Brain herniation is defined as the movement of brain tissue from one intracranial compartment to another. There are three key intracranial compartments created by two rigid in-folds of dura known as the falx cerebri and the tentorium. The falx cerebri separates each brain hemisphere. The tentorium separates the cranial vault into supratentorial and infratentorial compartments. The infratentorial compartment is also known as the posterior fossa. Within the posterior fossa are the cerebellum and brainstem. There are two openings in the infratentorial compartment: the foramen magnum and the tentorial notch. The tentorial notch allows communication between the infratentorial and supratentorial compartments. The foramen magnum allows passage out of the posterior fossa and into the spinal canal.
A tonsillar herniation is characterized by the descent of the cerebellar tonsils through the foramen magnum, which compresses the medulla against the clivus/odontoid process. It is described as "coning" as the brain tissue is squeezed down through the foramen like being squeezed into a cone. (See media image 1) Clinically, progressive compression of the medulla causes Cushing's reflex, eventually resulting in death.
Historically, LP was a more common cause of tonsillar herniation. If a patient has raised ICP, then lowering the pressure within the spinal canal can increase the pressure gradient between the posterior fossa and the spinal canal, resulting in tonsillar herniation through the foramen magnum. Before the advent of computed tomographic (CT) scan in the 1970s, patients with signs of raised intracranial pressure had "therapeutic lumbar taps," where a large volume of CSF was removed. In modern practice, if a patient is symptomatic with signs of raised ICP (headache, nausea, reduced consciousness, or focal neurological deficit), then intracranial imaging with CT scan or magnetic resonance imaging (MRI) is indicated before LP. This allows an assessment as to whether it is safe or not to proceed with an LP.
The incidence of tonsillar herniation is not documented, as it is a physiological response to a variety of underlying pathological processes. The most common cause is traumatic brain injury (TBI), but other causes include intracerebral hemorrhage and subarachnoid hemorrhage. The incidence of TBI worldwide is estimated to be 69 million individuals each year.
The Monroe-Kellie doctrine states that the contents of the cranial cavity have fixed volumes. There is approximately 1400 mL of brain tissue, 150 mL of CSF, and 150 mL of blood within the cerebral vasculature. The skull is a fixed structure once childhood sutures have fused at around 18 months of age. The only remaining outlet from the cranial vault is through the foramen magnum at the base of the posterior fossa. As per the Monroe-Kellie doctrine, if there is an increase in one of the components, the others must compensate. For example, if the brain parenchyma swells following an ischemic stroke, the total brain tissue volume increases. To compensate, the CSF and blood volumes decrease. Typically the volume of CSF is reduced first by being pushed out through the foramen magnum into the spinal canal. This is followed by venous and capillary blood, which is emptied predominantly via the jugular veins. This ability for the cranial cavity to compensate for increased volumes is known as intracranial compliance. Once this mechanism has been exhausted, brain parenchyma will herniate from one compartment to another, or posterior fossa herniation occurs through the foramen magnum. When this occurs, there is a loss of intracranial compliance.
A particularly important example for tonsillar herniation is a cerebellar hemorrhage, which can cause rapid herniation of the cerebellar tonsils down through the foramen magnum where it compresses the medulla. In 1901, Cushing published his paper "Concerning a definite regulatory mechanism of the vasomotor center, which controls blood pressure during cerebral compression", describing the physiological changes that occur during compression of the medulla. This later became known as Cushing's reflex, which gives rise to Cushing's triad of elevated blood pressure, bradycardia, and irregular breathing. Cushing demonstrated that increased intracranial pressure increases blood pressure to a level slightly above the pressure that is being exerted upon the medulla, and postulated that the increase in blood pressure was a reflex response to brainstem ischemia.
Cerebral perfusion pressure (CPP) is equal to the mean arterial pressure (MAP) of the carotid arteries minus the ICP [CPP = MAP - ICP]. As the ICP increases, the CPP drops. CPP is the force that pushes blood through the cerebral vasculature. Intracranial pathology that increases the ICP reduces blood flow to the brain tissue, and thus, a sympathetically driven compensatory rise in MAP is initiated to increase the CPP. There are two proposed theories to explain the subsequent bradycardia. First, the increased blood pressure activates baroreceptors in the aortic arch, which triggers parasympathetic activation. Second, intracranial compression of the vagus nerve causes bradycardia.
Regardless of which mechanism explains the bradycardia, this is considered a terminal stage as blood pressure continues to rise to perfuse the brainstem as it is compressed against the clivus/odontoid process by the cerebellar tonsils. As further damage occurs to the brainstem, the heart rate may become tachycardic, and breathing slows with apneic episodes. Eventually, the breathing may become agonal before the patient enters respiratory and cardiac arrest.
Generally, with raised intracranial pressure and brainstem compression, the patient is in a coma. On examination, the patient may have abnormal posturing, such as decortication or decerebration. However, with late tonsillar herniation, the patient will have flaccid paralysis. Pupils are likely to be abnormal with varying patterns depending on the underlying pathology. Midbrain compression tends to give midsized and unreactive pupils, while a pontine hemorrhage gives pinpoint and unreactive pupils. If a supratentorial lesion with tentorial herniation occurred first, the patient might have unequal pupils due to oculomotor compression.
