomized control trials or intervention studies on carbonated liquids
Exclusion criteria: Studies in which the outcome was not related to swallowing function

Pop. Fizz. Gulp…

No, I’m not renaming the Rice Krispy elves. Just thinking about those thirst-quenching moments when drinking one of those bubbly drinks creates a pretty sensation-filled picture. The popping that you instantly feel on your tongue. The fizzing bubbles that tickle your lips or nose. And then, how the actual carbonation, aka chemesthesis, makes you swallow ever so slightly different (are you a big gulper or slow slipper for these delectable drinks?!). If there was ever a magical cure in a can, could this be it?🤔

I’ve always been surprised to find a lacking of firm evidence regarding this unknown or misunderstood technique when it comes to swallowing disorders. While there’s some more research for all things sensory, this comprehensive review makes us look a little harder to notice some patterns and possibly even take a pause before we take that next sip!🤯

Why Carbonation?

The authors graciously share the rationale behind some other sensory compensatory modifications such as sour, cold, and/or capsaicin stimuli that can result in changes in swallow function for various measures such as pressure and cortical activation along with improvements in the PAS scores (with great references included!😉).

But carbonation? There a lot of unknowns when it comes to the impact of this alteration. And one thing EVERYONE agrees on in our field is that we need more answers!!

So the investigators did their diligent duties to conduct a systematic review on all the evidence published before March of 2021 across various reputable databases and using multiple MeSH terms (check the article out to see what these were to maybe use in your own search!). After picking and choosing studies based off abstracts and then full-texts with 3^rd party consensus as needed, out of a whopping 328 studies, based on the inclusion/exclusion criteria, the list eventually got weaned down to a total of 19 studies that were included in the qualitative analysis (systematic review), and 5 included in the quantative analysis (meta-analysis).

To help lessen the confusion about what carbonation meant, “A carbonated liquid was defined as any liquid with carbonic acid, commercial or self-made, and plain or sweetened.” As far as what the actual 19 studies looked like:

• 586 participants total across all studies
• 8 RCTs
• 11 intervention studies
• 5 studies = participants with swallowing/neurological impairments
• 14 studies = participants without swallowing problems (healthy adults)
• Most participants = young adults (5 studies focused on older adults)
• Carbonated liquids included commercial soft drinks, sweetened beverages, thickened liquids
• Control liquids = [citric] acids, alcohols, and/or water (with/without thickener)
• 2 studies looked at gas volumes/temperatures
• 1 study looked at pharyngeal electrical stimulation

As we get more into it, there’s some shock🤯 and awe😲. Then some Hmm🤔 and Uhh🤨…

While the authors provide their full list of all 19 studies and all the related characteristics (design, comparisons, outcomes, etc.) to glance over yourself in more detail, they broadly put all these outcomes into the objective and subjective categories listed below while also classifying all the results into their own subjective groups as “Favorable,” “No difference,” and “Unfavorable.”

“Outcomes were classified as ‘favorable’ if the result showed that carbonation was effective in
preventing aspiration or if it enhanced swallow safety or efficacy with statistical significance.”

“Conversely, if the effect of carbonated water was negative in preventing aspiration or improving swallowing function with statistical significance, it was classified as ‘unfavorable.’

“Lastly, outcomes were classified as ‘no difference’ in cases where there was no statistically significant difference in the results.” p.2

While this no doubt helps us clinicians know the overall result in an easy-peasy way, it also creates some limitations to keep in mind later on🤫. . .

Outcomes

Objective Outcomes ^{_{(from the selected studies)}}:

• Penetration/Aspiration
• Global swallowing
• Pharyngeal motor-evoked potentials
• Sip volume
• Volume for elicitation of swallowing reflex
• Amplitude of hyolaryngeal elevation
• Pharyngeal retention
• Linguopalatal swallowing pressure
• Pharyngeal occlusive pressure
• Swallowing Function Parameter

“Most results indicated favorable effects of carbonation on swallowing. Notably, the amplitude of
hyolaryngeal elevation for carbonated liquids did not differ from that of water, but it was lower than that of lemon juice concentrate. Pharyngeal retention for carbonated liquids was similar to that of water.” p.3

Time-related objective Outcomes:

• Swallowing time
• Reaction time
• Pharyngeal reaction time
• Pharyngeal transit time
• Oral transit time
• Swallowing apnea duration
• Duration of laryngeal elevation
• Duration of hyolaryngeal elevation
• Duration of suprahyoid muscle contraction
• Duration of UES opening

“The effects of the carbonated liquids were not consistent with the swallowing reaction time and pharyngeal transit time. The research indicated no difference between carbonated and control liquids when swallowing apnea was measured. Hyolaryngeal elevation did not differ between water and carbonated liquids but was increased with sour drinks.” p.3

Subjective Outcomes:

• Only 3 studies reported on palatability with mixed results

If all that seems like a LOT of different info, that’s because it is!! When we can’t even agree on certain terms or definitions (e.g. “duration of laryngeal vs hyolaryngeal elevation”), much less how we’re determining the degree of penetration/aspiration, or when studies use too vague terminology, it all makes the job of having a consensus-style answer that much harder.

Put another way, when you have a bunch of different apples (gala, granny, honeycrisp, etc.🍏🍎🍏🍎) in a basket and you have to determine which one is the “Best” based on a bunch of different possible criteria (best for your gut? best for your brain?? best for strong bones???), it can be confusing, frustrating, and overwhelming.😓

But, we push through because finding out what we don’t know at least leads us closer to knowing more!

