The clinical picture of the classic form of Lesch-Nyhan disease usually includes three things: uric acid crystals or stones, neurological disability, and difficult behaviors. But not all patients have all three problems. Some are spared the behavioral problems and self-injury. Others have only uric acid problems. These individuals with less severe problems are called the Lesch-Nyhan variants (LNV), to distinguish them from the more severely affected patients.
The condition with only problems due to too much uric acid is called 'HPRT-related hyperuricemia' (HRH). Patients who also have neurological problems, but no behavioral symptoms and self-injury, have 'HPRT-related hyperuricemia with neurological dysfunction' (HRND). See this figure for a schematic representation of the clincial spectrum of these conditions.
The reasons that some patients are less severely affected are an important area of current research. They may hold clues for how to spare others from getting all three of the problems.