Utility of measuring CSF hypocretin-1 level in patients with suspected narcolepsy

The patho-aetiology of narcolepsy Type I (NT1) is the loss of hypocretin-1 secreting neurons in the hypothalamus. Diagnostic criteria for NT1 include excessive daytime sleepiness (EDS) for at least three months not explained by any other condition, cataplexy and cerebrospinal fluid (CSF) hypocretin-1 concentrations lower than 110 pg/ml. In this study we evaluated the utility of measuring CSF hypocretin-1 levels in patients with suspected narcolepsy (N).

The study included 29 consecutively recruited patients at a tertiary sleep centre presenting with EDS for exclusion of N. All patients were examined using an extensive clinical interview followed by two weeks of actigraphy and sleep diary recordings, polysomnography (PSG) and multiple sleep latency testing (MSLT). Additionally, HLA-typing, urinary screening for substances of abuse and a lumbar puncture to measure CSF hypocretin-1 expression using radioimmunoassay were carried out.

In sum, 19 patients (66%) had a CSF hypocretin-1 level <110 pg/ml, of whom two had current severe depression without any features of narcolepsy except EDS. The predictive potential of hypocretin-1 measurement in diagnosing narcolepsy revealed a positive predictive value (PPV) of 89%, a specificity of 83%, with both negative predictive value (NPV) and sensitivity equal to 100%.

Despite a high sensitivity and specificity, the MSLT is not always a reliable diagnostic test for narcolepsy and where this uncertainty exits, CSF hypocretin-1 concentrations <110 pg/ml can be useful. However, due to a lower PPV and specificity at this cut-off, it may also not be entirely reliable as a stand-alone diagnostic test, particularly in the context of severe depression.