Investigations and diagnosis

Investigations carried out in patients with suspected obstructive sleep apnoea syndrome (OSAS) include:

• overnight oximetry
  • mean sensitivity and specificity of the test is only 87% and 65% respectively
  • an oxygen desaturation of 4% is conventionally used to indicate apnoea
  • it is recommend that in patients with a typical history and more than 10 desaturations of 4% an hour, treatment can be started without any additional detailed studies
  • further testing is not required in patients with fewer than five desaturations an hour and no sleep fragmentation
  • due to the high false-negative rate, 'normal' oximetry is not sufficient to exclude OSAS
    
    
• limited respiratory monitoring
  • contains a reduced combination of the full range of variables present with a full PSG and usually includes one or more of the following - oximetry; thoraco-abdominal respiratory movement and airflow; recordings of snoring, heart rate or general video
  • can often be performed at home by the patient making it more cost-effective
    
    
• polysomnography (PSG)
  • a full PSG is the ideal method for diagnosis of OSAS
  • requires overnight admission for supervised multichannel recording which include ECG, electroencephalography (EEG), eye movements, and electromyography etc (1,2)
  • due to the restricted availability and high cost of PSG, limited respiratory monitoring are more widely used

NICE state with respect to the diagnostic tests for OSAHS (Obstructive sleep apnoea/hypopnoea syndrome)

• offer home respiratory polygraphy to people with suspected OSAHS
  
  
• if access to home respiratory polygraphy is limited, consider home oximetry for people with suspected OSAHS. Take into account that oximetry alone may be inaccurate for differentiating between OSAHS and other causes of hypoxaemia in people with heart failure or chronic lung diseases
  
  
• consider respiratory polygraphy or polysomnography if oximetry results are negative but the person has significant symptoms
  
  
• consider hospital respiratory polygraphy for people with suspected OSAHS if home respiratory polygraphy and home oximetry are impractical or additional monitoring is needed
  
  
• consider polysomnography if respiratory polygraphy results are negative but symptoms continue
  
  
• use the results of the sleep study to diagnose OSAHS and determine the severity of OSAHS (mild, moderate or severe)
  
  
• Severity of OSAHS
  • is determined using the apnoea/hypopnoea index (AHI) value, as follows:
    • Mild OSAHS: AHI of 5 or more to less than 15
    • Moderate OSAHS: AHI of 15 or more to less than 30
    • Severe OSAHS: AHI of 30 or more

Note:

• further investigations are not required in
  • uncomplicated snoring or occasional apnoeas without a history of excessive sleepiness
  • unrefreshing sleep or daytime sleepiness due to inadequate sleep time (identified from the history)

Reference:

• (1) Greenstone M, Hack M. Obstructive sleep apnoea. BMJ. 2014;348:g3745
• (2) NICE (August 2021). Obstructive sleep apnoea/hypopnoea syndrome and obesity hypoventilation syndrome in over 16s