Panic attacks need not necessarily be a bolt from the blue. They could be seen at least an hour ahead, in waves of significant physiological instability. But the patients themselves might be unaware of the accumulating pattern of subtle changes.
In a rare study in which patients were monitored around-the-clock, portable recorders captured changes in respiration, heart rate and other bodily functions.
"The results were just amazing," said Alicia E Meuret, an assistant professor of Psychology at the Southern Methodist University, Dallas and lead researcher on the study.
"We found that in this hour preceding naturally occurring panic attacks, there was a lot of physiological instability. These significant physiological instabilities were not present during other times when the patient wasn't about to have a panic attack."
"The changes don't seem to enter the patient's awareness," Meuret said. "What they report is what happens at the end of the 60 minutes — that they're having an out-of-the blue panic attack with a lot of intense physical sensations. We had expected the majority of the physiological activation would occur during and following the onset of the panic attack. But what we actually found was very little additional physiological change at that time."
The diagnostic standard for psychological disorders, the DSM-IV, defines panic attacks as either expected or unexpected. Those that are expected, or cued, occur when a patient feels an attack is likely, such as in closed spaces, while driving or in a crowded place.
"But in an unexpected panic attack, the patient would say they were sitting watching TV when they were suddenly hit by a rush of symptoms, and there wasn't anything that made it predictable."
To sufferers and researchers alike, the attacks are a mystery. Meuret and her colleagues discovered the significant physiological instabilities using change-point analysis, a statistical method that searches for points when changes occur in a "process" over time.
"This analysis allowed us to search through patients' physiological data recorded in the hour before the onset of their panic attacks to determine if there were points at which the signals changed significantly," said David Rosenfield, an associate professor of Psychology with the same Methodist varsity and lead statistician on the project.
The study is significant not only for panic disorder, but also for other medical problems where symptoms and events have seemingly "out-of-the blue" onsets, such as seizures, strokes and even manic episodes.
"I think this method and study will ultimately help detect what's going on before these unexpected events and help determine how to prevent them," Meuret said. "If we know what's happening before the event, it's easier to treat it."
The duo have reported their results in the journal Biological Psychiatry in the article "Do Unexpected Panic Attacks Occur Spontaneously?"
By definition, the majority of the 13 symptoms of panic attack are physiological: shortness of breath, heart racing, dizziness, chest pain, sweating, hot flashes, trembling, choking, nausea and numbness. Only three are psychological: feeling of unreality, fear of losing control and fear of dying.
"Most patients obviously feel that there must be something going on physically," Meuret said. "They worry they're having a heart attack, suffocating or going to pass out. Our data doesn't indicate there's something inherently wrong with them physically, neither when they are at rest nor during panic. The fluctuations that we discovered are not extreme; they are subtle. But they seem to build up and may result in a notion that something catastrophic is going on."
Notably, the researchers found that patients' carbon dioxide, or C02, levels were in an abnormally low range, indicating the patients were chronically hyperventilating. These levels rose significantly shortly before panic onset and correlated with reports of anxiety, fear of dying and chest pain.
To capture the physiological data, 43 patients wore the monitoring devices for 24 hours on two separate occasions. The researchers collected 1,960 hours of ambulatory monitoring data, including 13 unexpected panic attacks.
Participants, all of whom suffer from panic disorder, were each outfitted with an array of electrodes and sensors attached to various parts of their bodies.
The ambulatory monitoring device was toted in a small waist pack the patients wore. Also included was a portable capnometer to measure CO2 collected from exhaled breath. The physiological responses were recorded continuously as digital data in a time series.
Each monitoring pack included a "panic button." Patients were instructed to press the button if they had an attack and to write down their symptoms. By triggering the panic button, patients inserted a marker into the time-series data, marking the moment the attack began.
The sensors measured eight physiological indices, including changes in respiration, such as how deep, fast or irregular people were breathing; cardiac activity; and evidence of sweating.
The study's results invite a reconsideration of the DSM diagnostic definition that separates "expected" from "unexpected" attacks, Meuret said.
Also, the study might explain why medication or interventions aimed at normalizing respiration for treating panic are effective, she said. Medication generally buffers arousal, keeping it low and regular, thereby preventing unexpected panic attacks.
"This study is a step toward more understanding and hopefully opening more doors for research on medical events that are difficult to predict. The hope is that we can then translate these findings into new therapies," she said.