1

What is the diaphragm?

The diaphragm is a dome-shaped musculofibrous septum that separates the thoracic and abdominal cavities.

2

How does the diaphragm develop embryologically?

The pleuroperitoneal membrane originates from the body wall, dorsal mesentery of the oesophagus and septum transversum (Figure 1A).

At week three, the folded septum picks up its segmental nerve supply from C3, 4 and 5.

During the subsequent weeks, the dorsal end of the embryo grows faster than its ventral part, resulting in apparent descent of the septum to a thoracic level, stretching the phrenic nerve with it.

The septum transversum does not completely separate the thoracic and peritoneal cavities but leaves large defects on either side, known as the pericardioperitoneal canals (Figure 1B).

At week eight, the pleuroperitoneal membrane forms over the canals and closes the connection between the thoracic and peritoneal cavities.

The superior part of the septum transversum gives rise to the central tendon, while myoblasts from the septum invade the pleuroperitoneal membrane resulting in the muscular part of the diaphragm (Figure 1C).

3

What are the origins and insertions of the diaphragm?

Origins:

a)

sternal – posterior surface of the xiphoid process;

b)

costal – lower six ribs bilaterally, interdigitating with the origin of transversus abdominis;

c)

lumbar – medial and lateral arcuate ligaments;

d)

lumbar vertebrae 1, 2 and 3, which give origin to the two crura.

Insertion – central tendon, which blends with the fibrous pericardium.

4

What are the anatomical components of the diaphragm (Figure 2)?

Central tendon – which is the flat aponeurotic part at the summit of the dome of the diaphragm.

Muscular part – circumferential muscular slips that interdigitate with the anterior abdominal muscles and intercostal muscles (Figure 3).

Crura (musculotendinous structures):

a)

right crus – which arises from the right half of the anterior surface of the L1-3 vertebral bodies and intervertebral ligaments (covered by the anterior longitudinal ligament). It completely surrounds the oesophagus and is larger and longer than the left crus;

b)

left crus – which arises from the left half of the L1-2 vertebral bodies as a tendinous ligament but changes into a flat muscle that inserts into the central tendon.

Median arcuate ligament – which represents a tendinous arch formed by the insertion of the left and right crura. It acts as the roof of the aortic hiatus.

Medial arcuate ligament (medial lumbocostal arch) – which represents thickening of the thoracolumbar fascia over the upper part of the psoas major muscle and extends from the crus to the transverse process of the L1 vertebral body.

Lateral arcuate ligament (lateral lumbocostal arch) – which is a thickening of the thoracolumbar fascia over the upper part of the quadratus lumborum muscle and forms a tendinous arch between the transverse process of L1 and the 12th rib.

The diaphragm is covered by pleura and peritoneum except at the ‘bare area’ of the liver.

5

Describe the structures that pass through the openings in the diaphragm (Figure 4)

Caval hiatus (T8) – opening in the central tendon:

a)

inferior vena cava (IVC) – which dilates during inspiration, as the diaphragm is adherent to the margins of the inferior vena cava;

b)

right phrenic nerve.

The left phrenic nerve pierces the central tendon of the left hemidiaphragm.

Oesophageal muscular hiatus (T10) – opening through the right crus:

a)

oesophagus – as the fleshy fibres of the right crus form the oesophageal sphincter, contraction of the diaphragm prevents reflux;

b)

anterior and posterior vagal nerves – which are composed of the mixing branches of the right and left vagal nerves, forming the oesophageal plexus, that reunite into the two anterior and posterior vagal nerves in the abdomen.

Aortic hiatus (T12) – opening under the median arcuate ligament between the right and left crura:

a)

descending thoracic aorta – as the aortic hiatus does not anatomically pierce the diaphragm, the aorta is not affected by its contraction;

b)

azygos vein;

c)

thoracic duct.

