Truncus Arteriosus (Common Arterial Trunk)

Truncus arteriosus — also called common arterial trunk, the term Anderson’s morphological school prefers — is a rare but well-characterized anomaly in which a single arterial vessel leaves the heart and directly gives rise to the systemic, pulmonary, and coronary circulations, rather than these arising from separate aortic and pulmonary roots. It accounts for roughly 1–4% of congenital heart disease, with an estimated incidence of 9–11 per 100,000 live births.

Definition and the Single-Outlet Family

A common arterial trunk is one member of a broader family of “single outlet” ventriculoarterial connections — alongside pulmonary atresia with a solitary aorta, and aortic atresia with a solitary pulmonary trunk, each of which has a second, atretic vessel that can’t be traced back to a ventricle. Truncus arteriosus itself has no such second atretic vessel: systemic, pulmonary, and coronary arteries all arise directly from the single truncal root through one common truncal valve. This should be distinguished from an aortopulmonary window, which describes a communication between separate aortic and pulmonary valves — a genuinely different lesion despite some superficial resemblance.

Two Classification Systems, Confusingly Sharing the Same Numerals

Two classification systems remain in active clinical use, and it’s worth knowing both — not least because they reuse the same numbering for different anatomic categories, a real source of confusion if you’re not aware of it.

Collett and Edwards classified by the origin of the pulmonary arteries:

• Type I — a short main pulmonary artery segment arises from the trunk and then divides into the branch pulmonary arteries.
• Type II — both branch pulmonary arteries arise adjacent to one another directly from the trunk, separated by a rim of truncal tissue, typically from its leftward/posterior aspect. This is the most common pattern.
• Type III — the branch pulmonary arteries arise from either side of the trunk, remote from one another (lateral aspects).
• Type IV — historically included as “no pulmonary artery arises from the ascending trunk; pulmonary circulation is supplied by collateral vessels from the descending aorta.” This is no longer considered a form of true truncus arteriosus at all — it’s now recognized as a form of tetralogy of Fallot with pulmonary atresia and systemic-to-pulmonary collateral supply, and shouldn’t be diagnosed or classified as truncus.

Van Praagh and Van Praagh, whose system is more typically used today, classify differently:

• Type 1 — the branch pulmonary arteries arise from a short main pulmonary artery (equivalent to Collett-Edwards Type I).
• Type 2 — the branch pulmonary arteries arise directly from the trunk through separate orifices (merging what Collett-Edwards splits into Types II and III).
• Type 3 — only one branch pulmonary artery arises from the ascending trunk; the contralateral lung is usually supplied by collateral vessels instead — a genuinely distinct category with different surgical implications.
• Type 4 — truncus arteriosus with aortic arch hypoplasia, coarctation, or interruption (usually type B interruption, between the left common carotid and left subclavian arteries), typically with a well-formed main pulmonary artery, a small ascending aorta, and a patent ductus arteriosus. This “Type 4” is a completely different entity from Collett-Edwards’ “Type IV” — the shared numeral is a coincidence of two independent classification traditions, not a cross-reference, and conflating them is a genuine pitfall.

Each Van Praagh type also carries an “A” (with VSD) or “B” (intact ventricular septum) modifier — truncus without a VSD is exceedingly rare.

A Shared Developmental Origin: The Conotruncal Defect Family

The aortopulmonary septum — derived from cardiac neural crest cells — normally divides the embryonic outflow tract into separate aortic and pulmonary channels. Failure of this process produces truncus arteriosus; partial or related disruption of the same neural crest lineage produces other conotruncal defects, including tetralogy of Fallot, double-outlet right ventricle, and transposition of the great arteries. This shared origin explains why truncus arteriosus clusters with these other lesions genetically and, at times, anatomically.

The 22q11 Deletion: A Major Association Requiring Active Screening

Roughly 35–40% of truncus arteriosus patients carry a chromosome 22q11 microdeletion (DiGeorge syndrome, also called velocardiofacial syndrome) — a substantial enough proportion that current recommendations call for screening every patient diagnosed with truncus arteriosus for this deletion, not just those with additional syndromic features. A right aortic arch, an abnormal aortic arch branching pattern, or both together further increase the likelihood of the deletion being present. Beyond the cardiac defect itself, 22q11 deletion/DiGeorge syndrome can include facial dysmorphism, thymic hypoplasia, parathyroid hypoplasia (with resulting hypocalcemia), learning disabilities, and later psychiatric disorders — sometimes grouped under the mnemonic CATCH-22 (Cardiac defects, Abnormal facies, Thymic hypoplasia, Cleft palate, Hypocalcemia).

Interrupted Aortic Arch: A Genuine, Bidirectional Association

Interrupted aortic arch occurs in roughly 19% of truncus arteriosus patients — nearly always type B interruption (between the left common carotid and left subclavian arteries), corresponding to Van Praagh Type 4 above. The association runs both directions: approximately 12% of patients with interrupted aortic arch also have truncus arteriosus. A right aortic arch, distinct from arch interruption, is present in roughly a third of truncus patients (~33%).

