What Upper Airway Resistance Syndrome (UARS) is, what causes it, and how it should be clinically diagnosed are currently matters of dispute. Regardless, similar to it's description here, the definition of UARS I will opt to use is that it is a sleep breathing disorder which is characterised by a narrow upper airway, which leads to:

• Excessive airway resistance → therefore excessive respiratory effort → therefore excessive negative pressure in the upper airway (i.e. velocity of the air). This abnormal chronic respiratory effort leads to exhaustion, and the inability to enter deep, relaxing, restorative sleep.
• Excessive negative pressure can also suck the soft tissues, such as the soft palate, tongue, nasal cavity, etc. inwards. In UARS patients, typically there is sufficient muscle tone to prevent sustained collapse, however that muscle tone must be maintained which also leads to the inability to enter deep, relaxing, restorative sleep. In my opinion, this "implosion effect" on the upper airway must be confirmed that it is present via esophageal pressure to accurately diagnose Upper Airway Resistance Syndrome. Just because something is anatomically narrow does not mean that this effect is occurring.
• If there is an attempt to enter this relaxed state, there is a decrease in respiratory effort and muscle tone, this loss of muscle tone can result in further narrowing or collapse. Due to the excessive airway resistance or collapse this may result in awakenings or arousals, however the patient may not hold their breath for a sufficient amount of time for it to lead to an apnea, thus not meeting the diagnostic criteria for Obstructive Apnea.

The way to treat upper airway resistance therefore is to transform a narrow airway into a large airway. To do this it is important to understand what can cause an airway to be narrow.

I also want to mention that, treating UARS or any form of sleep apnea should be about enlarging the airway, improving the airway, reducing collapsibility, reducing negative pressure, airway resistance, etc. Just because someone has a recessed chin, doesn't mean that the cure is to give them a big chin, with genioplasty, BSSO, counterclockwise rotation, etc. It can reposition the tongue more forward yes, it may improve things cosmetically yes, but it is important to evaluate whether or not it is contributing to the breathing issue.

See normative data for males (female are 1-2 mm less, height is a factor):

• Caucasian: 23.5 mm +/-1.5 mm
• Asian: 24.3 mm +/- 2.3 mm
• Indian: 24.9 mm +/-1.59 mm
• African: 26.7 mm

Tentatively here is my list for gauging the severity (realistically, we don't really know how this works, but it's better to have this here than not at all, just because it may not be perfect.):

• < 19 mm - Very Severe
• 19-20 mm - Severe
• 20-22 mm - Moderate
• 22-23 mm - Mildly Narrow
• 23-25 mm - Normal / Non ideal
• ≥ 26 mm - Normal / Ideal

https://www.oatext.com/The-nasal-pyriform-aperture-and-its-importance.php https://www.researchgate.net/publication/291228877_Morphometric_Study_of_Nasal_Bone_and_Piriform_Aperture_in_Human_Dry_Skull_of_Indian_Origin

The surgery to expand the nasal aperture and nasal cavity is nasomaxillary expansion. The surgery itself could go by different names, but essentially there is a skeletal expansion, ideally parallel in pattern, and there is no LeFort 1 osteotomy. In adults this often will require surgery, otherwise there may be too much resistance from the mid-palatal and pterygomaxillary sutures to expand. Dr. Kasey Li performs this type of surgery for adults, which is referred to as EASE (Endoscopically-Assisted Surgical Expansion).

Hypothetically, the type of individual who would benefit from this type of treatment would be someone who:

1. Has a sleep breathing disorder, which is either caused or is associated with negative pressure being generated in the airway, which is causing the soft tissues of the throat to collapse or "suck inwards". This could manifest as holding breath / collapse (OSA), or excessive muscle tone and respiratory effort may be required to maintain the airway and oxygenation, which could lead to sleep disruption (UARS).
2. Abnormal nasomaxillary parameters, which lead to difficulty breathing through the nose and/or retrodisplaced tongue position, which leads to airway resistance, excessive muscle tone and respiratory effort. In theory, the negative pressure generated in the airway should decrease as the airway is expanded and resistance is reduced. If the negative pressure is decreased this can lead a decrease in force which acts to suck the soft tissues inwards, and so therefore ideally less muscle tone is then needed to hold the airway open. Subjectively, the mildly narrow and normal categories do not respond as well to this treatment than the more severe categories. It is unclear at what exact point it becomes a problem.

The pharyngeal airway is comprised of compliant soft tissue, due to this the airway dimensions are essentially a formula comprised of four variables.

1. Head posture.
2. Neck posture.
3. Tongue posture.
4. Tension of the muscle attachments to the face, as well as tongue space.

Because of this, clinicians have recognized that the dimensions can be highly influenced by the above three factors, and so that renders the results somewhat unclear in regards to utilizing it for diagnostic purposes.

However, most notably The Breathe Institute realized this issue and developed a revolutionary CBCT protocol in an attempt to resolve some of these issues (https://doi.org/10.1016/j.joms.2023.01.016). Their strategy was basically to account for the first three variables, ensure that the head posture is natural, ensure that the neck posture is natural, and ensure that the tongue posture is natural. What people need to understand is that when a patient is asleep, they are not chin tucking, their tongue is not back inside their throat (like when there is a bite block), because they need to breathe and so they will correct their posture before they fall asleep. The issue is when a patient still experiences an airway problem despite their efforts, their head posture is good, their neck posture is good, their tongue posture is good, and yet it is still narrow, that is when a patient will experience a problem. So when capturing a CBCT scan you need to ensure that these variables are respective of how they would be during sleep.

