The airflow resistance provided by the airways during breathing is essential for good pulmonary function. The nose is responsible for almost two thirds of this resistance. Most of this resistance occurs in the anterior part of the nose. This region is called the nasal valve, and it acts as a flow-limiter.
The nasal valve was originally described by Mink in 1903. It is divided into external and internal portions. The external nasal valve is formed by the columella, the nasal floor, and the nasal rim (or caudal border of the lower lateral cartilage). The nasalis muscle dilates this portion during inspiration. The internal nasal valve accounts for the larger part of the nasal resistance. It is located in the area of transition between the skin and respiratory epithelium, and it is usually the narrowest part of the nose. The internal nasal valve is the better-known valve and is often referred to as the nasal valve.
The 2 terms should be differentiated because the nasal valve accounts only for the aperture between the nasal septum and the caudal border of the upper lateral cartilage (ULC). The angle formed between them is normally 10-15° in white patients. This measure was believed for many years to be universal. However, in 2006, Miman et al found the angle to be 22.5-52º in Turkish patients without nasal obstruction symptoms or septal deviation.  These measurements where made with nasal endoscopy in living subjects.
The nasal valve area (as in the image below) is formed by the nasal septum, the caudal border of the ULC, the head of the inferior turbinate, and the pyriform aperture and the tissues that surround it. This area is responsible for more than two thirds of the resistance produced by the nose.
History of the Procedure
In 1894, Franke performed nasal-flow experiments in models and cadavers and found that whirl formation occurred near the head of the turbinate during calm breathing.  The term nasal valve was first coined by Mink in 1903.  He developed this concept further in 1920, suggesting that the greatest area of resistance was in the limen nasi or the union of the lobular cartilage and ULCs.
In 1940, Uddstromer found that 70% of the resistance of the nose was produced in the nasal valve area and the remaining 30% was due to the nasal fossa. Van Dishoeck further investigated the mechanisms of the nasal valve in 1942, and in 1970, Bridger and Proctor wrote about a "flow-limiting segment" that included the limen nasi and the pyriform aperture. In 1972, Bachman and Legler found the pyriform aperture to have the smallest cross-sectional area of the nasal airway.
In 1983, Haight and Cole continued Bridger and Proctor’s studies and demonstrated that the maximal nasal resistance was localized near the pyriform aperture and depended on engorgement of the head of the inferior turbinate. 
As many as 13% of the patients with chronic nasal obstruction have nasal valve collapse. Of these patients, 88% have unilateral collapse.
Constantian found, however, that up to 50% of the patients post rhinoplasty had internal nasal valve obstruction. 
External nasal valve collapse can be found in patients without a history of trauma or surgery. These patients commonly have an overprojecting nose with extremely narrow nostrils. Another cause, though uncommon, can be an extremely wide columella, as seen in the image below.
Internal nasal valve collapse can be divided depending on the structure that caused the collapse. In many cases, more than one structure is affected. The most common cause is probably septal deviation, as seen in the image below. The second cause is collapse secondary to rhinologic surgery, especially after removal of the nasal roof. Khosh found, in 53 patients, the following causes of nasal valve collapse: previous rhinoplasty (79%), nasal trauma (15%), and congenital anomaly (6%). 
Deviations of the caudal septum are the most common cause of valvular collapse. They are usually secondary to trauma. The septum can be overly thick in the valvular area, decreasing the space in it. Also, an absence of cartilage in this area leaves a flaccid septum that moves during inspiration.
Upper lateral cartilage
Thickened cartilage can compromise an adequate aperture. The cartilage can also be twisted, deflected, or associated with excessive return of the caudal border. An absence of cartilage, either congenital or iatrogenic, can produce a flaccid valve that collapses during inspiration.
Lower lateral cartilage
Overresection during rhinoplasty can weaken the cartilage and cause inspiratory collapse. Deformation of the cartilage can be a result of trauma or congenital malformations of the cartilage. Also, scarring due to surgery can produce the lateral crura to obstruct the nasal valve, as shown below.
Trauma or previous surgery can create webs or stenosis in the valvular area. The tissue can also be too thick, reducing the lumen of the valve. This can be a result of inflammation or hypertrophy. Minman et al describe the presence of a septal body in the valvular area. Benign tissue similar to the turbinates, it can produce stenosis of the nasal valve if the nasal cycle is in the congestive stage.