Truncal Valve and Coronary Anatomy

The truncal valve most commonly has three leaflets, though two or four are common variants, and valves with five or even six leaflets have been reported. Truncal valve insufficiency is common, typically from thickened, dysplastic leaflets, contributing diastolic volume overload on top of the VSD’s systolic shunt; stenosis is less common. The valve usually overrides the ventricular septum roughly equally between the two ventricles, but can be committed predominantly — or even exclusively — to one ventricle.

Coronary artery origins are frequently abnormal — even in three-sinus hearts, the majority of coronary orifices sit above the sinotubular junction or close to the sinus margins, rather than in the typical location for a normal aortic valve. A prominent conal branch off the right coronary artery, distributing a vascular network across the right ventricular outflow tract, and coronary arteries coursing through the infundibular free wall, are both recognized variants — the latter specifically complicates later placement of the right-ventricle-to-pulmonary- artery conduit used in surgical repair. An intramural coronary course opening directly into the pulmonary artery has also been described. A single coronary artery and unusual epicardial distribution patterns are both common. See Coronary Arteries for how to image the origins and proximal courses by echo.

The VSD

Nearly always present (truncus with an intact ventricular septum is exceedingly rare) and located subtruncally, in the “Y” formed by the septomarginal trabeculation — anatomically similar in position to the VSD of tetralogy of Fallot. The key structural difference from TOF: the conal septum is absent entirely in truncus (rather than deviated, as in TOF), the truncal valve is typically in direct fibrous continuity with the mitral valve, and the tricuspid valve is usually separated from the truncal valve by the muscularized posterior limb of the septal band — protecting the conduction tissue in most cases. Rarely, the defect is small enough to be functionally closed by the truncal valve leaflets during diastole.

Clinical Findings

• Auscultation — a loud, constant truncal valve ejection click, a single S2 (reflecting the single truncal valve), a harsh systolic murmur with thrill from the VSD, and — when truncal regurgitation is present — a diastolic decrescendo murmur.
• ECG — biatrial and biventricular enlargement.
• Chest X-ray — cardiomegaly with increased pulmonary vascular markings from pulmonary overcirculation, and a right aortic arch in the roughly one-third of cases noted above.

Pathophysiology

Systemic, pulmonary, and coronary blood all mix at the level of the common trunk, which overrides the large VSD. With unrestricted pulmonary blood flow (the typical situation, since there’s no protective pulmonary valve or right ventricular outflow obstruction), pulmonary overcirculation, pulmonary hypertension, and heart failure develop unless surgically corrected — this is why early diagnosis and prompt repair matter so much. Left untreated, truncus arteriosus is usually fatal: a mean age of death around 2.5 months, with roughly 80% of affected children dying within the first year, driven by progressive, irreversible pulmonary vascular disease from chronic unprotected pulmonary blood flow.

Echocardiographic Findings

Truncus arteriosus can be accurately diagnosed with 2D and Doppler echocardiography, and in most cases echo alone provides a complete preoperative assessment — but getting a systematic, organized study matters, since it’s easy to miss a detail if the exam isn’t structured deliberately. A complete preoperative study should evaluate: presence of an atrial communication; AV valve anatomy and function, including any straddling chordae or valve tissue across the VSD; VSD location and size, and whether additional defects are present; ventricular size and function; truncal valve morphology, location, and function; pulmonary artery anatomy (whether a main pulmonary artery segment is present, and branch position/size); other sources of pulmonary blood flow such as a ductus or aortopulmonary collaterals — especially important when a branch pulmonary artery doesn’t arise from the ascending aorta at all; aortic arch anatomy (sidedness, hypoplasia, or interruption); coronary artery origin and proximal course, and the relationship between the coronary ostia and the origins of the pulmonary arteries and truncal valve leaflets; and associated lesions such as a persistent left SVC, anomalous pulmonary venous connection, or an aberrant subclavian artery origin.

The Key Diagnostic Clue, and a Systematic View-by-View Approach

The subxiphoid view is the preferred initial approach in many centers, since it provides an image of nearly every aspect of the anatomy in sequence: the atrial septum and pulmonary venous connections at the atrial level, then — sweeping the transducer superiorly — the truncal valve overriding the VSD, and the branch pulmonary arteries with their origin from the truncus (the right pulmonary artery is usually seen just inferior and posterior to the common trunk).

The parasternal long-axis view supplies the single most recognizable diagnostic clue: an overriding single semilunar valve, often with visibly thickened leaflets, together with a malalignment-type VSD. This view often also shows the main pulmonary artery arising from the posterior aspect of the trunk, and the truncal valve itself is well seen here, with regurgitation assessable by color Doppler. Truncal stenosis can be suggested by the valve’s anatomic appearance (thickened leaflets, reduced excursion) from this view — but the parasternal long-axis view is specifically inadequate for quantifying stenosis severity, since the Doppler cursor can’t be aligned parallel to flow here. If an associated arch interruption is present, the ascending aorta can be seen arising from the rightward aspect of the truncus in this same view.

