Mechanism of nasal irrigation

Whilst it's unclear exactly how nasal irrigation achieves these health benefits, it is hypothesised that it does so by one or more of the following.[1]

Mechanically clearing mucus

Nasal irrigation (especially jala-neti) mechanically clears excess mucus. Even thick and encrusted mucus (which is difficult for the cilia to move) is flushed away. It is not hard to imagine that nasal secretions, nasal congestion, post-nasal drip, (tickly) cough and encrusted mucus could all be alleviated to some extent simply by the removal of excess mucus. It's harder to see how sinus headaches could be helped by this mechanism, but it could be that removing excess mucus from the nasal passages helps the sinuses to drain. In the case of allergic rhinitis and exposure to air-borne irritants, all symptoms may be reduced simply because the allergens or irritants that have been captured by nasal mucus are flushed away.

Decreasing concentrations of inflammatory mediators

It has been shown that performing nasal irrigation (specifically jala-neti using isotonic saline with a pulsatile device) reduces the concentrations of inflammatory mediators (histamine and leukotriene C4) in the noses of people with allergic rhinitis.[2] Histamine concentrations are reduced for around six hours after performing an irrigation, whereas leukotriene C4 concentrations are reduced for around four hours.

Inflammatory mediators cause the lining of the nasal passages to swell (mucosal edema) and excess mucus to be produced. Reducing their concentrations should therefore mean a reduction in swelling and mucus production leading to reduced symptoms.

Nasal irrigation may reduce inflammatory mediator concentrations simply by flushing away allergens that have become trapped in the nasal mucus, as previously mentioned.

Decreasing mucosal edema

Mucosal edema is swelling of the lining of the nasal passages and sinuses. It leads to narrower nasal passages and an increased production of mucus, which can lead to various sino-nasal problems such as nasal congestion, post-nasal drip, rhinorrhea (a runny-nose), sinusitis etc. Any decrease in mucosal edema should therefore reduce these symptoms.

In the case of allergic rhinitis, nasal irrigation may decrease mucosal edema by reducing the concentrations of inflammatory mediators. Nasal irrigation with hypertonic (but not isotonic) saline may be able to decrease mucosal edema for other forms of rhinitis as well. As the salinity of a hypertonic solution is (by definition) greater than that of the nasal mucosa, an osmotic potential should exist leading to water being drawn out of the mucosa, actively reducing any swelling.

Improving mucociliary function

Mucociliary function (or mucociliary clearance) refers to the ability of the cilia in the airway to transport mucus away (usually towards the throat where it can be swallowed). When mucociliary function is impaired mucus may start to build up and cause nasal congestion and other sino-nasal symptoms. There are two main factors affecting mucociliary function, ciliary beat frequency (the frequency at which the cilia `beat') and mucus rheology (the fluidic properties of the mucus, viscosity, elasticity etc.).

Nasal irrigation with hypertonic saline has been shown to improve mucociliary function[3, 4, 5, 6, 7] by improving mucus rheology[8, 9] (thinning the mucus - reducing viscosity). Ciliary beat frequency however, is unaffected.[8] The same osmotic mechanism put forward to explain how hypertonic solutions might decrease mucosal edema is believed to play a role in improving mucus rheology, as water would be drawn out of the swollen nasal lining and into the mucus. It seems that the greater the salinity, the greater the improvement in mucociliary clearance but very concentrated solutions (12% and over) are associated with nasal irritation.[7, 9]

References
[1]
L. T. Tomooka, C. Murphy, and T. M. Davidson. Clinical study and literature review of nasal irrigation. Laryngoscope, 110(7):1189-93, 2000.
[2]
J. W. Georgitis. Nasal hyperthermia and simple irrigation for perennial rhinitis. changes in inflammatory mediators. Chest, 106(5):1487-92, 1994.
[3]
E. Daviskas, S. D. Anderson, I. Gonda, S. Eberl, S. Meikle, J. P. Seale, and G. Bautovich. Inhalation of hypertonic saline aerosol enhances mucociliary clearance in asthmatic and healthy subjects. Eur Respir J, 9:725-32, 1996.
[4]
D. S. Parsons. Chronic sinusitis: a medical or surgical disease? Otolaryngol Clin North Am, 29(1):1-9, 1996.
[5]
A. R. Talbot, T. M. Herr, and D. S. Parsons. Mucociliary clearance and buffered hypertonic saline solution. Laryngoscope, 107(4):500-3, 1997.
[6]
J. J. Homer, R. J. England, A. D. Wilde, G. R. Harwood, and N. D. Stafford. The effect of pH of douching solutions on mucociliary clearance. Clin Otolaryngol Allied Sci, 24(4):312-5, 1999.
[7]
J. J. Homer, A. C. Dowley, L. Condon, P. El-Jassar, and S. Sood. The effect of hypertonicity on nasal mucociliary clearance. Clin Otolaryngol Allied Sci, 25(6):558-60, 2000.
[8]
P. J. Wills, R. L. Hall, W. Chan, and P. J. Cole. Sodium chloride increases the ciliary transportability of cystic fibrosis and bronchiectasis sputum on the mucus-depleted bovine trachea. J Clin Invest, 99(1):9-13, 1997.
[9]
M. Robinson, A. L. Hemming, J. A. Regnis, A. G. Wong, D. L. Bailey, G. J. Bautovich, M. King, and P. T. Bye. Effect of increasing doses of hypertonic saline on mucociliary clearance in patients with cystic fibrosis. Thorax, 52(10):900-3, 1997.