Cushing described four stages of intracranial hypertension:
Neurological assessment in patients with TBI is based on the Glasgow coma scale, which is accepted worldwide. However, it does have some criticisms; in particular, the scale is not ordinal or linear. For instance, losing two points in speech may represent a focal neurological deficit with injury to a brain region responsible for speech as opposed to representing a disorder of consciousness. A head computed tomography (CT) scan is mandatory in patients presenting with coma unless a known and reversible medical or metabolic cause is found. Patients with reduced consciousness and unequal pupils are very likely to have a so-called "surgical lesion." Therefore, urgent imaging is required to diagnose intracranial pathology that may be amenable to surgical intervention.
Signs of mass effect on the head CT scan include midline shift, obliteration of the basal cisterns, effacement of the ventricles, obstructive hydrocephalus, and sulcal effacement. The mass effect should prompt the clinician to look for possible herniation. Tonsillar herniation is visualized on coronal imaging by crowding of the foramen magnum, effacement of the CSF cisterns surrounding the brainstem and, on sagittal views, the descent of the cerebellar tonsils below the level of the foramen magnum. There may also be signs of obstructive hydrocephalus due to compression of the fourth ventricle.
If the patient needs acute management, the placement of an ICP monitoring system may be indicated. This is usually in the form of an ICP bolt or intraventricular catheter. With herniation syndromes, ICP is typically high, at least over 20 mm Hg. The waveform will likely indicate a loss in intracranial compliance. On the ICP waveform, there are three waves; the first (P1) represents systole and is known as the "percussion wave" or "arterial kick." The second (P2) represents the "tidal wave" of brain compliance, and the third (P3) is the venous wave or "dicrotic notch." In normal physiology, P1 should be higher than P2, which is higher than the P3. (see media image 2) When P2 is the highest wave, compliance is lost. This indicates an intracranial compartment that is no longer able to compensate for increased volumes, and so, the pressure rises. An additional benefit to placing an intraventricular catheter to monitor the ICP is that CSF can be therapeutically drained, as required, to lower ICP.
A tonsillar herniation is indicative of an underlying pathology that may include trauma, hemorrhage, tumor, or hydrocephalus. Therefore, treatment is directed at the underlying pathology. Firstly, as with all comatose patients, supportive measures are initiated. These include:
Surgical interventions are based on three broad principles:
Congenital abnormalities like Chiari malformation show tonsillar descent below the foramen magnum and may be found incidentally on imaging. Though Chiari can be symptomatic, it is not an acute phenomenon and does not cause acute brainstem compression.
Autonomic dysreflexia can also cause high blood pressure with reflexive bradycardia; however, these patients are not comatose, and their breathing is unaffected.
Acute spinal injury with neurogenic shock, if cervical, can cause bradycardia, but with associated hypotension.
Tonsillar herniation is the final stage of an aggressive intracranial process. However, Cushing described four stages of tonsillar herniation, and some studies have suggested surgical intervention early in the process could prevent mortality. One case series looked at early recognition of tonsillar herniation in posterior fossa tumor cases and showed death could be avoided with early surgical decompression.
Tonsillar herniation is typically considered a terminal event if not treated promptly. When it occurs, the chance of recovery is significantly reduced. When a herniation syndrome produces respiratory compromise, as found in Cushing's triad, there is no chance of a meaningful recovery. Brain herniation following TBI has a poor prognosis, and herniation with signs of brainstem compression is "uncommonly reversible". Patients in a coma after gunshot wounds with clinical evidence of brainstem impairment had a 100% mortality.
Other than the underlying neurological deficit that may follow tonsillar herniation (permanent brain damage, brain death, respiratory arrest, coma, and death), other complications include the sequelae of critical illness. These include ventilator-associated pneumonia, deep vein thrombosis, pulmonary embolus, and critical illness myopathy. Approximately 5% of patients with TBI have an associated visual system injury. TBI with skull base fractures can also present later with hypopituitarism.
Measures to reduce road traffic collisions, workplace accidents, and traumatic brain injury are vital at a national public health level. Such measures include mandatory wearing of seatbelts and helmets on motorcycles and bicycles.
Tonsillar herniation and all other cerebral herniation syndromes cause a significant burden to the patient and the relatives and produce a substantial economic cost to the health care system. A prolonged hospitalization and extensive rehabilitation to the patient are the principal factors involved.
Tonsillar herniation is usually considered a terminal event if not treated promptly.
Prevention and adequate management of increased ICP is necessary to reduce the risk of tonsillar herniation.
Cerebral perfusion pressure is equal to mean arterial pressure minus the intracranial pressure. [CPP = MAP - ICP]
Cushing’s triad includes:
Tonsillar herniation is a medical emergency and requires urgent assessment. Intervention may require a wide variety of interprofessional team members, including emergency medicine clinicians, anesthesiologists, intensivists, traumatologists, and neurosurgeons. Patients with intracranial pathologies require close nursing observations and monitoring. Nurses will identify early signs of herniation and will consult with the intensivist and neurosurgery for adequate management. If the patient survives the acute episode, rehabilitation will likely be required. This includes a variety of healthcare professionals, including physiotherapists, occupational therapists, speech therapists, dieticians, and psychologists. If timely treated, a functional recovery is indicated by a return to activities of daily living.
Collaboration between the healthcare team members and communication are crucial elements for good results. The interprofessional care provided to the patient must follow an evidence-based approach to improve outcomes.
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