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Comprehensive Pediatrics Glossary

X

X-linked Agammaglobulinemia (Bruton’s Agammaglobulinemia)

  • Definition: Rare X-linked primary immunodeficiency characterized by near-complete absence of B lymphocytes and all immunoglobulin isotypes.

  • Etiology: Mutation in the BTK gene affecting B-cell development.

  • Clinical Features: Recurrent bacterial infections (especially Streptococcus and Haemophilus species), absence of tonsils and lymph nodes, failure to thrive.

  • Investigations: Low/absent B cells on flow cytometry, markedly reduced immunoglobulins (IgG, IgA, IgM), genetic testing.

  • Management: Lifelong immunoglobulin replacement therapy (IVIG or SCIG), prompt treatment of infections, prophylactic antibiotics in some cases.

  • Complications: Chronic lung disease (bronchiectasis), recurrent infections, autoimmune manifestations.

X-linked Hyper IgM Syndrome

  • Definition: Primary immunodeficiency caused by mutation in the CD40 ligand gene, leading to defective class-switching of B cells.

  • Clinical Features: Recurrent bacterial and opportunistic infections, neutropenia, chronic diarrhea, failure to thrive.

  • Investigations: Elevated IgM, low IgG/IgA/IgE, flow cytometry for CD40L expression, genetic testing.

  • Management: IVIG replacement therapy, prophylactic antibiotics, hematopoietic stem cell transplantation in severe cases.

  • Complications: Chronic infections, liver disease (sclerosing cholangitis), malignancy risk, early mortality without treatment.

X-linked Lymphoproliferative Disease (XLP)

  • Definition: Rare immunodeficiency characterized by abnormal response to Epstein-Barr virus (EBV) infection.

  • Clinical Features: Fulminant infectious mononucleosis, hemophagocytic lymphohistiocytosis (HLH), dysgammaglobulinemia, lymphoma predisposition.

  • Investigations: Genetic testing for SH2D1A or XIAP mutations, EBV PCR, immunoglobulin levels.

  • Management: Supportive care for infections, IVIG, hematopoietic stem cell transplantation (curative).

  • Complications: HLH, lymphoma, severe EBV infections, high mortality if untreated.

X-linked Adrenoleukodystrophy (X-ALD, Childhood Form)

  • Definition: Peroxisomal disorder caused by mutation in ABCD1 gene, resulting in accumulation of very long-chain fatty acids (VLCFA) and progressive neurological deterioration.

  • Clinical Features: Behavioral changes, progressive cognitive decline, spasticity, adrenal insufficiency (hyperpigmentation, fatigue), vision/hearing loss.

  • Investigations: Plasma VLCFA measurement, genetic testing (ABCD1 mutation), brain MRI (white matter changes), adrenal function tests.

  • Management: Supportive care, dietary therapy (Lorenzo’s oil), hematopoietic stem cell transplantation in early-stage cerebral ALD, corticosteroid replacement for adrenal insufficiency.

  • Complications: Severe neurological disability, adrenal crisis, early mortality in untreated cerebral form.

Xanthomas (Pediatric Lipid Disorders)

  • Definition: Deposits of lipid-laden macrophages in skin or tendons due to hyperlipidemia.

  • Etiology: Familial hypercholesterolemia, hypertriglyceridemia, secondary causes (biliary atresia, nephrotic syndrome).

  • Clinical Features: Yellowish nodules or plaques on elbows, knees, Achilles tendon, eyelids (xanthelasma).

  • Investigations: Lipid profile (cholesterol, triglycerides, LDL, HDL), genetic testing for familial disorders.

  • Management: Dietary modification, lipid-lowering therapy (statins in older children), treatment of underlying cause.

  • Complications: Premature atherosclerosis, cardiovascular disease, pancreatitis (in severe hypertriglyceridemia).

Xeroderma Pigmentosum (XP)

  • Definition: Rare autosomal recessive disorder of defective nucleotide excision repair causing extreme sensitivity to UV light.

  • Clinical Features: Severe photosensitivity, freckling, poikiloderma, early onset skin cancers, ocular involvement (photophobia, keratitis), neurological manifestations in some cases.

  • Investigations: Clinical diagnosis, genetic testing for nucleotide excision repair genes, UV sensitivity assays, skin biopsy for suspected malignancy.

  • Management: Strict sun protection (clothing, sunscreen, avoidance of sunlight), regular dermatologic surveillance, early excision of skin cancers, vitamin D supplementation.

  • Complications: Multiple skin cancers (basal cell, squamous cell, melanoma), ocular damage, progressive neurodegeneration.

Xerophthalmia

  • Definition: Eye disorder caused by vitamin A deficiency leading to conjunctival and corneal dryness.

  • Clinical Features: Night blindness, Bitot spots (foamy conjunctival plaques), corneal ulceration, keratomalacia in severe cases.

  • Investigations: Clinical diagnosis, serum retinol levels if available.

  • Management: Vitamin A supplementation (high-dose therapy in severe cases), treatment of infections, nutrition education.

  • Complications: Corneal ulceration, blindness, secondary infections.

 

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