A six month old boy presented with gradually progressive abdominal distension since late neonatal period. There was no history of persistent fever, vomiting, abnormal bowel movements, pallor, jaundice, bleeding, rash or additional swelling. Acquisition of all developmental milestones was delayed. Seizures and altered consciousness were absent. Antenatal and perinatal periods were normal. Birth was at term with a weight of 2.3 kg. He was the second issue of non-consanguineous, healthy, hindu parents. A male sibling had expired at 14 months of age with similar illness.

Investigations revealed mild normocytic normochromic anemia (Hb 9.5 gm/dL) with normal total and differential leukocyte, platelet and reticulocyte counts. Liver function tests were deranged; total bilirubin 1.4 mg/dL, direct bilirubin 0.4 mg/dL, Aspartate amniotransferase 790 IU. Alanine amniotransferase 907 IU, alkaline phosphatase 1481 IU). Serum cholesterol level was 180 mg/dl (normal for age 65-175 mg/dL). Abdominal ultrasonography confirmed liver and spleen enlargement with normal echotextures and normal portal vein diameter. Hearing and visual evaluation (by BERAphone screening and Visual Evoked Response) and skeletal survey were normal. Bone marrow aspiration revealed cellular bone marrow with normoblastic erythroid series, myeloid series and megakaryocytes. Storage cells were not detected even on bone marrow biopsy. Thyroid Function tests and MRI cranium were normal. Sequential enzyme assays were planned. Serum levels of ASM were undetectable which was diagnostic of Niemann-Pick disease, the phenotypes suggestive of type A (Classical infantile neuronapathic form).

During follow up, neuro-developmental status remained static. Hepatosplenomegaly progressively increased but without further enzyme derangement. At 11 months, he contracted severe pneumonia and succumbed enroute to hospital. Permission could be obtained only for a post mortem liver biopsy, the histopathology of which revealed NP cells. Gene sequencing of the Spingomyelin phosphodiesterase 1 (SMPD1) gene revealed compound heterozygosity for stop codon mutations R443X and R542X in exon 3 and 5, respectively (Fig. 1). Origin was maternal in the former and paternal in the latter.

Fig. 1 Gene sequencing of patient demonstrating paternal origin of mutation R542X and maternal origin of R443X.

In type A and B NPD, the affected enzyme is encoded by the　SMPD1　gene located on chromosome bands 11p15.1-p15.4, resulting in primary ASM deficiency with activity 1-10% of normal. Pathological sphingomyelin deposition results in infiltration of bone marrow, spleen, liver and lymph nodes with NP cells. In developed countries, diagnosis is established by enzyme assay and mutation analysis, rather than more invasive alternatives like BMA. However, in developing countries, these modalities are expensive and not easily available. Common practice in such circumstances is demonstration of NP cells by BMA.

Organomegaly has been reported as the commonest presentation of NPD-A, with a median age of diagnosis 6 months [1]. Elevated cholesterol was considered an early marker of the disease [1]. Since reticuloendothelial organs are completely infiltrated with NP cells originating from lipid-accumulating bone marrow progenitor cells, non-demonstration of NP cells in symptomatic patients is unusual. Extensive literature revealed no prior　studies reporting absence of NP cells in BMA. Suboptimal sensitivity of BMA has been previously reported in children with type C NPD presenting with cholestatic jaundice. The overall sensitivity of 64% decreased to 57% when BMA was performed during the first year [2]. This can be explained by the later age of onset in type C. The clinical profile of our case (static neuro-developmental status, hepatosplenomegaly without increasing enzyme deterioration and normal cholesterol) reflects early illness with probably less extensive infiltration of the bone marrow. This questions the reliability of BMA in early illness or young infants with NPD-A.

Mutational analysis has revealed many disease associated alleles in NPD-A. Most are sporadic with only a few having ethnic predilection like p.R496L, p.L302P and p.P330fs in Ashkenazi population and c.677delT in Israeli Arabs [3]. Most mutations are single base substitutions and small deletions with or without a frameshift [4]. Small deletions or nonsense mutations result in truncated ASM polypeptide and missense mutations render the enzyme non-catalytic in NPD-A, whereas enzyme is defective with residual catalytic activity and milder phenotype in NPD-B [5]. In this case, mutation R542X (arginine to stop codon at amino acid 542) is novel whereas mutation R443X (arginine to stop codon at amino acid 443) has been reported earlier in a homoallelic patient, also of Indian origin [6]. Only further studies will be able to confirm a possible Indian predilection.

In resource limited settings, demonstration of storage cells on BMA is the preliminary investigation despite its invasiveness. Confirmatory enzyme assays are performed subsequently. Absence of NP cells in the bone marrow usually leads to considering alternative diagnoses. When strong clinical suspicion of NPD-A exists, a normal BMA should not exclude the diagnosis without an enzyme assay. If services are unavailable locally, blood can be collected as ‘spots’ on 903S&S filter paper (GE) which remain sufficiently stable to be transported to the testing laboratory for enzyme analysis [7].

Acknowledgements: Dr Ashwin Dalal, Director, Diagnostics Division, Centre for DNA Fingerprinting and Diagnostics, Hyderabad for providing his valuable expertise.

Contributors: SBM, MP and SK were involved in clinical diagnosis and manuscript writing. SBM and SK will stand as guarantors. TPP was involved in the genetic tests and their interpretation. All authors approved the final manuscript.

Funding: None; Competing interests: None stated.

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