Cushing reflex (also referred to as the vasopressor response, the Cushing effect, the Cushing reaction, the Cushing phenomenon, the Cushing response, or Cushing's Law) is a physiological nervous system response to increased intracranial pressure (ICP) that results in Cushing's triad of increased blood pressure, irregular breathing, and bradycardia. It is usually seen in the terminal stages of acute head injury and may indicate imminent brain herniation. It can also be seen after the intravenous administration of epinephrine and similar drugs. It was first described in detail by American neurosurgeon Harvey Cushing in 1901.
Definition
The Cushing reflex classically presents as an increase in systolic and pulse pressure, reduction of the heart rate (bradycardia), and irregular respiration. It is caused by increased pressure inside the skull. These symptoms can be indicative of insufficient blood flow to the brain (ischemia) as well as compression of arterioles.
In response to rising intracranial pressure (ICP), respiratory cycles change in regularity and rate. Different patterns indicate a different location of the brain where the injury occurred. The increase in ventilation is exhibited as an increase in rate rather than depth of ventilation, so the Cushing reflex is often associated with slow, irregular breathing. As a result of the now defective regulation of heart rate and blood pressure, the physiologic response is decreased blood flow peripherally, which can present as Mayer waves. These are simply pathologic waves seen in HR tracings (i.e., arterial lines, electrocardiograph (ECG, etc.), which reflect decreased intravascular blood flow. This decreased flow often causes a reflexive HTN, or hypertension (increase in blood pressure) despite the actual decrease in intravascular volume.
Differential diagnosis
Whenever a Cushing reflex occurs, there is a high probability of death in seconds to minutes. As a result, a Cushing reflex indicates a need for immediate care. Since its presence is a good detector of high ICP, it is often useful in the medical field, particularly during surgery. During any neurosurgery being performed on the brain, there is always a likelihood that raised intracranial pressure may occur. Early recognition of this is crucial to the well being of the patient. Although direct measurement of ICP is possible, it is not always accurate. In the past, physicians and nurses have relied on hemodynamic changes or bradycardia, the late phase of the reflex, to identify the ICP increase. Once the initial stage of the Cushing reflex (bradycardia combined with hypertension) was discovered, it offered a much more reliable and swift warning sign of high ICP. It was found that hypertension and bradycardia occurred 93% of the time when cerebral perfusion pressure (CPP) dropped below 15 mmHg due to raised ICP. Also, the Cushing reflex is known to arise only from acute prolonged raises in ICP. Thus, it can be used as a tool by physicians to differentiate acute and chronic rises in ICP.
It has also been reported that the presence of a Cushing reflex due to an ICP increase could allow one to conclude that ischemia has occurred in the posterior cranial fossa. Finally, the Cushing reflex may be one of many ways to identify if a patient has rejected a transplanted organ. Aside from the innate autoimmune response, ischemia in the cranial region has been detected with a transplanted organ that is being rejected. As such, the presence of a Cushing reflex due to ICP can indicate that ischemia may be occurring due to foreign organ rejection.