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may be lacking, and physical examination may reveal only lethargy Elevation of the carboxyhemoglobin level is diagnostic Renal Disorders (See Chapter 100 Renal and Electrolyte Emergencies ) A young infant may also appear extremely ill because of renal failure or dysplasia Posterior urethral valves, especially in males, can cause bladder outlet obstruction and resultant renal failure Approximately one-third of these cases are diagnosed by week of age, but more than half go undetected for the first few months of life The parents may give a history of vomiting, poor appetite, inadequate weight gain, or an abdomen that appears swollen A careful review of systems may reveal a weak urinary stream On physical examination, hypertension or an abdominal mass (hydronephrosis) may be detected, as well as urinary ascites A suprapubic ultrasound may demonstrate the dilated posterior urethra and bladder and a voiding cystourethrogram will show a dilated posterior urethra, hypertrophy of the bladder neck, and trabeculated bladder The serum creatinine and blood urea nitrogen levels may be markedly elevated Urosepsis is a possible complication of posterior urethral valves Hematologic Disorders Any infant with severe anemia caused by aplastic disease, hemolytic process, or blood loss can look quite ill (see Chapter 93 Hematologic Emergencies ) Disorders of hemoglobin such as methemoglobinemia can also cause an infant to appear toxic and may be inherited or acquired Transient methemoglobinemia in infants is occasionally caused by environmental toxicity from oxidizing agents such as nitrates found in well water or oxidant drugs (e.g., topical benzocaine in teething gels) This intoxication presents with cyanosis, poor feeding, failure to thrive, vomiting, diarrhea, and lethargy In other patients, the oxidant stress is less obvious, causing severe diarrhea and metabolic acidosis It is postulated that the infectious agent that causes the diarrhea or the secondary metabolic acidosis may produce an oxidant stress that leads to methemoglobin formation These infants are lethargic, with hypothermia, tachycardia, tachypnea, and hypotension, and often appear mottled, cyanotic, or ashen One key to the diagnosis of methemoglobinemia is that oxygen administration does not affect the cyanosis (similar to CHD with right-to-left shunt) Laboratory tests show a profound acidosis (pH 6.9 to 7.2), yet the PaO is normal despite the cyanosis Leukocytosis and thrombocytosis are present, and prerenal azotemia may be noted The blood itself may appear chocolate brown (most easily noted when a drop of blood on filter paper is waved in the air and compared with a normal
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