Absent inferior vena cava, symmetrical liver, splenic agenesis, and situs inversus, and their embryology.

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Abstract
Conditions found when there is a break in the normal inferior vena cava with drainage through the azygos vein (absent inferior vena cava) were described. There are close associations in the incidence of situs inversus, bilateral superior vena cava, the absent inferior vena cava, agenesis of the spleen, and symmetry of the liver, and especially between the last 3. Four-fifths of the subjects with complete situs inversus appear to be normal in every other way, but l/5th have other malformations of the heart. The other 4 malformations discussed each occur in about 1/4 of this last group. More than half of those with splenic agenesis, symmetry of the liver, or an absent inferior vena cava, have also 1 or both of the remaining 2 malformations. Experimental work on animals shows that whether the situs is normal or inverted is settled at a very early stage of embryonic life, corresponding with the 3rd week of human development. It must be even earlier when there is a genetic cause. Three of the other malformations are settled at or before the end of the 5th week, and bilateral superior vena cava towards the end of the 7th week. The close association of these malformations is probably explained because situs inversus is a predisposing cause of the other 4, though they can occur without it. A symmetrical liver does so very rarely, splenic agenesis does so sometimes, and the 2 venous malformations do so more often, though they are commoner with situs inversus than with a normal situs. All these malformations are especially liable to occur when the situs is neither completely inverted nor completely normal. Splenic agenesis generally occurs, and a symmetrical liver nearly always occurs, under these conditions. Some possible but speculative explanations as to how splenic agenesis and a symmetrical liver may be produced by the vascular changes found with situs inversus are discussed.