Why does the baby girl "quake and tremble all this day"?

During the two-minute episode, the baby's eyes were open and not notably deviated. The episode was followed by a cry. She was easily comforted, they tell you, and her condition promptly returned to baseline. No loss of consciousness, they report, and no fever, cyanosis, or abnormal eye movements.

In the ED, the baby has seven more such episodes, lasting between 30 seconds and six minutes; the longer episodes are associated with oxygen desaturation down to the 70s and 80s. These episodes are similar to the one at home, the parents confirm, except that now her right side is more involved than the left side. Each episode is preceded by what you take to be gagging noises. The patient remains alert and appears normal between episodes.

Your patient was born by spontaneous vaginal delivery at 41 weeks' gestation, with a birth weight of 4,400 g (>95th percentile for gestational age). Delivery was complicated by meconium staining, but no suction or resuscitation was required. The Apgar score was seven at one minute and eight at five minutes. The Wisconsin newborn screen for hypothyroidism, congenital adrenal hyperplasia, cystic fibrosis, hemoglobinopathies, multiple inborn errors of metabolism, and other conditions was administered and found to be normal.

The newborn was discharged with her mother on day 2 of life, without medication. She has fed well since discharge, taking three or four ounces of infant formula (Similac with Iron) every three hours. She has had no fever and had not exhibited other signs of illness before the shaking episode at home.

You perform the initial physical examination. Here is an alert, vigorous, slightly jittery but easily consolable infant. Weight is 4.8 kg (>95th percentile); head circumference, 37.2 cm (95th percentile). The baby is afebrile and other vital signs are normal. The anterior fontanelle is soft and flat. You do not observe dysmorphic features. Skin shows no jaundice. The remainder of the general physical examination is normal.

Neurologically, cranial nerves II-XII are intact. Motor examination reveals normal strength, tone, and bulk. She withdraws appropriately from sensory stimulation on all limbs. Deep-tendon reflexes are brisk, 3+, and symmetric, with two to three beats of clonus at each ankle. Plantar stimulation causes upgoing great toes bilaterally. She bears weight, briefly, on each leg. Suck, Moro, palmar grasp, plantar grasp, and stepping reflexes are all intact, as is a weak rooting reflex.

Shortly afterward, you review the initial laboratory results (see the table on this page). In addition, venous blood gas analysis reveals a pH of 7.39; CO₂, 40 mm Hg; O₂, 51 mm Hg (on a FIO₂ of 30%); bicarbonate, 23.8 mmol/L; and base deficit, 0.6. A complete blood count is normal. The urine toxicology screen is negative for amphetamines, barbiturates, benzodiazepines, cocaine metabolites, phencyclidine, and opiates.