Isolated CNS Hemophagocytic Lymphohistiocytosis in Children:What We Know, What We Don't Know ?

The presence of central nervous system involvement has a profound impact on the prognosis, treatment, and clinical outcome of the primary hemophagocytic lymphohistiocytosis (pHLH). However, isolated CNS-HLH is a challenging disease with a high mortality and morbidity , possibly resulting from a spsecific neuroinflammation that leads to isolated disease only without systemic activation under some additional genetic modifiers. In this study, we retrospectively reviewed our isolated CNS-HLH cases and there were 73 patients (36 male, 37 female) with a median age of 20 months (range, 1- 226 months) diagnosed as primary hemophagocytic lymphohistiocytosis at Hacettepe University Faculty of Medicine, between January 2005 and June 2021. Among these, 39 (53%) patients had central nervous system involvement either on admission or during the recurrence. On admission, the number of patients who had both CNS and sytemic involvement was 19 (49%), moreover 2 had CNS infiltration both initially and during the course of relapse. 8 patient did show CNS involvement only during the relapse. Ten (25%) patients (5 male, 5 female) with isolated CNS involvement are the main subject of this study and none of them had infectious trigger. What we know is they were presented with mostly unexplained neurological findings and /or cranial nerve paralysis. In this group median age at presentation was 101 months ( range 6 - 180 months). They all had primary HLH associated patogenic mutation and in some of them diagnosed was also confirmed by brain biopsy. Neither family history and /or consanguinity nor HLH criteria are fullfilling in this devastating disorder. Cranial MRI gives many clues during admission in experienced hands. Two of our published cases were initially diagnosed as lymphomatoid granulomatosis and acute disseminated encephalomyelitis ; they were diagnosed as hemophagocytic lymphohistiocytosis after developing systemic symptoms 3 and 12 months later. Interestingly 6 of 10 patients in this group never developed systemic symptomps, 7 patients underwent allogeneic bone marrow transplantation. Spinal cord involvement was determined in 8 patients(20%), including 4 at diagnosis and 4 during follow up ; including one previously published case, 4 out of 8 had isolated cases did show spinal involvement as well. Even though few number of cases with isolated CNS-HLH has been reported in the recent years, we believe that the number of such cases is not limited to those who have been reported because it is rather difficult to diagnose patients with isolated CNS symptoms, which leads to misdiagnosis and/or mistreatment. What we don't know is how to specifically treat patients with CNS directed therapy, and exactly which mutations are associated with isolated CNS-HLH or whether there is a known tendency in this group and perhaps unknown mutations ? Does it have a facilitating effect ?