Best Practices for IH Diagnosis
Making a diagnosis of IH involves a careful clinical history, with particular attention to the possibility of other disorders with similar symptomatology, and objective testing with actigraphy, polysomnography, and multiple sleep latency testing.
According to the International Classification of Sleep Disorders (ICSD), third edition, the diagnostic criteria for IH are as follows:1,2
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) can be considerably more difficult. Cataplexy is absent in both disorders, and other components of the “narcolepsy tetrad” (ie, sleep paralysis and sleep-related hallucinations) are commonly seen in patients with NT2 and about 1/4 of patients with IH. As a result, these features are not useful for ruling out a diagnosis of IH. Additionally, similar to IH, long sleep times may be present in NT2.3 As a result of this symptom overlap, the differentiation between IH and NT2 rests entirely on the presence or absence of at least 2 sleep-onset rapid eye movement periods (SOREMs) on PSG/MSLT (with 0–1 SOREMs in patients with IH).
Once established, IH is usually stable and long-lasting; however, fluctuations in disease severity may be observed as documented by changes in MSLT results over multiple year intervals and spontaneous remissions reported in several series.4,5 The spectrum of IH and fluctuations in disease require that the individual with IH be continually monitored and followed over the long term to ensure appropriate management.6
Diagnostic Challenges
Patients often describe their sleepiness using vague terms such as “tired,” “fatigue,” or “lack of energy.”7 For clinicians it is important to distinguish between sleepiness and fatigue. Fatigue is a physical or psychological 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 their 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.8
Additionally, clinicians may experience difficulty in diagnosis based on clinical signs and symptoms alone. As onset of narcolepsy and IH may be gradual, a diagnosis may not occur until a catastrophic event due to sleepiness, such as a car accident or reduced work productivity, which leads to patient presentation in the clinic.
