User:Philnomar23/The Cushing Reflex

This is the Wikipedia proposal for "Cushing Reflex":

Cushing Reflex: One Page Proposal

The goal of this project is to modify the existing Wikipedia article of 'Cushing Reflex' to the standing of a Good Article. Cushing Reflex is a nervous system response to an increase in intracranial pressue. This reflex was discovered by Harvey Cushing, a doctor who worked in both endocrinology and neurology. Cushing observed a distinct relationship between increasing cerebral compression and increasing blood pressure. This increase of pressure has been shown to affect both the sympathetic and parasympathetic portions of the autonomic nervous system. The primary three effects of the Cushing Reflex have been dubbed Cushing's Triad. This refers to the increase in blood pressure, irregular breathing, and bradycardia. This nervous system reflex is relevant in instances ranging from severe head trauma to using it as a warning sign during neurosurgery.

Outline

Here is our general outline of subtopics that is subject to change as additional references are acquired:

1. Introduction (with a picture included)
2. History/Discovery
3. Anatomy and Location of Structures involved
   1. Brainstem
4. Mechanism
   1. How it occurs (ICP etc.)
5. Importance of Reflex
   1. Warning sign for brain ischemia particularly during an neuroendoscopy
   2. Concussions
   3. Hemorrhages
6. Symptoms/Signs of the reflex
   1. Cushing's Triad
7. Current Research/Where future research is headed
8. See also
9. References

The work will be divided as follows: Sean Dikdan will research History and Anatomy, Philip Johnson will research the Mechanism and its Importance, and Cynthia Cepeda will research the Signs and Symptoms of the Cushing Reflex along with Current Research. These will all be compiled together as is seen in the preliminary outline above. This should offer a broad and thorough overview of the Cushing Reflex.

Causes

As first postulated by Harvey Cushing, raised ICP is the primary cause of the Cushing Reflex. Furthermore, sustained moderate increases in ICP, as opposed to rapid and large ones, allows for the Cushing Reflex to occur. These dramatic pressure rises do not allow for the mechanism of the reflex to sufficiently take place. This elevated ICP can occur due to numerous pathways of brain impairment such as subarachnoid hemorrhages, ischemia, trauma, including concussions, hypoxia, tumors, and stroke. In addition, during typical neurosurgical procedures on patients involving neuroendoscopic techniques, frequent washing of the ventricles have been known to cause high ICP.

The Cushing Reflex also results from low cerebral perfusion pressure (CPP), specifically below 15 mmHg. CPP is defined as the difference between mean arterial blood pressure (MABP) and ICP. Either an increase in ICP or decrease MABP can reduce CPP, resulting in the reflex. Typically, it is considered as a global number, reflecting an equal distribution within the intracranial vault. With normal conditions of MABPs of 80 to 100 mm Hg and ICPs of 5 to 10 mm Hg, CPP can be expected to be 70 to 85 mmHg. Raised ICP was also found in combination with a drop in CPP, which preceded the emergence of brain plateau waves. These plateau waves were subsequently erased after a Cushing Reflex response occurred 10-15 seconds prior to this.

In a 71 year old male case study, it was concluded that the physiological conditions brought on by ischemia alone could also potentially spark a Cushing reflex response (7). Induced subarachnoid hemorrhage studies on cats via injection of autologous blood into the cisterna magna of the brain confirmed that ICP causes mechanical distortion of bulbar sensors in the medulla (10,11). This was then followed by sympathetic nervous system over activity, characteristic of the reflex.

Physiological Significance

Raised ICP 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 CR 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 CBP and CBF under stressful situations such as ischemia or subarachnoid hemorrhages.

A case report on a patient who underwent a spontaneous subarachnoid hemorrhage demonstrated that the CR played a part in maintaining CPP and CBF as evidenced by the absence of neurological deterioration during neurological stress. Eventually, the CPP drops to a level range where a state of induced hypertension in the form of the CR is no longer required. The CR was then aborted, and CPP was maintained. This effect is protective, especially during increased ICP.

Clinical Importance

One of the more prominent warning signs of the CR is that patient death will likely occur, sooner rather than later. As a result, when a CR is detected, immediate care is needed for the patient to survive. Unfortunately, death may be inevitable if the cause of a CR and ICP is unknown.

Since the presence of a Cushing Reflex 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 ICP 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 CR. Once the initial stage of the CR was discovered, tachycardia combined with hypertension, offered a much more reliable and swift warning sign of high ICP. Wan et al. also reported that the presence of a CR due to an ICP could allow one to conclude that ischemia has occurred in the posterior cranial fossa. The reflex begins when some event causes increased intracranial pressure (ICP). This increases the hydrostatic pressure of cerebrospinal fluid to the point that it meets and gradually exceeds mean arterial pressure (MAP). As the ICP exceeds the MAP, the cerebral arterioles become compressed, diminishing blood supply to the brain, a condition known as cerebral ischemia. This constriction raises the total peripheral resistance of blood flow and elevates blood pressure causing hypertension in an attempt to restore perfusion to the ischemic brain. The sympathetic stimulation also increases heart contractility (tachycardia) and cardiac output. This combined with hypertension is the first stage of the Cushing reflex.

Meanwhile, baroreceptors in the carotid arteries detect the increase in blood pressure and trigger a parasympathetic response via vagal stimulation. This induces bradycardia, and signifies the second stage of the reflex. Bradycardia may also be caused by increased ICP impinging on the vagal nerve, mechanically stimulating a parasympathetic response. An irregular respiratory pattern and/or apnea is typically the result of herniation or increased pressure on the brainstem. This is the third and final stage of the reflex.

Commonly, in various pressor reflexes, the central chemoreceptors of the brain and the baroreceptors 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.

References

Cushing H. "Concerning a definite regulatory mechanism of the vasomotor centre which controls blood pressure during cerebral compression." Bull Johns Hopkins Hosp. 126 (1901): 289–92. 

Dickinson, C. "REAPPRAISAL OF THE CUSHING REFLEX THE MOST POWERFUL NEURAL BLOOD PRESSURE STABILIZING SYSTEM." Clinical science 79.6 (1990): 543-50.

Erol, Demet. "A Risk during an Elective Repair of Craniosynostosis: The Cushing Reflex." Paediatric anaesthesia 17.5 (2007): 496-7.

Fodstad, H., P. Kelly, and M. Buchfelder. "History of the Cushing Reflex." Neurosurgery 59.5 (2006): 1132-7.

Fox, J., et al. "THE CUSHING REFLEX IN THE ABSENCE OF INTRACRANIAL HYPERTENSION." Annals of clinical research.Supplement.47 (1986): 9-16.

Grady, P., and O. Blaumanis. "PHYSIOLOGIC PARAMETERS OF THE CUSHING REFLEX." Surgical neurology 29.6 (1988): 454-61.

Kalmar, A., et al. "Value of Cushing Reflex as Warning Sign for Brain Ischaemia during Neuroendoscopy." British journal of anaesthesia 94.6 (2005): 791-9.

Meyer, G., T. Ducker, and L. Kempe. "THE CUSHING REFLEX." Transactions of the American Neurological Association 94 (1969): 358-9.

---. "THE CUSHING REFLEX." International Congress Series (1969): 103.

Molnr, Csilla, et al. "Harvey Cushing, a Pioneer of Neuroanesthesia." Journal of anesthesia 22.4 (2008): 483-6.