Cataplexy


 * Not to be confused with Catalepsy.

Cataplexy is a sudden and transient episode of muscle weakness accompanied by full conscious awareness, typically triggered by emotions such as laughing, crying, terror, etc. It is the cardinal symptom of narcolepsy with cataplexy affecting roughly 70% of people who have narcolepsy, and is caused by an autoimmune destruction of the neurotransmitter hypocretin, which regulates arousal and wakefulness. Cataplexy without narcolepsy is rare and the cause is unknown.

The term cataplexy originates from the Greek κατά (kata, meaning "down"), and πληξις (plēxis, meaning "stroke").

Presentation
Cataplexy manifests itself as muscular weakness which may range from a barely perceptible slackening of the facial muscles to complete muscle paralysis with postural collapse. Attacks are brief, most lasting from a few seconds to a couple of minutes, and typically involve dropping of the jaw, neck weakness, and/or buckling of the knees. Even in a full-blown collapse, patients are usually able to avoid injury because they learn to notice the feeling of the cataplectic attack approaching and the fall is usually slow and progressive. Speech may be slurred and vision may be impaired (double vision, inability to focus), but hearing and awareness remain normal.

Cataplexy attacks are self-limiting and resolve without the need for medical intervention. If the patient is reclining comfortably, he or she may transition into sleepiness, hypnagogic hallucinations, or a sleep-onset REM period. While cataplexy worsens with fatigue, it is different from narcoleptic sleep attacks and is usually triggered by strong emotional reactions such as laughter, anger, surprise, orgasm, awe, and embarrassment, or by sudden physical effort, especially if the person is caught off guard.

Occurrence
In a 24-hour period, cataplectic attacks usually occur between the hours of 10 am and 9 pm and very few attacks occur between the hours of 10 pm and 9 am. Attacks can last from a few seconds up to ten minutes, and may occur up to several times per week [4-6 times]. Cataplexy is considered “typical” when it is always of short duration (< 5 minutes).

Physiology


Cataplexy is considered secondary when it is due to specific lesions in the brain that cause a depletion of the hypocretin neurotransmitter. Secondary cataplexy is associated with specific lesions located primarily in the lateral and posterior hypothalamus. Cataplexy due to brainstem lesions is uncommon particularly when seen in isolation. The lesions include tumors of the brain or brainstem and arterio-venous malformations. Some of the tumors include astrocytoma, glioblastoma, glioma, and subependynoma. These lesions can be visualized with brain imaging, however in their early stages they can be missed. Other conditions in which cataplexy can be seen include ischemic events, multiple sclerosis, head injury, paraneoplastic syndromes, and infections such as encephalitis. Cataplexy may also occur transiently or permanently due to lesions of the hypothalamus that were caused by surgery, especially in difficult tumor resections. These lesions or generalized processes disrupt the hypocretin neurons and their pathways. The neurological process behind the lesion impairs pathways controlling the normal inhibition of muscle tone drop, consequently resulting in muscle atonia.

Theories for episodes
A phenomenon of REM sleep, muscular paralysis, occurs at an inappropriate time. This loss of tonus is caused by massive inhibition of motor neurons in the spinal cord. When this happens during waking, the victim of a cataplectic attack loses control of his or her muscles. As in REM sleep, the person continues to breathe and is able to control eye movements.

Studies
A study of 40 cataplectic patients (age range 13–23 years) reported that sagging of the jaw, inclined head, drooping of the shoulders, and transient buckling of the knees were the most common presentations. Slurred speech may be present. However, diaphragmatic paralysis resulting in central apneas has not been reported. There is an isolated form that involves facial muscles exclusively. Cataplexy may rapidly reoccur repeatedly, giving birth to "status cataplecticus", and to the "limp man syndrome" as described by Stalh et al. "Status cataplecticus" is rare and can be extremely disabling to the individual. Cataplexy also occurs more frequently in times of emotional stress and when patients are deprived of napping while sleepy.

