What does hypermagnesemia in neonates primarily cause?

Hypermagnesemia in neonates primarily causes neuromuscular and respiratory depression due to the role of magnesium in neuromuscular transmission and muscle contraction. Elevated levels of magnesium can inhibit the release of acetylcholine at the neuromuscular junction, leading to muscle weakness and decreased responsiveness. This can impact respiratory function, as the respiratory muscles may not contract effectively, causing respiratory depression.

Neonates, being particularly vulnerable, can show signs of decreased muscle tone and diminished reflexes, which are critical for adequate respiratory effort. Hypermagnesemia can often be associated with other underlying conditions or interventions, such as magnesium sulfate treatment for pre-eclampsia in the mother, which can lead to transiently high levels of magnesium in the neonate.

The other options do not align with the primary effects of hypermagnesemia. While hypotonia and lethargy may be symptoms related to neuromuscular depression, they are not the primary characteristic compared to the direct impact of respiratory depression in this context. Increased metabolic rate is contrary to what hypermagnesemia typically causes, as it generally leads to a state of decreased activity and function. Similarly, dehydration and constipation are not directly linked to hypermagnesemia in neonates.