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Hemidiaphragm

The term "hemidiaphragm" refers to one half (left or right) of the dome-shaped musculotendinous diaphragm that separates the thoracic and abdominal cavities. Clinically, it is most commonly encountered as the elevated hemidiaphragm - an important radiological sign with a broad differential.

Anatomy

The diaphragm is a thin musculotendinous structure filling the inferior thoracic aperture. Peripheral muscle fibers converge on a central tendon, to which the pericardium is attached.

Attachments:

Sternal - xiphoid process
Costal - 6th to 12th ribs (costal margin + ends of ribs XI and XII)
Lumbar - arcuate ligaments, right crus to L3, left crus to L2; median arcuate ligament crosses the aorta

Key openings (hiatuses):

Opening	Level	Contents
Caval hiatus (central tendon)	T8	Inferior vena cava, right phrenic nerve
Oesophageal hiatus	T10	Oesophagus, vagus nerves
Aortic hiatus (posterior to diaphragm)	T12	Aorta, thoracic duct, azygos/hemiazygos veins

Innervation: Phrenic nerves (C3, C4, C5) - "C3, 4, 5 keep the diaphragm alive." They innervate the diaphragm from its abdominal surface. The sensory supply of the periphery is from the lower intercostal nerves (T5-T11), which is why subphrenic irritation can refer pain to the shoulder tip (phrenic) or the anterior thoracic/abdominal wall (intercostal).

Blood supply:

From above: pericardiacophrenic and musculophrenic arteries (branches of internal thoracic arteries); superior phrenic arteries from the thoracic aorta
From below: inferior phrenic arteries (the largest supply, directly from the abdominal aorta)
Gray's Anatomy for Students, p. 198

Normal Radiological Appearances

On a PA chest X-ray, each hemidiaphragm is a smooth, curved line convex upward. The right typically lies at the level of the 5th or 6th anterior rib interspace at full inspiration.

In >90% of normal people, the right hemidiaphragm is higher than the left (difference ~15 mm, up to 30 mm is still normal). This is because the liver elevates the right, and the gastric air bubble lowers the left.
The left hemidiaphragm is often partially obscured anteriorly by the heart.
The costophrenic angle should be sharply defined and acute - blunting suggests pleural fluid.
Grainger & Allison's Diagnostic Radiology, p. 78

Elevated Hemidiaphragm

Diaphragmatic paralysis should be suspected when an entire hemidiaphragm is elevated by >2 cm compared with the contralateral side.

Unilateral Elevation - Causes

The causes are organized by anatomical level:

Pulmonary/pleural (above the diaphragm):

Pulmonary collapse / atelectasis
Pulmonary hypoplasia
Pneumonia or pleurisy
Pulmonary thromboembolism

Diaphragm itself:

Phrenic nerve palsy (most important - see below)
Eventration (congenital focal weakness, forming a focal bulge)
Diaphragmatic rupture / traumatic hernia

Subphrenic (below the diaphragm):

Subphrenic abscess or infection
Subphrenic mass (liver tumour, hepatomegaly, splenic mass)
Gaseous distension of stomach or colon

Positional/structural:

Lateral decubitus position (dependent side)
Dorsal scoliosis
Rib fracture and other painful conditions

Mimics of elevated hemidiaphragm:

Subpulmonary pleural effusion
Large well-defined tumour adjacent to the dome
Combined middle and lower lobe collapse
Grainger & Allison's Diagnostic Radiology, Table 3.4

Bilateral Elevation - Causes

Cause
Supine position
Poor inspiratory effort
Obesity
Pregnancy
Abdominal distension (ascites, obstruction, mass)
Diffuse pulmonary fibrosis
Lymphangitis carcinomatosa
Disseminated SLE
Bilateral basal pulmonary emboli
Painful conditions (e.g., post-abdominal surgery)
Bilateral diaphragmatic paralysis

Grainger & Allison's Diagnostic Radiology, Table 3.3

Diaphragmatic Paralysis

Unilateral diaphragmatic paralysis (phrenic nerve palsy) is the most clinically significant cause of an elevated hemidiaphragm.

Common causes of phrenic nerve palsy:

Malignancy (lung cancer invading the mediastinum is the most common)
Cardiac/thoracic surgery (post-operative phrenic nerve injury - occurs in 2-20% of cardiac surgeries)
Cervical spine disease
Neuralgic amyotrophy (Parsonage-Turner syndrome)
Mediastinal pathology (lymphoma, aortic aneurysm)
Idiopathic

Diagnosis:

CXR: hemidiaphragm elevated by >2 cm compared with the normal side
Fluoroscopic sniff test (gold standard): paradoxical upward (cephalad) motion of the paralyzed hemidiaphragm with a sharp sniff maneuver, while the normal side descends
Ultrasound: paralyzed hemidiaphragm is thinner at rest (~1-2 mm vs normal ~2-3 mm), shows little thickening during inspiration, and may move paradoxically. The contralateral working hemidiaphragm thickens more than normal to compensate.
Fishman's Pulmonary Diseases and Disorders; Murray & Nadel's Textbook of Respiratory Medicine

Eventration

Eventration is a focal weakness or thinning of the diaphragmatic muscle resulting in a focal bulge or protrusion, most commonly on the right side (the most common diaphragmatic abnormality overall). A minor eventration is often an incidental finding. A large eventration (especially left-sided) can simulate diaphragmatic hernia and may contain viscera.

Diaphragmatic Hernias

When a hemidiaphragm is newly elevated following blunt trauma, traumatic diaphragmatic rupture with herniation must be excluded. Features:

More common on the left (the liver protects the right side)
Herniation of abdominal viscera may occur days to years after the trauma
CT/MRI in coronal and sagittal planes is the reference standard

Other hernia types: Hiatal (most common, through the oesophageal hiatus), Morgagni (anterior/medial), Bochdalek (posterior/central - more often diagnosed in infancy).

Fishman's Pulmonary Diseases and Disorders, p. 519

Quick Summary Table

Feature	Right Hemidiaphragm	Left Hemidiaphragm
Normal height	Higher (liver underneath)	Lower (gastric bubble)
Landmarks	5th-6th anterior rib space	Slightly lower than right
Distinction	Right dome clearly visible	Partially obscured by heart anteriorly
Phrenic nerve palsy	More commonly right (cardiac surgery)	Lung cancer invasion

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