Hiatus (anatomy)

A hiatus is a natural gap or cleft in an anatomical structure, usually one through which a vessel, nerve or organ passes from one compartment to another.

Hiatus is Latin for a gap or opening, and the word is used in Nomina Anatomica for several different structures that share a slit-like shape rather than a common function.

The best-known examples are the oesophageal and aortic hiatuses of the diaphragm, but the term also covers the adductor hiatus, the sacral hiatus and the maxillary hiatus.

Definition

• A hiatus is a slit or gap between two parts of the same structure or between two adjacent structures, and it is usually lined by the tissue that continues through it.
• Hiatus differs from foramen because a foramen is a rounded opening through bone, while a hiatus is often a gap between muscles, membranes or cartilages.
• Hiatus also differs from fissure because a fissure is a deeper cleft that separates two lobes or parts, whereas a hiatus is a passage rather than a boundary.
• The Latin plural is hiatus, unchanged, and English writers sometimes use hiatuses, although anatomical usage prefers the Latin form.
• A hiatus may be bounded by bone, ligament, muscle or aponeurosis, and its compliance depends on which of these forms the margin.

Examples

• The oesophageal hiatus is an opening in the right crus of the diaphragm at the level of the tenth thoracic vertebra and transmits the oesophagus and the vagal trunks.
• The aortic hiatus lies at the level of the twelfth thoracic vertebra between the crura of the diaphragm and transmits the aorta, thoracic duct and azygos vein.
• The adductor hiatus is the opening in the adductor magnus through which the femoral vessels pass from the adductor canal into the popliteal fossa.
• The sacral hiatus is the lower opening of the sacral canal, where the filum terminale and the sacral nerve roots emerge.
• The maxillary hiatus is the large opening of the maxillary sinus into the middle meatus of the nasal cavity, partly narrowed by the uncinate process and ethmoid bulla.
• The semilunar hiatus is a curved groove in the middle meatus that receives the openings of the frontal, maxillary and anterior ethmoidal sinuses.
• The hiatus for the greater petrosal nerve is a small opening on the anterior surface of the petrous temporal bone that transmits that nerve.
• The urogenital hiatus is the anterior opening of the pelvic floor through which the urethra and vagina pass, and its integrity is central to pelvic organ support.
• The hiatus of the facial canal, also called the hiatus Fallopii, transmits the greater petrosal nerve from the geniculate ganglion.
• The scapular hiatus is a gap in the supraspinatus muscle or fascia that can be used as a landmark in shoulder surgery.

Structure

• The margins of a hiatus are usually reinforced by a fibrous ring or by the tendinous edge of a muscle, which prevents excessive stretching.
• A hiatus that transmits a pulsating vessel is often surrounded by loose connective tissue that allows movement without friction.
• The size of a hiatus is not fixed, because the surrounding muscle can contract, relax or atrophy with age.
• The oesophageal hiatus is formed mainly by the right crus of the diaphragm, whose fibres decussate around the oesophagus.
• The phrenoesophageal ligament attaches the oesophagus to the diaphragm at the hiatus and helps maintain the gastro-oesophageal angle.

Function

• A hiatus allows a structure to cross a barrier without losing the barrier's separating function, as the diaphragm still separates thorax from abdomen around the oesophagus.
• The aortic hiatus permits the descending aorta to pass behind the diaphragm while the crura anchor the diaphragm to the vertebral column.
• The adductor hiatus lets the femoral vessels change direction from the anterior thigh to the posterior knee without being compressed by the adductor magnus.
• The sacral hiatus provides access to the sacral canal for caudal anaesthesia and is a landmark for identifying the sacral cornua.

Clinical significance

• Hiatus hernia occurs when part of the stomach moves through the oesophageal hiatus into the thorax, and it is classified as sliding or paraoesophageal.
• Large hiatus hernia can cause reflux, dysphagia, iron-deficiency anaemia and, in paraoesophageal hernia, gastric volvulus.
• Sacral hiatus is the target of caudal epidural injection, and an abnormally high or closed hiatus can make the procedure difficult.
• Adductor canal syndrome, also called Hunter canal syndrome, may compress the femoral vessels or saphenous nerve at the adductor hiatus.
• Maxillary hiatus obstruction contributes to maxillary sinusitis, and endoscopic middle meatal antrostomy enlarges the natural ostium.
• Urogenital hiatus enlargement is associated with pelvic organ prolapse and is assessed by perineal ultrasound and magnetic resonance imaging.
• A hiatal hernia can complicate pregnancy and chronic cough, because raised intra-abdominal pressure widens the hiatus.

Development

• The diaphragmatic hiatuses form as the pleuroperitoneal membranes close and the crura migrate around the oesophagus and aorta.
• The oesophageal hiatus reaches its adult position at about the tenth thoracic level as the stomach descends during the second trimester.
• Congenital hiatal hernia results from failure of the pleuroperitoneal canal to close or from abnormal crural development.
• The sacral hiatus is formed by failure of the laminae of the lower sacral vertebrae to fuse in the midline.
• The adductor hiatus appears when the adductor magnus separates into its pubic and ischial parts around the femoral vessels.

Comparative anatomy

• The oesophageal hiatus is present in all mammals, but its position relative to the vertebral column varies with posture and trunk length.
• Birds have a distinct arrangement of the diaphragm and air sacs, and the oesophageal hiatus is replaced by a different opening in the coelomic septum.
• The adductor hiatus is conserved in mammals with a large adductor magnus, but it is reduced in species whose hind limbs are adapted for digging.
• Reptiles and amphibians lack a muscular diaphragm and therefore lack true diaphragmatic hiatuses, although they have other gaps for the oesophagus and aorta.

History

• Classical anatomists described the diaphragmatic openings as perforations rather than hiatuses, and the modern term was fixed in later Latin nomenclature.
• The adductor hiatus was described with the adductor canal by John Hunter and later named in his honour.
• The sacral hiatus was used for caudal anaesthesia from the early twentieth century, after the technique was developed in obstetrics.
• Terminologia Anatomica lists hiatus aorticus, hiatus oesophageus and hiatus canalis nervi petrosi majoris as preferred terms.

Research and controversies

• The classification of hiatal hernia into sliding and paraoesophageal types is clinically useful but does not capture the continuum seen on imaging.
• Whether the phrenoesophageal ligament should be repaired routinely during anti-reflux surgery is still debated.
• Measurements of the urogenital hiatus by ultrasound are not standardised, so prevalence estimates for prolapse vary between studies.
• Some authors argue that the term hiatus should be restricted to openings between muscles and membranes, excluding bony openings that are better called foramina.