The lesion
In narcolepsy type 1, the great majority of the orexin-producing neurons in the lateral hypothalamus have been destroyed. The prevailing explanation is autoimmune: the evidence includes a very strong association with a specific immune-system genotype, and onset that often follows an immune challenge. It is a small, precise lesion with outsized consequences, because those neurons hold the entire wake-sleep switch steady.
The loss is permanent. Nothing currently available regrows the neurons, which is why the therapeutic strategy is to substitute for what they used to do.
What it feels like
The best-known symptom is excessive daytime sleepiness, but sleepiness is a poor word for it. Patients describe not drowsiness but a sleep pressure that arrives without warning and cannot be resisted, several times a day, regardless of how well they slept the night before.
The defining symptom is cataplexy: a sudden loss of muscle tone, triggered by emotion — most often laughter — while the person stays fully conscious. Cataplexy is what separates type 1 from type 2. It is the muscle paralysis of REM sleep arriving in the middle of waking life, which is exactly what you would expect when the latch holding the two states apart is gone.
The same boundary failure produces the rest of the syndrome: sleep paralysis, hypnagogic hallucinations at the edge of sleep, and — counterintuitively for a disease of sleepiness — badly fragmented night-time sleep.
How it is diagnosed
Diagnosis usually involves an overnight sleep study followed by a multiple sleep latency test the next day, which measures how fast a person falls asleep across a series of scheduled naps and whether they drop into REM sleep abnormally quickly.
There is also a direct measurement. Orexin can be assayed in cerebrospinal fluid, and a low or undetectable level is diagnostic of narcolepsy type 1. It requires a lumbar puncture, so it is not routine, but it is the closest thing in sleep medicine to a definitive test — it measures the missing molecule itself.
Why this class was built for it
Existing treatments work downstream of the problem. Wake-promoting agents push the wake system harder; oxybates consolidate night-time sleep and reduce cataplexy. Both help, and neither addresses the fact that a specific signal is absent.
An orexin agonist does. It is a replacement strategy rather than a compensation strategy, and narcolepsy type 1 is the cleanest possible test case, because the deficiency is documented and measurable in the individual patient.
That is why oveporexton, alixorexton and cleminorexton all ran their pivotal work here first.