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The above results looked at explaining all the outcome info in a narrative way, now let’s look at what the numbers mean…

Unfortunately, because we don’t live in a perfect SLP world, only 2 of the 19 studies selected could be used for a meta-analysis in order to combine all the math-related data and results to come up with an overall impact statement.

Why you might ask?🤔

Well, the authors did go through to assess for the risk of bias with their bias tools to tease out things like blinding, incomplete data, selective reporting, and confounding variables and more. So some studies just didn’t make the cut because of high risk of bias when it came down to it (particularly for confounding variables and lack of blinding of outcome assessments). An interesting note, the authors do mention they can’t rule out that selection bias^owhen the study participants differ from the group of people investigators want to studyTooltip content or detection bias^owhen assessors know the participant’s group assignmentTooltip content were at play which also could have affected the results for this.😬 (For a free tool to keep handy when it comes to this, checkout the Identify Biases Resource!)

Anyhoo, more unfortunate news is that out of all those outcomes listed above, only 2 could be used for the meta-analysis across the 5 studies: aspiration/penetration & duration of swallowing apnea.

Aspiration/Penetration:

Out of 2 studies of moderate quality that were included for aspiration and 3 for aspiration+penetration:

“The meta-analysis showed that carbonated liquids can prevent aspiration and penetration.”

“The present meta-analysis showed that the (number needed to treat) for aspiration was 3.7, which is a clinically important finding indicating that carbonated liquids can prevent aspiration in approximately one of four individuals.” p.8-9

The authors move forward to provide their rationales as to this now quantitative phenomenon, hypothesizing how a more sensitive swallowing reflex “can contribute to early pharyngeal flow, thereby preventing penetration/aspiration.”

I may be missing something, but I’ll be honest that it got harder for me to keep connecting all the dots from the authors’ attempts to explain all these relationships further, especially with less explanations on the relationship for swallowing anatomy/physiology…

Duration of Swallowing Apnea:

Out of 2 studies (1 of which looked at young/older adults separately) of low quality,

“The meta-analysis indicated that carbonated liquids changed the duration of swallowing apnea when compared to water.”

“The meta-analysis also demonstrated that carbonated liquids had no significant effect on the duration of swallowing apnea when compared to citric acid solution.” p.8

Ok, I’m not gonna lie that I had to read over this section a few times to make sense of some of the wording and still have a hard time figuring out just how much the apneic period changed (at the end of the day, I guess significant is significant?). But all in all, it seemed when looking at the numbers from a few studies, prolonging this much theorized airway-protective phenomenon might have some overall impact on swallow function.

Who woulda thought?

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In case your brain is still trying to sort out all this different info, know you’re not alone! I have many unanswered questions still– like why there was no mention of how things like penetration/aspiration or even the swallow function measures were even assessed (did most use things like MBSImP? PAS? Random subjective ratings??). I’m also yearning to know more about what the long-term or external outcomes might be for such a stimulating-sensory approach (Does this result in better patient adherence? Better hydration? Better staff training? Quality of life?). Yes, those questions are definitely outside of the scope of this article, but how can one help not asking more questions even when you get a couple answers??

What we DO know:

Yes, the results indicate carbonation can prevent aspiration in adults by increasing the duration of swallowing apnea. BUT, does that mean it will? We won’t know unless we test this out under direct visualization of the swallowing system (be it VFSS or FEES) for each individual patient (something I personally feel should have been emphasized much more in the original article).

Yes, many of the studies actually showed subjectively “favorable” changes in various objective outcomes related swallow function. BUT, these were few and far between, with small and very specific sample sizes, and about half showing no difference when it came to timing measures. Not to mention another huge limitation is that the populations they looked at varied so much across each study leaving little room to have a definitive statement like “X works well for A, B, C” or even so much as saying there were any differences across different groups like age, gender, diagnoses, etc.

Despite this being a systematic review with the quantification to back some of it up, I know it’s enticing but it still cannot lead us to have an end-all-be-all approach with these beloved bubbly beverages.😩🫤

While the authors do state “Carbonated liquids are potentially useful for rehabilitative or compensatory interventions in patients with swallowing problems,” I’m not arguing they cannot be helpful to some, for the right reasons, at the right time (which we still don’t know🫤), I’m still hesitant to start tossing these bubbly beverages to every resident or patient I come across with dysphagia.

My take? If you have a patient who you’re curious about trying this possible carbonation cure on, then I would rather recommend knowing what you’re looking for, why, and glance over some of the selected studies to see what fits (or even search on your own for maybe one that wasn’t included but better fits your patient population!). Do we have a little more reason to possibly try this sensory technique under instrumentation? Sure, why not?! Should we assume this will work for every patient regardless of presentation, diagnosis, age, preference, etc.?

Nonetheless, I’ll certainly be adding this topic to my SLP-Wish List for answers. I wish we could know for sure what this sensory strategy could do and specifically for who. Instead of just going off some mixed results which by the end still gives the open-ended conclusion “further research involving robust methodology and well-define cohorts is needed to clarify the influence of carbonation on swallowing function,” even just a very good quality, well-designed single-subject study can still be helpful to show us a small pathway forward with similar patients. And before you know it, others can start collecting their own data to help us all get some more answers!!

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Article Referenced:

Nagano, A., Maeda, K., Shimizu, A., Murotani, K. and Mori, N., 2022. Effects of Carbonation on Swallowing: Systematic Review and Meta‐Analysis. The Laryngoscope,. doi: 10.1002/lary.30019

***For more carbonated-related sensory articles, definitely check out the above article for the whole reference list along with a previous review from SLP R&R on Carbonation+Parkinson’s Disease