The International Classification of Sleep Disorders (ICSD) sets forth the diagnostic criteria for IH as follows:1,2
A diagnosis of IH will involve a careful examination and clinical history from your doctor. He/she will pay particular attention to the possibility of other disorders with similar symptoms. The doctor will also often conduct certain sleep related tests, including:
When comparing IH and narcolepsy type 1 (NT1; Table 1 below), the differences are fairly straightforward with regard to cataplexy, sleep quality and naps. However, the distinction between IH and narcolepsy type 2 (NT2) are more difficult. Cataplexy is absent in both disorders, and other components of the “narcolepsy tetrad”, ie, sleep paralysis and hallucinations, are commonly seen in NT2 and about one quarter of IH patients. Similar to IH, long sleep times may be present in NT2.3 As a result of this symptom overlap, distinguishing between IH and NT2 rests entirely on the presence or absence of <2 sleep-onset REM periods in IH (2 or more in NT2).
Patients often describe their sleepiness with terms such as “tired,” “fatigue,” or “lack of energy.”4 It is important to distinguish between sleepiness and fatigue. Fatigue is a physical feeling that may occur in a variety of other disorders, such as depression or Parkinson disease. Unlike sleepiness, patients who are fatigued do not fall asleep when involved in sedentary activities such as watching television or reading. This distinction is important because “sleepiness” indicates the presence of a sleep disorder or a problem with nighttime sleep. Patients may or may not be aware of their sleepiness before falling asleep, but it often significantly affects quality of life.
The Epworth Sleepiness Scale (ESS) can assist with the screening of individuals as the scores significantly distinguished normal subjects from patients in various diagnostic groups, including obstructive sleep apnea syndrome, narcolepsy, and IH.5
As the onset of IH and other sleep disorders such as narcolepsy may be gradual, diagnosis may not occur until a catastrophic event due to sleepiness (eg, a car accident or reduced work productivity) leads to a patient seeking treatment.
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