Paroxysmal sympathetic hyperactivity
Paroxysmal sympathetic hyperactivity (PSH) is a syndrome that causes episodes of increased activity of the sympathetic nervous system. Hyperactivity of the sympathetic nervous system can manifest as increased heart rate, increased respiration, increased blood pressure, diaphoresis, and hyperthermia. Previously, this syndrome has been identified as general dysautonomia but now is considered a specific form of it. It has also been referred to as paroxysmal sympathetic instability with dystonia, or PAID, and sympathetic storm. Recently, however, studies have adopted the name paroxysmal sympathetic hyperactivity to ensure specificity. PSH is observed more in younger patients than older ones. It is also seen more commonly in men than women. There is no known reason why this is the case, although it is suspected that pathophysiological links exist. In patients surviving traumatic brain injury, the occurrence of these episodes is one in every three. PSH can also be associated with severe anoxia, subarachnoid and intracerebral hemorrhage, and hydrocephalus.
Signs and symptoms[edit]
Characteristics of paroxysmal sympathetic hyperactivity include:
- fever
- tachycardia
- hypertension
- tachypnea
- hyperhidrosis or diaphoresis
- dystonic posturing
- pupillary dilation
- flushing
In cases where PSH episodes develop post-injury, specifically traumatic brain injury, symptoms typically develop quickly, usually within a week. Symptom onset has been seen to average 5.9 days post-injury. Episodes vary in duration and occurrence. Episodes can last as little as a few minutes or as long as ten hours, and they can occur multiple times a day. Episode duration has been seen to average 30.8 minutes and occur five to six times a day. Episodes can occur naturally or arise from external triggers. Common triggers include pain or stimulation, body turning or movements, and bladder distention. Bladder distention has been observed in patients being treated in intensive care units with the concurrent use of catheters. Symptoms of PSH can last from weeks to years following initial onset. As episodes persist over time, they have been found to become less frequent in occurrence but last for prolonged periods.
Causes[edit]
The number of events that can lead to the development of PSH symptoms is many. The exact pathways or causes for the development of the syndrome are not known. Traumatic brain injury, hypoxia, stroke, anti-NMDA receptor encephalitis (although further associations are being explored), injury of the spinal cord, and many other forms of brain injury can cause onset of PSH. Even more obscure diseases such as intracranial tuberculoma have been seen to cause onset of paroxysmal sympathetic hyperactivity. It is observed that these injuries lead to the development of PSH or are seen in conjunction with PSH, but the pathophysiology behind these diseases and the syndrome is not well understood.
Pathophysiology[edit]
A considerable number of theories exist as to the pathophysiology:
- Epileptiform discharges in the diencephalon, or the interbrain, are a potential theory for PSH. These discharges can be identified using electroencephalography.
- Increased intracranial pressure is another theory. Currently, this theory seems to be less likely than the others. Intracranial pressure has been seen to have no correlation to PSH episodes.
- Disconnection via lesions of the inhibitory efferent pathways from cortical and subcortical areas of the brain is a potential theory. This theory deals with inhibitory pathways being ablated or malfunctioning post-injury. This leads to sympathetic pathways from the cortical and subcortical areas being less controlled, resulting in a 'sympathetic storm'.
- Excitatory-inhibitory models suggest that lesions in the mesencephalic area lessen inhibition pathways from the brain. This is thought to lead to pathways that are usually non-nociceptive becoming nociceptive, which results in the peripheral sympathetic nervous system being over activated.
- Another theory deals with malfunction of the brainstem, specifically excitatory centers in the brainstem. In this case, rather than inhibitory pathways malfunctioning and allowing sympathetic pathways to propagate unhindered, excitatory centers are up-regulated, increasing sympathetic activity.
There are many theories dealing with the pathophysiology of paroxysmal sympathetic hyperactivity. It is possible that none or multiple of these theories are correct. Research that is being conducted on PSH is focused on figuring out these pathways.