General Medical Information

Anatomy And Physiology Of The Sinuses

A. Daniel Pinheiro
George W. Facer
Eugene B. Kern

The exact functions of the four paired paranasal sinuses are unknown. Some degree of nasal sinus respiration—maxillary nasal breathing—was proved by Cottle. The function of the sinuses includes dampening of sudden increased intranasal pressure, voice resonance, possible participation in olfaction, and humidification of inspired air as well as decreasing the weight of the skull. The maxillary and ethmoidal sinuses are present at birth, whereas the sphenoidal and frontal sinuses appear by the second to third years of life. Complete sinus development usually occurs by the eighteenth year of life. The frontal sinus varies greatly in size and shape. The nasofrontal duct drains into the frontoethmoidal recess in the middle meatus. Ten percent to 12% of adults have a rudimentary frontal sinus or lack pneumatization of the frontal bone.

The ethmoidal sinus is the most developed sinus at birth. The cells in the medial wall of the orbit in adults vary greatly in size and number. The ethmoidal sinus is separated from the orbit by a very thin layer of bone, the lamina papyracea. Infections can readily penetrate the thin bone and rapidly cause orbital complications. The anterior and middle ethmoidal cells drain into the middle meatus, whereas the posterior cells drain into the superior meatus. The ostia of the ethmoidal sinus are approximately 1 to 2 mm in diameter.

The maxillary sinus usually is present at birth. The fully developed maxillary sinus is pyramidal. The ostium drains through the membranous middle meatus. The average diameter is 2.5 mm, with a cross-sectional area of approximately 5 to 6 mm2. One or more accessory ostia frequently are present in the anterior middle meatus. The accessory ostia, when present, are located in the infundibulum or the membranous region of the medial sinus wall. The sphenoidal sinus begins to develop by the second or third year of life and usually is completely pneumatized by the seventeenth or eighteenth year of life. The sphenoidal sinus drains into the sphenoethmoidal recess.

The pertinent anatomic relations of the middle meatus are important to understanding the concept of sinusitis. It is now known that in most instances maxillary sinusitis is caused by disease in the ostiomeatal complex. Mucosal swelling and obstruction in this critical area can disturb the function of the sinus, and the disturbance can cause serious disease and symptoms.