Minor openings:

a)

lesser apertures of the right crus – greater and lesser right splanchnic nerves;

b)

lesser apertures of the left crus:

i)

greater and lesser left splanchnic nerves;

ii)

hemiazygos vein;

c)

behind the medial arcuate ligament – sympathetic trunk;

d)

behind the lateral arcuate ligament – subcostal (T12) neurovascular bundle.

6

What is the blood supply and innervation of the diaphragm?

Arterial supply:

a)

superior and inferior phrenic arteries;

b)

musculophrenic and pericardiacophrenic arteries (branches of the internal thoracic artery).

Venous drainage:

a)

musculophrenic and pericardiacophrenic veins (tributaries of the internal thoracic vein);

b)

inferior phrenic vein (tributary of the IVC on the right and suprarenal vein on the left).

Nerve supply:

a)

phrenic nerve (C3, 4 and 5 nerve roots);

b)

lower six intercostal nerves (peripheral 1-2 inches of the diaphragm perimeter).

7

What is the course of the phrenic nerve?

The phrenic nerve is formed from the 3rd, 4th and 5th cervical nerve roots, which unite behind the scalene muscles.

The nerve descends towards the thoracic inlet, crossing in front of the scalenus anterior muscle from its lateral to medial border.

Emerging from the medial border of the muscle, it descends between the subclavian artery and vein as it enters the thoracic inlet.

On the right side, the phrenic nerve continues to descend just anterolateral to the superior vena cava (SVC), anterior to the hilum of the lung and lateral to the right atrium, to continue descending lateral to the IVC passing through the caval hiatus.

On the left, it crosses the arch of the aorta to pass anterior to the hilum. On the arch, it crosses anterior to the superior intercostal vein, which joins the innominate vein. The phrenic nerve continues lateral to the pericardium until it pierces the central tendon of the left hemidiaphragm.

The phrenic nerve then divides into radially spreading branches, supplying both motor and sensory innervation to the hemidiaphragm, except the peripheral 1-2 inches of the diaphragmatic perimeter, which is segmentally supplied by the lower six intercostal nerves.

8

What is the physiological function of the diaphragm?

The diaphragm is the main muscle of respiration.

Contraction of the diaphragm causes it to descend, thereby increasing the longitudinal axis of the thorax (piston movement) and generating a negative pleural pressure. This causes the lungs to inflate during inspiration, as the negative pressure is translated into volume change.

Rib movement during inspiration increases the anteroposterior and lateral dimensions which, combined with the descent of the diaphragm, result in a three-dimensional increase in the thoracic volume.

Relaxation of the diaphragm causes it to ascend, thereby producing deflation of the lungs during expiration.

Secondary functions of the diaphragm include:

a)

abdominal straining, such as during defecation, micturition and parturition;

b)

improving venous return by:

i)

increasing the intra-abdominal pressure, thereby squeezing the abdominal veins;

ii)

increasing the negative intrapleural pressure, thereby aiding the suction of blood into the caval vessels and heart.

9

What are the landmarks of the diaphragm on a normal chest X-ray (Figure 5)?

The dome of the right hemidiaphragm is 1-2 intercostal spaces higher than the left hemidiaphragm (in mid-inspiration).

The left dome is never normally higher than the right.

The dome of the right hemidiaphragm intersects the mid-clavicular line at the level of the 6th rib anteriorly.

The diaphragm subtends an acute angle with the lateral chest wall, called the costophrenic angle. In patients with:

a)

severe emphysema, the angle is widened as the diaphragm becomes flat (splinting of diaphragm);

b)

pleural effusion, the costophrenic angle will be obliterated with 500mL of fluid.

The angle subtended between the right dome of the diaphragm and the right border of the heart is called the cardiophrenic angle. It can be obliterated by pericardial fat or a collapsed middle lobe.

The gastric bubble (air under the diaphragm) is a normal finding under the left hemidiaphragm but air under the right hemidiaphragm is not normal.

Very rarely, normal loops of small or large bowel intervene between liver and diaphragm.

10

What are the causes of diaphragmatic paralysis?

