Mystery: An infant with failure to thrive and vomiting

You are in the emergency department in the middle of an extremely busy winter night when you pick up yet another chart with the chief complaint of "vomiting." This time the patient is a two-month-old African-American male with no past medical history listed on the face sheet. Heading to the room you start thinking of your standard concerns-Is the vomiting bilious? Is it after every feed? Has the infant continued to make wet diapers?

On review of systems, Mom denies any recent fever or associated diarrhea. He has had no sick contacts, and does not sweat during feeds. There is no indication of cough or congestion causing post-tussive emesis. So far, his symptoms seem most consistent with gastroesophageal reflux disease (GERD), but the sudden onset and the lack of arching or fussiness seem a little odd.

On further exam

On physical exam, vital signs reveal an afebrile infant with a heart rate of 182 bpm, respiratory rate of 36 bpm, and blood pressure of 81/61 mm Hg. Weight at this visit is 6 pounds 9.6 ounces-only about 5 ounces above birth weight! You note he has gone from the 25th percentile to just under the third. Quickly plotting his length and head circumference, you find them to be 50th and 10th percentile, respectively. You are uncertain how this compares to previous values for his length and head circumference, but the weight comparison from birth to now alone concerns you.

Before going any further, you ask a few more questions. Going back to his difficulty regaining birth weight, Mom reports he did not achieve this milestone until four weeks of age. This gives you pause, as most infants regain birth weight by 2 weeks old, and no later than 3 weeks, if breastfeeding. No lab studies were done during that time, and he has not been seen since four weeks of age.

On exam, you confirm he appears thin with a sunken anterior fontanelle. Despite this, he's alert. His mucous membranes are slightly dry. There is no lymphadenopathy, and his neck is supple. Aside from his tachycardia, his cardiovascular exam is unremarkable. Breath sounds are clear bilaterally. His abdominal exam reveals a soft, nontender abdomen that is mildly distended. No discrete masses are palpable, and there is no hepatosplenomegaly. A brief neurologic exam shows normal strength and tone and symmetric, 2+ deep tendon reflexes. The infant has mildly erythematous dry patches in the right antecubital fossa and behind the ears.