Presently HELLP syndrome is considered to represent a form of severe preeclampsia. It is distinct in a number of ways, and work being done in some genetic labs around the world may better pinpoint the similarities and the differences between the two conditions.
There may be a different pathway to disease with HELLP syndrome compared to severe preeclampsia. A patient certainly can have both, or at least appear to have both. In the series of patients with HELLP syndrome from our institution published several years ago, the recurrence risk for HELLP syndrome was approximately 1:5 for HELLP and 2:5 for preeclampsia in any of its forms. At least one other group has found a similar recurrence risk for HELLP syndrome, contrasted to a third group which found a lower risk of recurrence around 1:20.
Like most disease potential, whether or not a patient gets the condition depends upon a host of factors -- in the case of HELLP syndrome it could depend upon the genetics of the fetus, the cardiovascular health of the patient, and so forth. There is no readily available test yet available to use to predict which patient is at risk to recur, but I think we will have one within 5 years.