Paralysis of the diaphragm can be unilateral or bilateral, an isolated phenomenon or part of a generalised neuromuscular disorder, acute or chronic, congenital or acquired, an upper motor neuron or a lower motor neuron lesion.

Central nervous system lesions:

a)

cord transection above C3, 4 and 5, including trauma and transverse myelitis;

b)

‘hangman’s fracture’, which represents a spinal fracture of both pedicles of the axis vertebra (C2) that may result in cord transection at the level of C2. Both phrenic nerves are lost and voluntary breathing is not possible;

c)

multiple sclerosis.

Motor neuron lesions:

a)

amyotrophic lateral sclerosis;

b)

polio;

c)

spinal muscular atrophy.

Nerve root lesions:

a)

cervical spondylosis;

b)

trauma – road traffic accident.

Peripheral nerve lesions (phrenic nerve):

a)

trauma:

i)

accidental – road traffic accident;

ii)

iatrogenic:

–

insertion of a central venous line (subclavicular or internal jugular);

–

pericardial iced cold saline to cool the heart during cardiac surgery;

–

harvesting the internal thoracic artery for coronary artery bypass grafting;

–

thymic or thyroid surgery (accidental or deliberate);

–

injury from diathermy or ultrasonic dissecting devices during lung or mediastinal tumour resection (direct thermal or indirect ischaemic damage);

–

deliberate phrenic nerve crush to reduce postoperative pleural space, when the lung is not expected to inflate fully;

–

systematic nodal dissection, especially stations 5 and 6;

b)

infection – viral neuritis (could completely recover in 6-12 months), Herpes virus, Lyme disease (bacterial infection from deer-tick, which is endemic in the USA);

c)

compression or invasion by local structures, including:

i)

benign lymph nodes, such as sarcoidosis;

ii)

malignant lymph nodes (commonest cause), such as in the aortopulmonary window on the left side and hilar nodes on the right side;

iii)

malignant tumours, such as lung, thymic or thyroid carcinoma;

iv)

pleural plaques;

d)

polyneuropathy:

i)

neuralgic amyotrophy (hereditary and non-hereditary) – autoimmune disease leading to neural degeneration, which mainly affects the brachial plexus, leading to shoulder pains but can also affect the phrenic and recurrent laryngeal nerves. The illness can recur and may lead to wasting of muscles (deltoid, diaphragm and vocal cords);

ii)

Guillain-Barré-like syndromes;

iii)

chronic inflammatory and intensive care polyneuropathy;

iv)

Charcot-Marie-Tooth disease (distal hereditary motor neuropathy);

v)

acute porphyria;

e)

collagen disease or vasculitis:

i)

systemic lupus erythematosus (SLE) – vanishing lung syndrome, which is a rare disease, where alveolar walls gradually disintegrate to form large bullae and air spaces leading to compression of normal parenchyma. Patients usually die of respiratory failure;

ii)

granulomatosis with polyangiitis;

f)

radiotherapy;

g)

chemotherapy;

h)

diabetic neuropathy;

i)

idiopathic (common).

Myopathic lesions:

a)

SLE;

b)

dermatomyositis;

c)

systemic sclerosis;

d)

endocrine:

i)

hypothyroidism;

ii)

hyperthyroidism;

iii)

Cushing’s disease;

e)

intensive care myopathy;

f)

amyloidosis;

g)

idiopathic.

11

What is the pathophysiology of diaphragmatic paralysis (Figure 6)?

Paralysis of a hemidiaphragm causes paradoxical ascent of the ipsilateral hemidiaphragm during inspiration.

As part of the energy of contraction is dissipated into the paradoxical elevation, as well as loss of rigidity of the central tendon to pull against, the contralateral hemidiaphragm does not descend to its previous level.

This results in reduced ventilation on the contralateral side, with minimal ventilation on the paralysed side

Associated with this, collapse of the ipsilateral lung adds to the physiological shunt and V/Q mismatch, resulting in dyspnoea, especially on effort or when the intra-abdominal pressure rises, such as in the supine position.