Overview
The
diaphragm (derived from the Ancient Greek term ‘diáphragma’ meaning
‘partition’) is a dynamic structure shaped like a parachute, which is composed
of skeletal muscle and fibrous tissue. Its two main functions are to separate
the two major cavities (acting as the inferior border of the thorax and the
superior border of the abdomen) and to aid in respiration. Contraction and
relaxation of the diaphragm creates changes in volume and pressure, thus
encouraging movement of air in and out of the thoracic cavity.
The
diaphragm is anchored in place by a number of tendons and is comprised of two
domes known as the left and right hemidiaphragms. It also has a number of
diaphragmatic openings transmitting important structures from the thorax to the
abdomen.
Gross Anatomy
As
previously mentioned, the diaphragm is said to have a left and right
hemidiaphragm, the right lying slightly higher than the left due to the large
size of the liver. The muscles fibres extend from the xiphoid process and costal
margin anteriorly, ribs 6-12 laterally and T12 and upper lumbar vertebrae (and
the anterior longitudinal ligament of the vertebral column) posteriorly. These
attachments converge at different points to form the following tendinous
structures:
· CENTRAL TENDON
o
Formed
by the convergence of the peripheral attachments, the central tendon is a
strong aponeurosis lying slightly anterior within the muscle
o
It
contains multi-directional muscle fibres which add to its strength
o
Continuous
with the inferior surface of a small area of fibrous pericardium
· RIGHT + LEFT CRURA
o
Are
continuous with the anterior longitudinal ligament of the vertebral column
o
Arise
from the anterior surfaces of the upper lumbar vertebrae
o
The
right crus is larger and longer than the left crus, and contains some fibres
which surround the oesophagus to act as a physiological sphincter
· INTERNAL ARCUATE LIGAMENT (or medial
lumbocostal arch)
o
Tendinous
arch derived from the fascia overlying the superior part of the psoas major
muscle
o
Attaches
to the body and transverse processes of the L1 or L2 vertebra
· EXTERNAL ARCUATE LIGAMENT (or
lateral lumbocostal arch)
o
Arches
across the superior part of the quadratus lumborum muscle
o
Attaches
to the transverse processes of L1 and to the lower margin of rib 12
Diaphragmatic Openings
The
diaphragm has 3 main apertures which allow passage of structures from the
thorax to the abdomen. The first is the caval
opening which passes through the central tendon. It is found at T8 level
and transmits the inferior vena cava
and some minor branches of the right phrenic nerve. The second is the oesophageal hiatus found at T10 level.
It lies posterior and slightly left of the central tendon and conveys the
oesophagus and the anterior and posterior vagal
trunks. The third is the aortic
hiatus found posteriorly between the left and right crura, which allows the
aorta, azygos vein and thoracic duct to pass into the abdomen.
Other
smaller apertures include the lesser apertures of the right and left crus,
which transmit the right greater and lesser splanchnic nerves and the left
greater and lesser splanchnic nerves (along with the hemiazygos vein)
respectively. The sympathetic trunk passes into the abdomen posteriorly to the
diaphragm under the internal arcuate ligament.
The
Diaphragm as a Muscle
Attachments |
Action |
Blood Supply/Drainage |
Innervation |
Xiphoid process, costal cartilages of ribs 6-12, lumbar
vertebrae + arcuate ligaments |
Contraction – increases volume of thoracic cavity,
decreases intra-thoracic pressure and draws air into lungs (Passive) Relaxation – decreases volume of thoracic
cavity, increases intra-thoracic pressure and air moves out of lungs |
Arterial: inferior
phrenic (also superior phrenic, musculophrenic + pericardiacophrenic
arteries) Venous: inferior
phrenic (also superior phrenic, musculophrenic + pericardiacophrenic veins |
Phrenic
nerve (C3, C4,
C5 from the cervical plexus) Also the lower intercostal nerves |
Clinical Anatomy
Pneumoperitoneum
Air within the peritoneal cavity, or
pneumoperitoneum, can be identified on an erect chest X-ray by the presence of
air under the diaphragm. It is a worrying sign as it is an indicator of a
perforated hollow viscus, such as a perforated peptic ulcer. However,
pneumoperitoneum can also be induced, as in laparoscopic abdominal surgery and
is still seen radiologically for up to 48 hours post-operatively.
Diaphragmatic paralysis
This often occurs secondary to a lesion of the
phrenic nerve, though could also result from interruption of innervation at the
cervical spinal cord level or at the brainstem. Causes include compression from
tumours (particularly bronchial cancers), trauma (including iatrogenic damage
from surgery), neuropathies and myopathies. Paralysis of the diaphragm gives
rise to converse diaphragmatic movement, and consequently the diaphragm moves
up on inspiration and down on expiration. This can be asymptomatic if a unilateral
paralysis is present and is often found incidentally on a chest X-ray. However,
if both sides are affected the patient can present with shortness of breath and
become easily fatigued. Management of this condition depends on the cause but
continuous positive airway pressure (CPAP) can be of benefit.
Herniation
A hiatus
hernia involves abnormal protrusion of the lower oesophagus and/or the
stomach through the oesophageal hiatus secondary to weakness or a tear in the
diaphragm itself. It is strongly associated with obesity and is a common cause
of acid reflux in adults due to an inability to maintain pressure on the lower
oesophageal sphincter. If symptomatic, management can range from conservative measures,
such as weight loss and avoiding lying flat after meals, to medical management including
proton pump inhibitors or H2 receptor antagonists. If the symptoms become very
severe surgery may be implicated, namely a procedure called fundoplication,
which involves wrapping the fundus of the stomach around the oesophagus to
prevent herniation.
A congenital
diaphragmatic hernia is a life-threatening condition that occurs when the
pleuroperitoneal membrane fails to fuse, leaving a space in which abdominal
contents can protrude into the thoracic cavity. The majority of cases occur on
the left and can lead to hypoplasia of the developing lung. The most common
form of congenital diaphragmatic hernia is known as a Bochdalek hernia, which
is due to a posterolateral defect in the diaphragm. Definitive management is
surgical.
Quick Anatomy
Key Facts
Aide-Memoire
Innervation
to the diaphragm:
C3, 4, 5
keeps the diaphragm alive
DiaPHRagm is innervated by the PHRenic nerve
The
diaphragmatic openings, their main constituents and vertebral level can be
remembered using this quick rule:
VENA
CAVA has 8 letters and the caval opening occurs at T8 level
OESOPHAGUS has 10 letters and the oesophageal hiatus occurs at T10 level. It also transmits the VAGUS NERVE (10 letters) and is the 10th cranial nerve
AORTIC
HIATUS has 12 letters and occurs at T12 level
Summary
The diaphragm is a thin flat muscle separating the thorax from the
abdomen and plays a vital role in respiration. It is derived embryologically by
fusion of the septum transversum and the pleuropericardial membrane. It is
composed of the two hemidiaphragms (the right is slightly larger due to the
liver) and attaches peripherally to the body walls by muscular fibres and
tendinous structures, such as the central tendon, right crus and left crus. A
number of diaphragmatic apertures transmit important structures, such as the
aorta and inferior vena cava, from the thoracic cavity to the abdomen.