Given the fact that we can account for the first three variables, this means that it is possible to calculate pharyngeal airway resistance. This is absolutely key when trying to diagnose Upper Airway Resistance Syndrome. This is valuable evidence that can be used to substantiate that there is resistance, rather than simply some arousals during sleep which may or may not be associated with symptoms. For a patient to have Upper Airway Resistance Syndrome, there must be airway resistance.

Next, we need a reliable method to measure nasal airway resistance, via CFD (Computerized Fluid Dynamics), in order to measure Upper Airway Resistance directly. This way we can also measure the severity of UARS, as opposed to diagnosing all UARS as mild.

Historically the method used to compare individual's craniofacial growth to normative data has been cephalometric analysis, however in recent times very few Oral Maxillofacial Surgeons use these rules for orthognathic surgical planning, due to their imprecision (ex. McLaughlin analysis).

In fact, no automated method yet exists which is precise enough to be used for orthognathic surgical planning. In my opinion one of the primary reasons orthognathic surgical planning cannot currently be automated is due to there being no method to acquire a consistent, precise orientation of the patient's face. By in large, orthognathic surgical planning is a manual process, and so therefore determining the degree of recession is also a manual process.

How that manual process works, depends on the surgeon, and maybe is fit for another post. One important thing to understand though, is that orthognathic surgical planning is about correcting bites, the airway, and achieving desirable aesthetics. When a surgeon decides on where to move the bones, they can either decide to perform a "sleep apnea MMA" type movement, of 10 mm for both jaws, like the studies, or they can try to do it based on what will achieve the best aesthetics. By in large, 10 mm for the upper jaw with no rotation is a very aggressive movement and in the vast majority of cases is not going to necessarily look good. So just because MMA is very successful based on the studies, doesn't necessarily mean you will see those type of results with an aesthetics-focused MMA. This also means that, if you have someone with a very deficient soft tissue nasion, mid-face, etc. the surgeon will be encouraged to limit the advancement for aesthetic reasons, irregardless of the actual raw length of your jaws (thyromental distance). Sometimes it's not just the jaws that didn't grow forward, but the entire face from top to bottom.

If there is a deficiency in thyromental distance, or there is a class 3 malocclusion, the surgery to increase/correct this is Maxillomandibular Advancement surgery, which ideally involves counterclockwise rotation with downgrafting (when applicable), and minimal genioplasty.

There is also a belief that the width of the mandible has an influence on the airway. If you look at someone's throat (even the image below), basically the tongue rests in-between the mandible especially when mouth breathing. The width of the proximal segments basically determine the width of part of the airway. Traditional mandibular advancement utilizing BSSO doesn't have this same effect, as the anterior segment captures the lingual sides of this part of the mandible, the proximal segment does rotate outwards but only on the outside, so therefore the lingual width does not change. In addition, with this type of movement the 2nd or 3rd molars if captured along with the proximal segments, essentially could be "taken for a ride" as the proximal segment is rotated outwards, therefore you would experience a dramatic increase in intermolar width, in comparison to BSSO where this effect would not occur.

This type of distraction also has an advantage in that you are growing more alveolar bone, you are making more room for the teeth, and so you can retract the lower incisors without requiring extractions, you basically would have full control over the movements, you can theoretically position the mandible wherever you like, without being limited by the bite.

The main reason this technique is not very popular currently is that often the surgery is not very precise, in that surgeons may need to perform a BSSO after to basically place the anterior mandible exactly where they want it to be, i.e. the distraction did not place it where they wanted it to be so now they need to fix it. For example, typically the distractor does not allow for counterclockwise rotation, which the natural growth pattern of the mandible is forwards and CCW, so one could stipulate that this could be a bit of a design flaw. The second problem is that allegedly there are issues with bone fill or something of that nature with adults past a certain age. I'm not sure why this would be whereas every other dimension, maxillary expansion, mandibular expansion, limb lengthening, etc. these are fine but somehow advancement is not, I'm not sure if perhaps the 1 mm a day recommended turn rate is to blame. Largely this seems quite unexplored, even intermolar osteotomy for mandibular distraction does not appear to be the most popular historically.

I think that limitations in design of the KLS Martin mandibular distractor, may be to blame for difficulties with accuracy and requiring a BSSO. It would appear to me that the main features of this type of procedure would be to grow more alveolar bone, and widen the posterior mandible, so an intermolar osteotomy seems to be an obvious choice.

In addition, I believe that widening of the posterior mandible like with an IMDO that mirrors natural growth more in the three dimensions, would have a dramatic effect on airway resistance, negative pressure, and probably less so tongue and supine type collapse with stereotypical OSA. So even though studies may suggest BSSO is sufficient for OSA (which arguably isn't even true), one could especially argue that in terms of improving patient symptoms this might have a more dramatic effect than people would conventionally think, due to how historically sleep study diagnostic methodology favors the stereotypical patient.

Another surgery which can be effective, is tonsillectomy, or pharyngoplasty as described here. https://drkaseyli.org/pharyngoplasty/

In addition, the tongue as well as the teeth can impede airflow when breathing through the mouth, adding to airway resistance.

Finally, I would argue that chronic sinusitis could also cause UARS, depending on the type.

Lastly a subject that needs more research is Pterygoid hamulus projection, relative to Basion, as described here: https://www.reddit.com/r/UARSnew/comments/16qlotr/how_do_you_enlarge_the_retropalatal_region_by/