CUSHING TRIAD

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Cushing’s Triad is a clinical triad described with bradycardia, Systolic hypertension and irregular breathing. It is named after Harvey William’s Cushing, who actually demonstrated cushing’s reflex (again him) first. 

Cushing’s Reflex is the physiological phenomena, in which raised intracranial pressure leads to irregular breathing, bradycardia and systolic hypertension/ increased wide pulse pressure.  Cushing’s Reflex is also known as Cushing’s Effect, Cushing’s Reaction, Cushing’s Phenomenon and Cushing’s Law.

MECHANISM

The Cushing reflex is complex and seemingly paradoxical. The reflex begins when some event causes increased intracranial pressure (ICP). Since cerebrospinal fluid is located in an area surrounded by the skull, increased ICP consequently increases the pressure in the fluid itself. The pressure in the cerebral spinal fluid eventually rises to the point that it meets and gradually exceeds the mean arterial blood pressure (MABP or MAP). When the ICP exceeds the MABP, arterioles located in the brain's cerebrum become compressed. Compression then results in diminished blood supply to the brain, a condition known as cerebral ischemia.
During the increase in ICP, both the sympathetic nervous system and the parasympathetic nervous system are activated. In the first stage of the reflex, sympathetic nervous system stimulation is much greater than parasympathetic stimulation. The sympathetic response activates alpha-1 adrenergic receptors, causing constriction of the body's arteries. This constriction raises the total resistance of blood flow, elevating blood pressure to high levels, which is known as hypertension. The body's induced hypertension is an attempt to restore blood flow to the ischemic brain. The sympathetic stimulation also increases the rate of heart contractions and cardiac output. Increased heart rate is also known as tachycardia. This combined with hypertension is the first stage of the Cushing reflex.
Meanwhile, baroreceptors in the aortic arch detect the increase in blood pressure and trigger a parasympathetic response via the vagus nerve. This induces bradycardia, or slowed heart rate, and signifies the second stage of the reflex.[Bradycardia may also be caused by increased ICP due to direct mechanical distortion of the vagus nerve and subsequent parasympathetic response. Furthermore, this reflexive increase in parasympathetic activity is thought to contribute to the formation of Cushing ulcers in the stomach, due to uncontrolled activation of the parietal cells. The blood pressure can be expected to remain higher than the pressure of the raised cerebral spinal fluid to continue to allow blood to flow to the brain. The pressure rises to the point where it overcomes the resisting pressure of the compressed artery, and blood is allowed through, providing oxygen to the hypoxic area of the brain. If the increase in blood pressure is not sufficient to compensate for the compression on the artery, infarction occurs.
Raised ICP, tachycardia, or some other endogenous stimulus can result in distortion and/or increased pressure on the brainstem. Since the brainstem controls involuntary breathing, changes in its homeostasis often results in irregular respiratory pattern and/or apnea. This is the third and final stage of the reflex.
Commonly, in various pressor reflexes, the central chemoreceptors, which transform chemical signals into action potentials, and the baroreceptors, which sense pressure changes of the carotid sinuses, work together to increase or decrease blood pressure. However, chemoreceptors do not play a role in the Cushing reflex. Thus, even in the presence of sympathetic stimulation from the brain, which would normally produce tachycardia, there is in fact bradycardia.

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FUNCTION

Raised intracranial pressure can ultimately result in the shifting or crushing of brain tissue, which is detrimental to the physiological well being of patients. As a result, the Cushing reflex is a last-ditch effort by the body to maintain homeostasis in the brain. It is widely accepted that the Cushing reflex acts as a baroreflex, or homeostatic mechanism for the maintenance of blood pressure, in the cranial region.

Specifically, the reflex mechanism can maintain normal cerebral blood flow and pressure under stressful situations such as ischemia or subarachnoid hemorrhages. A case report of a patient who underwent a spontaneous subarachnoid hemorrhage demonstrated that the Cushing reflex played a part in maintaining cerebral perfusion pressure (CPP) and cerebral blood flow. Eventually, the ICP drops to a level range where a state of induced hypertension in the form of the Cushing reflex is no longer required. The Cushing reflex was then aborted, and CPP was maintained. It has also been shown that an increase in mean arterial pressure due to hypertension, characteristic of the reflex, can cause the normalization of CPP. This effect is protective, especially during increased intracranial pressure, which creates a drop in CPP.

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