In our centre, bone marrow aspirates have proved to be a poor screening test for NPC presenting with infantile liver disease with a sensitivity of only 57% for those done in the first year of life. Strong clinical suspicion remained crucial in establishing the diagnosis and initiating definitive investigations.
Previous studies and correspondence have suggested that false negative bone marrow aspirates for NPC are unusual.5,14
This is at variance with our experience. Sampling or interpretive errors appear unlikely given that the bone marrow aspirate slides were reviewed by an experienced haematologist, who confirmed the diagnostic quality, adequacy of cellularity, and the original findings of all the samples.
A major strength of the current study is that a clearly defined population was assessed prospectively using a structured protocol with a high rate of clinical follow up. Therefore the chance of further missed cases was low. In contrast, previous studies have been retrospective, based on children in whom the diagnosis of NPC disease was already established, often on the basis of demonstrating storage material.5
We have found that storage cells are more likely to be found if the bone marrow aspirate was performed later. So, while a positive bone marrow aspirate is specific for NPC, marrow storage cells in NPC may only become apparent as the disease evolves.15
If this is so, relying solely on early bone marrow aspiration for diagnosis of NPC may be intrinsically flawed. Unfortunately our findings cannot suggest an obvious cut‐off age above which bone marrow aspirate is more likely to be diagnostic.
The overall sensitivity of liver biopsies for detecting storage cells in these children was 36%, which is comparable to earlier studies.5
Our findings suggest that liver histology findings in NPC also develop with time. The earliest liver biopsy that revealed the typical light microscopic features of Niemann–Pick disease was obtained at 5.5 months. Thus, early biopsies may also miss the diagnosis. This is a similar to what we have observed with bone marrow aspirates, and is consistent with previous reports of liver histology in NPC.16
Electron microscopy was suggestive of Niemann–Pick disease even before characteristic changes were seen in the corresponding light microscopy results. Although myelin figures can be a non‐specific finding in neonatal hepatitis, if they are present in large numbers and not confused with whorled structures seen in bile whenever there is cholestasis, they can be a helpful marker for targeting further investigations.
Interestingly, 4/5 patients biopsied before 2 months of age showed paucity of bile ducts; this resolved in two children who had follow up biopsies. Non‐syndromic bile duct paucity is a non‐specific finding and can occur from a variety of infective, metabolic, and genetic causes.17,18,19,20
This has only been reported in one previous case of NPC and may be more common than previously suspected. The mechanism of bile duct paucity in NPC is unclear, but may be related to the production of abnormal bile acids which have been reported in NPC.21
Whether these develop due to unusual metabolism of cholesterol as a consequence of its abnormal subcellular distribution or as a reflection of involvement of NPC1 in the conversion of cholesterol to bile acids is unknown. Our retrospective study cannot shed any light on this matter but it does emphasise the importance of considering NPC in the differential diagnosis of bile duct hypoplasia associated with raised γ‐glutamyl transferase.
Our findings highlight the need for a reliable screening test for NPC in children with infantile liver disease to determine who will require definitive investigations. Unfortunately there is no proven method. Serum or plasma chitotriosidase, an endo‐β‐glucosaminidase secreted by activated macrophages, has been shown to be moderately elevated22,23
in proven cases of NPC. This test is relatively easy and may be useful as a screening tool, although it has not yet been evaluated for this purpose.
In conclusion, bone marrow aspirates are not reliable as a screening test in the diagnosis of NPC presenting with infantile liver disease. While a positive result is a useful pointer to NPC, a negative result cannot exclude the diagnosis. In children with a high index of clinical suspicion, i.e. especially those presenting with cholestasis and unexplained splenomegaly, skin biopsy for fibroblast culture should be undertaken without delay to enable definitive tests to be performed. Reliable screening tests for NPC are needed.
What is already known on this topic
- Diagnosing NPC in infants presenting with liver disease can be difficult and a high level of clinical suspicion is necessary
- The definitive diagnostic method is to demonstrate abnormal cholesterol esterification in cultured fibroblasts or to detect pathogenic mutations
What this study adds
- Bone marrow aspirates are not reliable as a screening test for NPC presenting with infantile liver disease
- In children with a high index of clinical suspicion, definitive tests (skin biopsy for fibroblasts culture) should be undertaken without delay