However, "even in the presence of other specific causes of hypersomnia, one should carefully assess the contribution of these etiological factors to the complaint of EDS and when specific treatments of these conditions fail to suppress EDS, the additional diagnosis of IH should be considered. "The severity of EDS can be quantified by subjective scales, such as the Epworth Sleepiness Scale and the Stanford Sleepiness Scale, and also by objective tests, like actigraphy, psychomotor vigilance task, maintenance of wakefulness test (MWT), multiple sleep latency test (MSLT) although as per latest research studies, the effectiveness of MSLT has been called into question. Several groups of researchers have found normal MSLT results in patients who otherwise seem to have IH. Therefore, when IH is suspected, researchers suggest appending a 24-hour continuous polysomnography to the standard overnight/MSLT study in order to record total sleep time.The recent studies have also found that reports of sleepiness in IH relate more to mental fatigue rather than physiological sleepiness per se and the subjective scales like ESS, IH symptom diary (IHSD) and PGIC better captures the severity of symptoms consistently.It is also important to note that whereas narcolepsy is strongly associated with the HLA-DQB1*0602 genotype, "HLA typing is of no help in the positive diagnosis of idiopathic hypersomnia." This is "despite some reports that suggest an increase [sic] frequency of HLA Cw2 and DRS in idiopathic hypersomnia subjects. "In patients with IH, polysomnography typically shows short sleep latency, increased mean slow wave sleep, and a high mean sleep efficiency.
These symptoms may affect performance on perception, memory, learning, executive functions, language, constructive abilities, voluntary motor control, attention, and mental speed. Affected individuals can complain of forgetfulness, confusion, or inability to think clearly. Excessive sleep (9 hours or more over a full 24-hour period), without feeling refreshed after waking. Daytime naps can be up to several hours and are also unrefreshing.Some studies have shown increased frequencies of palpitations, digestive problems, difficulty with body temperature regulation, and other symptoms in patients with IH. Anxiety and depression are frequent, likely as a response to chronic illness. A case series in 2010 found that peripheral vascular symptoms, such as cold hands and feet (e.g., Raynaud Syndrome), were more common in people with IH than in controls.Other autonomic dysfunctional symptoms, such as fainting episodes, dizziness upon arising, headaches (possibly migrainous in quality), food cravings, and impotence may also be correlated with IH. Researchers have found that people reporting IH symptoms report high levels of autonomic dysfunction on par with other conditions of autonomic failure (i.e., MSA and diabetes). As of June 2021, there is no evidence to suggest such symptoms are related to IH in any way. Causes
Unlike narcolepsy with cataplexy, which has a known cause (autoimmune destruction of hypocretin-producing neurons), the cause of IH is largely unknown.