VSD Classification #1 — Perimembranous Defects (Outlet / Central / Inlet)

Perimembranous ventricular septal defects (pmVSDs) are defined by their anatomic “anchor” at the membranous septum and the fibrous continuity at the cardiac base. In the classic morphologic framework, the ventricular septum is considered as membranous + muscular, and the muscular septum is further subdivided into inlet, trabecular, and outlet (infundibular) components—a map that is directly useful in echo interpretation and surgical planning [1].

What your slide set captures perfectly is the practical rule:

“Perimembranous” tells you where the defect is anchored; the subtype (outlet / central / inlet) tells you where it mainly extends—and therefore what risks and operative priorities dominate. [1]

1) A surgeon’s “orientation map” for pmVSD

When describing a pmVSD, start with fixed landmarks, then define the dominant extension:

• Membranous septum (anchor zone): the key neighborhood for conduction-system proximity.
• RV geography: think inlet → trabecular → outlet (helps you predict where the shunt “opens” on the RV side) [1].
• Tricuspid valve relationship: septal/anterior leaflet tissue may partially cover the defect and can even participate in spontaneous restriction/closure (aneurysmal transformation) [2].
• Aortic root relationship (especially for outlet extension): informs risk of cusp prolapse and progressive aortic regurgitation (AR) [4].

2) Subclassification by dominant extension (the 3 phenotypes)

A) Outlet-type pmVSD

Morphology

• Perimembranous defect that extends toward the outlet (infundibular) septum and opens predominantly into the RVOT, while remaining anchored at the membranous septum [1].

Why it matters (high-yield)

• This phenotype is the one most tightly linked to right coronary cusp (RCC) prolapse and AR progression; morphologic “severity indices” of RCC prolapse/AR have been proposed to guide which outlet defects merit closure for valve preservation [4].

Operative priorities

• Protect the posteroinferior rim (conduction-risk zone), and avoid patch geometry that compromises aortic cusp support when the defect approaches the aortic annulus.

B) Central (Confluent) pmVSD

Morphology

• A large pmVSD that extends broadly into the trabecular septum, functionally communicating with multiple RV components (often described as “confluent” behavior across inlet/trabecular/outlet) [1].

Why it matters

• Predicts a larger patch footprint and a higher premium on a disciplined conduction-safe suture line (especially along the posteroinferior margin).

Operative priorities

• Plan exposure to keep the suture line controlled and reproducible; be intentional about tricuspid leaflet handling when it forms part of the border.

C) Inlet-type pmVSD

Morphology

• pmVSD with dominant extension into the inlet septum, opening mainly into the RV inlet near the septal/anterior tricuspid leaflets [1].

Why it matters

• Often implies closer interaction with tricuspid valve tissue and heightened sensitivity to conduction-axis proximity (again, posteroinferior margin discipline is everything).

Operative priorities

• Keep the patch contour “quiet” (no distortion of tricuspid apparatus), and maintain small, deliberate bites near the conduction-risk region.

3) Natural history (a key “textbook” nuance to add)

pmVSD anatomy can evolve over time—sometimes becoming restrictive or closing—often via aneurysmal transformation of adjacent tissue (commonly tricuspid valve tissue) that progressively covers the defect [2].

Practical implication: management should integrate hemodynamics + chamber remodeling + complications (AR, endocarditis risk, arrhythmias), not anatomy alone.

4) Complications that define decision-making

1) Conduction system risk is central (not optional)

Because pmVSDs sit at the membranous septum “hinge zone,” both surgical closure and device closure must be framed around AV block risk [1,5–8].

2) Aortic cusp prolapse / AR can become an indication by itself

Especially in outlet-leaning pmVSDs, progressive RCC prolapse and AR can drive closure even when shunt metrics alone do not tell the full story—because the goal becomes aortic valve preservation [4].

5) Contemporary therapy snapshot (balanced: surgery vs transcatheter)

Surgical closure (reference standard in many centers)

Modern surgical outcomes for isolated VSD repair are excellent, with very low mortality and very low rates of major rhythm complications in experienced programs [3].

In a large contemporary surgical series, perimembranous defects comprised ~80% of isolated VSD repairs, underscoring how often this morphology defines real-world practice [3].

Transcatheter closure (effective, but rhythm surveillance is the price of entry)

The major “signature” adverse event remains complete AV block (cAVB):

• In a major long-term series, pacemaker implantation for cAVB occurred in 5.7%, and the signal was strongest in younger patients (<6 years) [5].
• Other cohorts similarly emphasize AV block as a serious complication during/after device closure (e.g., reported AV block incidence 3.5% in one experience) [6], and late cAVB with clinically significant tricuspid regurgitation has been reported [7].
• Editorial commentary in the device era consistently reinforces the core message: proceed with caution and follow rhythm long-term [8].

6) “How to report” template (clinic → echo → OR)

A high-signal, surgeon-friendly line is:

“Perimembranous VSD with dominant [outlet / central-confluent / inlet] extension (membranous septal anchor), with assessment of aortic cusp support/AR, tricuspid involvement, and conduction-system risk.” [1,4–8]

References (PubMed-verified)

[1] Soto B, Becker AE, Moulaert AJ, Lie JT, Anderson RH. Classification of ventricular septal defects. Br Heart J. 1980;43(3):332-343.

[2] Anderson RH, Lenox CC, Zuberbuhler JR. Mechanisms of closure of perimembranous ventricular septal defect. Am J Cardiol. 1983;52(3):341-345.

[3] Scully BB, Morales DLS, Zafar F, McKenzie ED, Fraser CD Jr, Heinle JS. Current expectations for surgical repair of isolated ventricular septal defects. Ann Thorac Surg. 2010;89(2):544-549.

[4] Tomita H, Arakaki Y, Ono Y, Yamada O, Yagihara T, Echigo S. Severity indices of right coronary cusp prolapse and aortic regurgitation in ventricular septal defect in the outlet septum: which defect should be closed? Circ J. 2004;68(2):139-144.

[5] Butera G, Carminati M, Chessa M, Piazza L, Micheletti A, Negura DG, et al. Transcatheter closure of perimembranous ventricular septal defects: early and long-term results. J Am Coll Cardiol. 2007;50(12):1189-1195.

[6] Zhou T, Shen XQ, Zhou SH, Fang ZF, Hu XQ, Zhao YS, et al. Atrioventricular block: a serious complication in and after transcatheter closure of perimembranous ventricular septal defects. Clin Cardiol. 2008;31(8):368-371.

[7] Chen H, Liu J, Gao W, Hong H. Late complete atrioventricular block and tricuspid regurgitation after percutaneous closure of a perimembranous ventricular septal defect. J Thorac Cardiovasc Surg. 2010;140(3):e60-e61.

[8] Forbes TJ. The race to close perimembranous ventricular septal defects (PVSD): proceed with caution. J Interv Cardiol. 2014;27(3):273-274.