Craniopagus twins (CPT) are an uncommon, highly fascinating accident of nature. 
The clinical pathology of this complex entity is reviewed and placed in 
perspective. A logical classification aids understanding of the anomaly, and is 
essential to gauge outcome from separation attempts. 'Partial forms' lack 
significant shared dural venous sinuses (SDVS) and 'Total forms' with SDVS also 
exhibit more severe compressional brain distortion. Our classification consists 
of Partial Angular (PA), Partial Vertical (PV), Total Angular (TA) and Total 
Vertical (TV, formerly O'Connell Types I-III). Total vertical has a continuous 
cranium, and inter-twin axial facial rotation <40 degrees (Type I), 140-180 
degrees (Type II) or intermediate (Type III). The term 'Angular' denotes an 
inter-twin longitudinal angle below 140 degrees , regardless of axial rotation. 
Our review categorized 64 well-delineated CPT, including 41 operative separation 
attempts in small children since initial success in 1952. Just over one-half 
were TV, almost one-third TA, and partial forms accounted for the remaining 
one-sixth. About 30% of CPT had shared or fused brain tissue, and a similar 
percentage of TA twins shared a posterior fossa. Partial forms had significantly 
higher birth weights, were separated at an earlier age (6 versus 11 months) and 
had lower mortality and better outcome compared with Total forms. A multi-staged 
surgical separation for Total CPT had a significantly better mortality than 
single-staged separation. Discussion emphasizes embryological, anatomical and 
clinical aspects of the malformation, with emphasis upon obstacles to a 
successful outcome.