Hypertrophy of the inferior turbinate can significantly increase nasal resistance. Several studies have demonstrated that the head of the turbinate is responsible for most of this increase. In comparison, the body and tail of the turbinate play minor roles in nasal resistance. The increase in size can be secondary only to mucosa or bone hypertrophy.
Although uncommon, some patients may have deformities of the pyriform aperture that reduce the space of the nasal valve. The first description the authors found of a congenital stenosis of the pyriform aperture was made by Brown et al in 1989;  other reports have followed, such as Ramadan, 1995; Fornelli, 2000;  and Lee, 2002.  A more common cause of obstruction is the osteotomy made during a rhinoplasty. In particular, the type known as low-to-low is blamed for excessive narrowing of the pyriform aperture. Some modifications to this procedure allow an osteotomy to be performed without compromising the space in the valve.
Rhinoplastic procedures are particularly prone to disturbing the nasal valve area. Hump removal affects the nasal valve in several ways. If the hump is particularly large, separation of the ULC can be necessary. Resection of the T-shaped area of the dorsal border of the septum produces a narrower area in the roof. If the mucosa in the valve is not protected during the surgery, which occurred with the use of many older techniques, scarring of the valve can lead to structure formation or stenosis of the valve.
In reduction rhinoplasties, the cross-sectional area of the overall nose is reduced. This increases the resistance to airflow. If the nasal valve is not properly repaired during the surgery, patients may report nasal obstruction after the surgery, even if this was not reported preoperatively. Overresection of the lower lateral cartilage can lead to pinching and inspiratory collapse.
Age is another factor to consider. The relaxation of tissues may eventually produce a flaccid valve. In these cases, surgery of the valve can correct the loss of patency.
The internal nasal valve works as a flow-limiting area. The fixed part of the valve is composed of the septum and the pyriform aperture. The ULC and the mucosa of the turbinate act as the mobile part. When air is inspired, it is forced through this narrow area, increasing its speed and pressure. Just after passing the valve, the air expands in the bony cavum, creating turbulence that promotes contact between the air and the mucosa. In this way, the inspired air is cleansed of particles, humidified, and heated or cooled (depending on its temperature).
Because it is the narrowest part of the nose, the nasal valve can be affected by minute alterations of the nasal anatomy that would not be important in other areas. The angle between the ULC and the nasal septum is 10-52°. Internal nasal valve collapse occurs when, for some reason, this angle is diminished. The result is an increase in nasal resistance to airflow; consequently, the patient reports nasal obstruction. The opposite is known as ballooning. In this case, the nasal valve is excessively open.
The increase in nasal resistance is also related to abnormalities of pulmonary function. These changes in pulmonary function return to normal after septal surgery is performed and nasal resistance is decreased.
Patients primarily report nasal obstruction. Other symptoms are crusting and bleeding, but these are more often associated with septal deviation.
Diagnosis can be difficult if the physician does not visualize the valvular area. Examining the valve without disturbing it with a nasal speculum is important because the speculum usually opens the valve. Sometimes, trimming the vibrissae is necessary to obtain a clearer view of the valve. Another method is to use a 0° endoscope.
The Cottle test is useful to evaluate nasal valve stenosis. The cheek of the evaluated side is gently pulled laterally with 1-2 fingers, which opens the valve. The examiner then asks the patient to breathe and evaluates if breathing is better before or after pulling the cheek. A positive test result is when the patient feels less resistance with the valve opened. This test is easy and quick to perform (see the image below).
The internal nasal valve is limited medially by the nasal septum. Laterally, in its superior part, it is limited by the caudal border of the ULC, where it forms the limen nasi with the cephalic border of the lobular cartilage. The angle between the septum and the ULC is 10-15° in the noses of white persons. Persons of other races demonstrate great variability in this nose measurement, as previously mentioned (22.5-52º). In this area, epithelium shifts from the skin of the vestibule to the respiratory mucosa of the bony cavum.
The pyriform aperture continues the limit of the valve from the ULC to the floor. The head of the inferior turbinate is immediately posterior to the pyriform aperture and plays an important role in the function of the valve, which is the reason it is also considered part of the internal nasal valve (see the image below). In some patients, particularly white persons, the caudal border of the ULC scrolls externally in what is called the returning of the ULC. When this returning is excessive, it can produce valve collapse.
In addition to the typical reasons to avoid surgery (eg, bleeding disorders), other contraindications include the following:
Excessive scarring due to multiple previous surgeries, which may compromise the outcome of the procedure
The presence of cheloid scarring
Unrealistic patient expectations
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