The parasternal short-axis view, with the transducer angled superiorly above the truncal valve, shows the pulmonary arteries arising from the left posterior aspect of the trunk — this view is excellent for determining which anatomic type is present (per the classification systems above) by directly imaging the branch pulmonary artery pattern, and — unlike the long-axis view — is well suited for accurate Doppler gradient measurement across the truncal valve when stenosis is suspected, including flow direction across the valve. 3D echocardiography is particularly helpful here for characterizing truncal valve commissures and leaflet morphology in detail.

The apical four- and five-chamber views, obtained by angling the transducer anteriorly toward the outflow tract, are where the large VSD and the degree of truncal override are best assessed, and — like the parasternal short-axis view above — the Doppler beam can be aligned parallel to flow here too, making the apical five-chamber view another reliable site for quantifying truncal regurgitation and stenosis. The relationship between the pulmonary artery origin and the truncal root is also clearly seen by angling further anteriorly from this position.

VSD-specific detail: at the level of the truncal valve, the defect sits within the “Y” of the septal band. A rim of muscle sometimes appears adjacent to the tricuspid valve on short-axis imaging; when the VSD instead extends all the way to the tricuspid valve — with fibrous continuity between the tricuspid and truncal valves — this indicates involvement of the membranous septum specifically.

Coronary artery imaging benefits from a specific technique: rotating the transducer clockwise from the parasternal short-axis view at the truncal valve level brings the coronary arteries into view, and angling the transducer superiorly and inferiorly from there images the coronary artery and pulmonary artery origins in the same plane — directly relevant given how often coronary anatomy is abnormal in this lesion. The parasternal long-axis and subxiphoid coronal views are also useful for this same relationship.

Suprasternal notch views provide the best imaging of the aortic arch and are used to determine arch sidedness. Long-axis views help demonstrate arch interruption when present, but short-axis views are specifically the most useful for confirming it. When the arch is truly interrupted, the descending aorta is instead supplied by the ductus arteriosus, forming a “ductal arch” — this can be distinguished from a true aortic arch by tilting the transducer to the left of the sagittal plane to image the ductal arch specifically, then sweeping right and left to confirm the ascending aorta is a separate structure. Parasternal and suprasternal views together assess for a patent ductus arteriosus — typically only present alongside arch hypoplasia or interruption, though rarely a duct can persist with an otherwise normal arch.

Doppler interrogation of the abdominal aorta (from the subxiphoid long-axis view) and the descending aorta characteristically shows diastolic flow reversal — reflecting either pulmonary run-off into the low-resistance pulmonary vascular bed, truncal valve regurgitation, or both together, which this sign alone can’t distinguish between.

Prenatal Diagnosis

Truncus arteriosus is now frequently diagnosed by fetal echocardiography, and pregnancies are typically referred for detailed fetal cardiac assessment for one of several reasons: a family history of congenital heart disease (particularly a relative with a conotruncal defect specifically); an abnormal genetic screening result, most often a 22q11 microdeletion identified by FISH testing; extracardiac findings on routine obstetric ultrasound that raise concern (such as a cleft palate, given the 22q11 association); or an incidental cardiac finding on routine screening. Reported diagnostic accuracy is high for the overall identification of truncus arteriosus, including VSD position, aortic arch anatomy, truncal valve pathology, and pulmonary artery architecture — though, as with any prenatal diagnosis, findings should still be confirmed and detailed further after birth.

Treatment Considerations

Primary surgical repair in early infancy is standard, generally performed at diagnosis rather than delayed, given the natural history above. Repair involves: separating the pulmonary arteries from the truncal root, closing the VSD to route left ventricular output through the truncal root (which becomes the neo-aorta), and establishing a new pulmonary blood supply — typically a valved conduit (often Dacron-based) connecting the right ventricle to the reconstructed pulmonary arteries. Coronary artery anatomy should be carefully mapped preoperatively given how frequently it’s abnormal, since this directly affects surgical approach and risk. See Ventricular Septal Defect for the isolated-lesion picture of the VSD component, and Aortic Regurgitation for the broader framework relevant to truncal valve insufficiency assessment.

Postoperative Follow-Up

A structured postoperative echocardiographic checklist should specifically cover: the presence, size, and flow velocity of any residual VSD(s); truncal valve function and truncal root dimension; right-ventricle-to-pulmonary- artery conduit stenosis or regurgitation (conduits require replacement over time as the patient grows and the prosthetic material degenerates — see Prosthetic Heart Valves for the broader picture of conduit and prosthesis follow-up); branch pulmonary artery stenosis; RV systolic pressure; AV valve function; ventricular size and function; and the aortic arch specifically, if obstruction or interruption was noted at initial presentation. Modern surgical outcomes are markedly better than the historical natural history: current aggregate operative mortality runs around 9.5%, with many high-volume centers now achieving under 5%.

Clinical Importance

Truncus arteriosus sits within a broader family of neural-crest-derived conotruncal defects, and its strong, bidirectional associations with 22q11 deletion and interrupted aortic arch mean that diagnosis of one should prompt active screening for the others — not passive awareness. See Tetralogy of Fallot for the related conotruncal lesion sharing much of the same developmental origin and VSD anatomy.