Failure to thrive
Failure to thrive is a term used to describe poor weight gain. It can be defined as weight <3rd percentile for age and sex. Alternatively, a child has FTT if a previously normal growth curve crosses two percentile lines on a standard growth chart. FTT describes postnatal poor weight gain. Poor growth prenatally falls under the category of intrauterine growth retardation (IUG
Severe combined immune deficiency (SCID) - Patients present in the first months of life with failure to thrive, chronic diarrhea, and opportunistic infections (severe oral thrush, PJP, severe respiratory virus infections). The diarrhea is typically infectious in nature (rotavirus, picornavirus, adenovirus, coxsackievirus). Recurrent severe infections lead to nutrient malabsorption and increased energy expenditure.
Serious T Cell or Combined Immune Deficiencies
Serious T cell or combined immune deficiencies - Patients may present similarly to patients with SCID with FTT, diarrhea, and opportunistic infections. Examples of such disorders include Wiskott-Aldrich syndrome, MHC Class II deficiency, ZAP 70 deficiency, and complete Digeorge syndrome.
IPEX syndrome - Patients develop intractable chronic diarrhea secondary to autoimmune small bowel inflammation (biopsy reveals villous atrophy). These symptoms begin during infancy and lead to FTT from nutrient malabsorption. Additional features of this disease include atopic dermatitis, endocrinopathies (Type I diabetes and thyroid disease), and autoimmune cytopenias. Immunosuppressive medications (ex. FK506, sirolimus) are needed to treat bowel disease and G-tube placement or TPN may be required to maintain adequate nutrition. CD25 deficiency patients present with an IPEX-like phenotype of diarrhea, FTT, dermatitis, and autoimmune disease.
Dyskeratosis congenita - The severe infant form of the disease (Hoyeraal-Hreidarsson syndrome) presents with FTT, enteropathy, IUGR, cerebellar hypoplasia, and combined immunodeficiency
HIV
HIV infection - Pediatric patients with HIV present very similarly to patients with SCID: FTT, chronic diarrhea, and opportunistic infections (severe oral thrush, PJP, severe respiratory virus infections). This diagnosis should always be considered when evaluating a patient for suspected SCID (and vice versa).
Cystic fibrosis - Patients can present with FTT and chronic diarrhea due to fat malabsorption from pancreatic insufficiency. In addition, there is increased energy expenditure from chronic respiratory tract infections.