Paediatric sleep investigations: polysomnography, sleep-study interpretation and MSLT — branching viva

Opening — choosing the test

The examiner begins: a four-year-old snores nightly with witnessed pauses and is inattentive by day. What investigation would you choose and why? [1]

I would arrange attended in-laboratory polysomnography, the reference standard, because it records brain and breathing together and can classify and grade the events rather than merely detect desaturation. It answers a breathing question, so the overnight study, not a daytime test, is what I need. If laboratory access were limited I would use oximetry or a limited study knowing a negative result would not exclude apnoea in so symptomatic a child. [3] [2]

Branch A — interpreting the report

The study shows an obstructive apnoea-hypopnoea index of 9, a central index of 0.3, a saturation nadir of 84 per cent and a peak carbon dioxide of 45 mmHg. Interpret it. [6]

This is moderate-to-severe obstructive sleep apnoea, because in a child an obstructive apnoea-hypopnoea index above one event per hour is abnormal and nine is well into the treatable range, with the low central index confirming the events are obstructive. The saturation nadir shows a significant hypoxic load and the carbon dioxide is not sustainedly high, so there is no hypoventilation. I would treat rather than watch, using paediatric thresholds throughout. [6] [2]

Branch B — obstructive versus central and the role of carbon dioxide

How do you distinguish an obstructive from a central event, and when does carbon dioxide change your interpretation? [7]

The effort channels make the distinction: in an obstructive event airflow falls while chest and abdominal effort continue, often paradoxically, whereas in a central event airflow and effort stop together. Carbon dioxide changes everything when it stays high across sleep rather than dipping in events, because that is sleep hypoventilation, the hallmark of central and neuromuscular disease, and it can be present with a normal saturation, so I never read a normal oximetry as normal ventilation. [7] [10]

Branch C — the sleepy adolescent

Now a fifteen-year-old has severe daytime sleepiness and knee-buckling when he laughs. His overnight breathing study is normal. What next? [9]

His cataplexy-like episodes and sleepiness raise narcolepsy, so I would arrange a multiple sleep latency test, but only after preparing properly: two weeks of documented adequate, regular sleep on a diary or actigraphy, withdrawal of rapid-eye-movement-suppressing drugs where safe, and an adequate overnight study the night before to exclude apnoea and confirm sufficient sleep. A short mean sleep latency with two or more sleep-onset rapid-eye-movement periods would support narcolepsy. [5] [8]

Closing — the single safety and interpretation rule

Give me the single rule you would apply to any sleep investigation. [2]

Choose the test from the question and interpret it through the child: an overnight study answers a breathing problem and a nap test answers a sleepiness problem, a negative simple study never overrules a strong clinical story, and carbon dioxide, not saturation, is what proves hypoventilation. [2] [10]