A survey of 100 cataplectic patients from the Stanford Sleep Disorders Clinic (age range 14–24 years) reported that 93 percent of the attacks lasted less than two minutes, 6 percent reported events lasting up to five minutes, and 0.94 percent reported events lasting longer than five minutes. There is a bimodal pattern of the age of onset of symptoms; either at 15 or 35 years. It has also been reported past the age of forty. Guilleminault et al. investigated 51 prepubertal children with narcolepsy; in 10 subjects (5 years and younger) cataplexy was the symptom first recognized. Cataplectic symptoms in general tend to decrease with age. A review of 100 patients with cataplexy at the Stanford Sleep Disorders Clinic (age range 12–20 years) reported that 62 of these patients stopped taking anti-cataplectic medications after 10 years. However, the general decrease in cataplectic symptoms with aging may be reversed after the experience of a significant emotional upset, such as a loss of spouse in older subjects.

Hypocretin
The hypothalamus region of the brain regulates basic functions of hormone release, emotional expression and sleep. A study in 2006 in "Tohoku Journal of Experimental Medicine" concluded that the neurochemical hypocretin, which is regulated by the hypothalamus, was significantly reduced in study participants with symptoms of cataplexy. Orexin, also known as Hypocretin, is a primary chemical important in regulating sleep as well as states of arousal. Hypocretin deficiency is further associated with decreased levels of histamine and epinephrine, which are chemicals important in promoting wakefulness, arousal and alertness.

Treatment
Cataplexy is treated pharmacologically. There are no behavioral treatments for cataplexy. The cholinergic and noradrenergic neurotransmitter systems are targeted in the treatment of cataplexy. Despite its relation to narcolepsy, in most cases, cataplexy must be treated differently and separate medication must be taken. For many years, cataplexy has been treated with tricyclic antidepressants such as imipramine, clomipramine or protriptyline. The main feature of tricyclics is their ability to inhibit the reuptake of norepinephrine and serotonin at the nerve endings. However these can have unpleasant side-effects and so have been generally replaced by newer drugs such as venlafaxine.

For cataplexy associated with narcolepsy, Xyrem (sodium oxybate) is often recommended.

Monoamine oxidase inhibitors may be used to manage both cataplexy and the REM sleep-onset symptoms of sleep paralysis and hypnagogic hallucinations.

Wise (2004) noted that people with narcolepsy will often try to avoid thoughts and situations that they know are likely to evoke strong emotions because they know that these emotions are likely to trigger cataplectic attacks.

A newer class of antidepressants with selective serotonergic reuptake blocking properties known as the selective serotonin reuptake inhibitors fluoxetine, paroxetine, sertraline, citalopram has become popular for the treatment of cataplexy. This class of drugs has an active metabolite with norepinephrine reuptake blocking properties (such as nor-fluoxetine). Serotonin reuptake inhibitors (SSRIs) have fewer side effects compared to the tricyclics and can be used in adults and children. A side effect worth mentioning regarding tricyclic antidepressants and SSRIs is the risk of development of REM behavior disorder (RBD) due to elimination of the normal REM sleep atonia. These drugs are known to decrease stage REM sleep. They can also decrease muscle atonia associated with REM sleep and consequently dissociate REM sleep. As a consequence, the subject may act out his or her dreams and cause harm to himself/herself or others.

Emerging therapies
Emerging therapies include Hypocretin Gene Therapy and Hypocretin Cell Transplantation for narcolepsy-cataplexy.

The newest agent for the treatment of cataplexy is sodium oxybate (gamma-hydroxybutyrate [GHB]), known commercially as Xyrem. Although its mechanism is unknown, it reduces cataplectic attacks and other manifestations of REM sleep. GHB increases slow wave sleep, decreases nighttime awakenings, and consolidates REM sleep. Sodium oxybate is the only medication that will improve both cataplexy and daytime sleepiness. Cataplectic symptoms are improved much faster. Because it can cause daytime sleepiness, during this time, sodium oxybate should be taken concomitantly with a stimulant.