Sinus (anatomy)

A sinus is a sac or cavity within an organ or tissue, and the word is also used for an abnormal cavity or a passage that connects two surfaces.

In everyday usage sinus usually means a paranasal sinus, the air-filled cavity in the facial bones that opens into the nasal cavity.

Because the same word covers venous channels, arterial swellings, lymphatic spaces and organ recesses, the precise meaning must always be taken from context.

Etymology

• Sinus is Latin for a bay, pocket, curve or bosom, and the anatomical sense developed from the idea of a recess or hollow.
• The word entered English through medical Latin and was applied to cavities that seemed to form a bay within a larger structure.
• Sinusitis means inflammation of a sinus, and it is distinct from a fistula, which is an abnormal tract between two epithelial surfaces.
• The plural is sinuses in English and sinus in Latin, and anatomical usage varies between the two forms.
• The adjectival form sinusoid describes a small sinus or a sinusoidal capillary, and it is not interchangeable with sinus.

Sinuses in the body

• The paranasal sinuses are the maxillary, ethmoid, sphenoid and frontal sinuses, and they are paired and air-filled.
• The maxillary sinus is the largest paranasal sinus, and it lies in the body of the maxilla below the orbit.
• The ethmoid sinuses are a group of small air cells between the orbits and the upper nasal cavity.
• The sphenoid sinus lies within the body of the sphenoid bone and is related to the pituitary gland and the cavernous sinus.
• The frontal sinus lies in the frontal bone above the orbit and drains into the middle meatus through the frontonasal duct.
• The dural venous sinuses are channels between the layers of the dura mater that drain blood from the brain into the internal jugular veins.
• The superior and inferior sagittal sinuses, straight sinus, transverse sinuses and sigmoid sinuses form the main dural drainage pathway.
• The cavernous sinus lies beside the sella turcica and contains the internal carotid artery and several cranial nerves.
• The carotid sinus is a dilation of the internal carotid artery at its origin, and it contains baroreceptors that sense blood pressure.
• The coronary sinus is a venous channel in the posterior heart that collects most cardiac venous blood and opens into the right atrium.
• The renal sinus is the fat-filled space around the renal pelvis, and it contains the renal vessels, nerves and collecting system.
• The costodiaphragmatic recess is a pleural sinus, a potential space between the lung and the diaphragm at the lower border of the pleural cavity.
• Lymph node sinuses include the subcapsular, trabecular and medullary sinuses, and they provide a route for lymph and antigen-presenting cells.

Paranasal sinuses

• The paranasal sinuses develop as outpouchings of the nasal mucosa into the surrounding bones, and they enlarge through childhood and adolescence.
• The maxillary sinus is present at birth, while the frontal and sphenoid sinuses become pneumatised later in childhood.
• Sinus pneumatisation varies greatly between individuals, and asymmetry is common and usually harmless.
• The functions of the paranasal sinuses are not fully established, and proposed roles include voice resonance, air humidification and skull weight reduction.
• The sinuses may also act as shock absorbers for facial trauma and as thermal insulators for the orbit and brain.
• Sinus development is clinically relevant because incomplete pneumatisation can mimic disease on imaging.

Function

• The paranasal sinuses lighten the facial skeleton and may improve resonance of the voice.
• They add moisture and warmth to inhaled air and contribute to the conditioning of air before it reaches the lower airway.
• The dural venous sinuses collect cerebral venous blood and cerebrospinal fluid and direct them toward the internal jugular veins.
• The carotid sinus baroreceptors detect stretch and trigger reflex changes in heart rate and vascular tone.
• Lymph node sinuses slow lymph flow and allow macrophages and dendritic cells to sample antigens.
• The renal sinus accommodates the renal pelvis and allows the kidney to change volume without compressing its vessels.

Clinical significance

• Acute sinusitis is usually viral and lasts less than four weeks, with congestion, facial pain and purulent discharge.
• Chronic rhinosinusitis lasts twelve weeks or longer and is now classified by inflammatory endotype rather than symptoms alone.
• Maxillary sinusitis can spread to the orbit or to the cavernous sinus, and ethmoid sinusitis is especially dangerous in children.
• Cavernous sinus thrombosis is a rare but serious complication of facial infection, and it produces headache, ophthalmoplegia and proptosis.
• Dural venous sinus thrombosis can cause intracranial hypertension and venous infarction, and it is diagnosed by magnetic resonance venography.
• Carotid sinus hypersensitivity can cause syncope through excessive vagal activation, and carotid sinus massage is used diagnostically.
• Renal sinus fat increases with obesity and may compress the renal pelvis or vessels, although its clinical significance is debated.
• Functional endoscopic sinus surgery enlarges sinus ostia and removes obstructing tissue while preserving mucosa.

Development

• The paranasal sinuses develop from nasal cavity diverticula that invade the maxilla, ethmoid, sphenoid and frontal bones.
• The maxillary sinus is the first to appear, and it reaches adult size only after the permanent teeth have erupted.
• The frontal sinus develops from the frontal recess and may remain small or absent in some adults.
• The dural venous sinuses develop from the primitive meningeal plexus and are remodelled as the skull grows.
• The carotid sinus develops from the proximal internal carotid artery and becomes functional as a baroreceptor in fetal life.

Comparative anatomy

• Paranasal sinuses are present in mammals, but their size and number vary with skull shape, diet and respiratory needs.
• Birds have air sacs rather than paranasal sinuses, and their respiratory system is organised around a different flow pattern.
• Reptiles and amphibians have simpler nasal cavities and fewer pneumatised spaces than mammals.
• The dural venous sinuses are vertebrate features, and their arrangement differs with the shape of the braincase and the pattern of jugular drainage.
• The carotid sinus is well developed in mammals and birds, and it is part of the baroreflex that stabilises blood pressure during posture change.

History

• The Latin sinus was used by classical anatomists for any hollow or recess, and its meaning narrowed only in modern nomenclature.
• Nathaniel Highmore described the maxillary sinus in the seventeenth century, and it is still called the antrum of Highmore.
• The dural venous sinuses were mapped in detail in the eighteenth and nineteenth centuries as anatomy and surgery developed.
• The carotid sinus reflex was described in the twentieth century, and carotid sinus massage became a clinical test for syncope.
• Functional endoscopic sinus surgery in the 1980s changed the treatment of chronic sinusitis and renewed anatomical study of sinus ostia.

Research and controversies

• The function of paranasal sinuses is still debated, and no single explanation accounts for their diversity across species.
• Chronic rhinosinusitis is being reclassified by inflammatory endotype, but the relationship between endotype and anatomical pattern remains unclear.
• The sinonasal microbiome is studied as a contributor to chronic inflammation, but causality has not been established.
• Whether asymptomatic sinus opacities on imaging should be treated is a common clinical dilemma.
• Anatomical studies of sinus ostia vary in measurement technique, which complicates comparison of surgical outcomes.