We have recently studied a case of complete heart block in which there was considerable difficulty in deciding whether it was of congenital or acquired origin. This was because the heart block wasfirst discovered at the age of 2 years in the course of acute diphtheria. Though diphtheritic infections are known to affect the cardiac conduction system, authenticated cases of post-diphtheritic block persisting after the infection are rare, and, furthermore, histopathological study in this case revealed discontinuity between the atrial tissues and the more peripheral parts of the atrioventricular conduction tissues. This has been more commonly observed in congenital cases of complete heart block and it has been postulated on theoretical grounds that this could be the basis for congenital heart block; on the other hand, it has been noted in a single case thought to be of acquired origin. Review of the evidence availablefailed to allow accurate classification of the case into either congenital or acquired categories. The normal, segmental development of the atrioventricular node, each segment being of different embryological origin, is discussed and the case presented is understandable in the light of this.The distinction between congenital and acquired heart block, while of considerable clinical importance (Ayers, Boineau, and Spach, I966), is often difficult to make. This difficulty is illustrated by a recent case we have studied, in which heart block was first observed at the age of 2 (I972). Indeed, such block had been predicted on theoretical grounds by Kung and Mobitz (I930) and Mahaim (I93i). None the less, it is less readily comprehensible on the basis of nodal structure and formation as propounded by James (i96i, I970). In view of these considerations, we are reviewing the possible morphogenesis of this case and other varieties of heart block in terms of presently demonstrated architecture of the normal atrioventricular node.
Case reportThe patient was a girl, bom in I932 after an uneventful pregnancy and normal delivery. No hospital records were available, but she was apparently normal until she contracted scarlet fever at the age of 2 years followed by diphtheria at the age of 2j years. She was said to have been very ill, and was found to have heart block at the time. In her recovery period she had temporary paralysis of one arm and the face on the same side. A detailed cardiological investigation was not made at this time. She subsequently made a complete recovery and remained well during the rest of her childhood. There was no restriction of her physical activities. At the age of IS years she complained of episodes of tiredness, vertigo, and breathlessness. She was noted to have a slow pulse and complete heart block was again diagnosed.