I hope that you recognise that a perimembraneous ventricular septal defect (PMVSD) is not a hole in the septum but a defect at its edge where the infundibular septum has failed to fuse with the interventricular septum (see the presentation on cardiac embryology, and the echo below). There may be mal-alignment of the two septa. The infundbulum can be displaced to the right: this is the case in tetralogy of Fallot, when there is associated hypoplasia and hypertrophy of it and the RV free wall (see note below the echo!). More rarely the infundibular septum can be displaced to the left: then it can narrow the LV outflow tract.
A PMVSD with preserved alignment is the commonest form of VSD. Despite normal alignment it is perhaps no surprise that the infundibular septum may not be entirely normally formed. This case brought it home to me, in the days when echo had not been invented. It is an unusual angiogram: the catherterisation was done to evaluate a VSD but there was suspicion of some pulmonary stenosis and a right ventricular injection was perfomed in RAO projection. There is uncontrasted inflow from the left ventricle in diastole allowing the VSD to be seen as a negative shadow (white arrows). Note that the crista supraventricularis is concave (black arrows) instead of its normal convex shape (see case 6 in the "seminal" section), taking the shape of the upper border of the VSD. Negative VSD flow can be seen quite often if you look for it but it seldom defines the whole of the defect as well as it has done here.
In old nomenclature, PMVSD was called "sub-cristal", still not a bad name for it when it is of moderate size as here. Remember that it may well be larger and it may extend downwards into the inflow septum or forwards into the right ventricular outflow tract.
Below is a long axis echocardiogram of a PMVSD: the black star is on the infundibular septum and the defect is immediately apical to it.
Note: were you confused when I described the infundibular septum in tetralogy of Fallot as both hypoplastic and hypertrophied? I hope not, but if you were then can I say that I am trying to be correct in that the opposite of "hypoplasia" is "hyperplasia", and the opposite of "hypertrophy" is "atrophy". In tetralogy, the infundibulum is essentially hypoplastic, and this does not stop it becoming hypertrophied as well. Still confused? It took me a while to understand the implications, so keep at it.