Differential Diagnosis: Failure to Thrive (FTT) / Failure to Gain Weight in a Child

Definition: FTT is broadly defined as weight consistently below the 3rd-5th percentile for age, or a sustained drop across 2 major percentile lines on a growth chart. ~90% of cases are nonorganic; ~10% are organic (or mixed).

Classification Framework

FTT is classically divided into three pathophysiologic categories:

Mechanism	Question to ask	Examples
Inadequate intake	Is the child getting enough calories?	Psychosocial, feeding errors, poverty, anatomic
Inadequate absorption	Can the child absorb what it eats?	Celiac, CF, short gut, SIBO
Increased metabolic demand	Is the child burning too much?	CHD, chronic infection, hyperthyroidism, malignancy

1. NONORGANIC CAUSES (~90%)

Psychosocial / maternal deprivation - most common overall cause; parental neglect, lack of nurturing, parental depression or mental illness
Errors in feeding technique - improper formula preparation (over-diluted), incorrect positioning, poor latch (breastfeeding)
Inappropriate diet - excessive juice, early introduction of solids that displace calories, food restriction by parents
Poverty / food insecurity - lower socioeconomic status, lower parental education level
Behavioral feeding disorders - oral aversion, food refusal, sensory processing issues

2. ORGANIC CAUSES (~10%)

Inadequate Caloric Intake

GERD / esophagitis - feeding refusal, vomiting, pain
Oropharyngeal dysphagia - cleft palate, cricopharyngeal achalasia, laryngomalacia, choanal atresia
Congenital heart disease (cyanotic) - fatigue during feeding, increased metabolic demand
Neurologic disorders - cerebral palsy, hypotonia, neuromuscular diseases (poor suck/swallow)
Lead poisoning - inner-city environments with older housing stock
Prematurity - use corrected age on growth charts

Malabsorption / GI Causes

Celiac disease - anti-tTG antibodies, biopsy; classic presentation with diarrhea, bloating
Cystic fibrosis - exocrine pancreatic insufficiency, chronic pulmonary infections, sweat chloride test
Cow's milk protein intolerance / food allergy - rectal bleeding, eczema, failure to gain in infants
Short bowel syndrome - post-surgical
Small intestinal bacterial overgrowth (SIBO)
Chronic diarrhea (any cause) - parasites (Giardia, Cryptosporidium), inflammatory bowel disease
Pancreatic insufficiency - Shwachman-Diamond syndrome in children
Lysosomal storage/fat malabsorption disorders - intolerance to breast milk/formula is often first sign

Increased Metabolic Demand / Chronic Disease

Chronic infections - HIV, TB, recurrent otitis media, UTI, parasites
Congenital heart disease - especially cyanotic lesions
Chronic renal disease / renal tubular acidosis - RTA is a classic cause in infants; check BUN/creatinine, urinalysis
Hypothyroidism - check TSH/T4; also consider hyperthyroidism
Diabetes mellitus / inborn errors of metabolism - check glucose, metabolic screen
Malignancy - leukemia, lymphoma
Obstructive uropathy (UPJO) - FTT is a recognized presentation in infants

Chromosomal / Genetic

Turner syndrome - short stature, webbed neck, absent breast development
Trisomy 21 (Down syndrome)
Prader-Willi, Russell-Silver, Noonan syndromes
Intrauterine growth restriction (IUGR)

Diagnostic Approach

Key history points: feeding history, stool character, parental heights, psychosocial environment, medications, travel, recurrent infections.

Physical exam: pattern of growth (weight only vs. weight + height vs. all three parameters affected guides diagnosis); dysmorphic features; organomegaly; signs of neglect.

Initial investigations:

CBC, ESR
Urinalysis + urine culture (rule out UTI/RTA)
BUN, creatinine
TSH, T4
Serum lead level (inner-city, older housing)
Liver enzymes
Bone age radiograph (hand/wrist)
Stool for ova and parasites
Based on history: sweat chloride (CF), anti-tTG IgA (celiac), HIV, TB testing

Diagnostic algorithm from Swanson's Family Medicine Review:

Key Pearls

~90% of FTT is nonorganic - always assess the psychosocial environment first
Nonorganic FTT: weight is disproportionately low vs. height; organic FTT affecting all parameters suggests a systemic cause
Children with a history of FTT remain at increased risk of recurrence even after normalization - long-term follow-up is important
Appetite stimulants (cyproheptadine, megestrol) are NOT recommended unless significant underlying disease is identified
Early intervention matters: FTT without treatment causes lasting neurodevelopmental sequelae
If hospitalized and the child gains weight rapidly, this strongly supports nonorganic (psychosocial) etiology

Sources: Swanson's Family Medicine Review, pp. 557-560; Tietz Textbook of Laboratory Medicine 7e; Swanson's Family Medicine Review Chapter 104

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