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.[3]

Cushing described four stages of intracranial hypertension:[2]

1. Compensation - intracranial pressure rises without physiological changes
2. Early manifestations - headache and irritability develop
3. Maximum manifestation - severe symptoms with raised blood pressure, bradycardia and slowed breathing
4. Paralysis - deep coma with low blood pressure and tachycardia before apnea and death

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.[16] 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.[17] 

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.[3][17] 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.[3] 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:

• Airway protection if Glasgow coma scale (GCS) of 8 or less.[18]
• Oxygenation and avoidance of hypoxia (O2 saturations target >90%).[19]
• Adequate ventilation for a target paCO2 of 35-40 mmHg. A short period of hyperventilation with a paCO2 of 30-35 mmHg is only used as a temporizing measure to lower paCO2, causing arterial vasoconstriction and reducing cerebral blood flow. This reduces ICP acutely as a life-saving method and may be especially useful for a patient who is acutely deteriorating with signs of herniation while arranging a CT scan or transfer to the operating room.[18][20][21][22][23] A paCO2 below 25 mmHg will cause cerebral ischemia.[23][24]
• Fluid balance with MAP targets of 60 to 70 mm Hg. Avoidance of hypotension (systolic blood pressure <90mm Hg) as it doubles mortality in TBI.[25]
• Osmotic therapy is another temporizing measure that can be initiated when a patient has acute signs of herniation. Again, this works by lowering the ICP and thus improving brain physiology and increasing the CPP. Typical agents used are mannitol (0.25 - 1 gm/ kg body weight) or hypertonic 3% saline (2-5 mL/kg over 10-20 minutes).[18][26]
• Normothermia, treatment of sepsis, and adequate nutrition are initiated.[27][28]

Surgical interventions are based on three broad principles:

1. Removal of the lesion causing mass effect such as hematoma or tumor.[29] 
2. CSF diversion to reduce ICP, such as insertion of an external ventricular drain.
3. Decompressive craniectomy to combat the Monroe-Kellie doctrine of a fixed space allows intracranial contents to swell out of the craniectomy site. There is level III evidence for the use of decompressive craniectomy for refractory raised ICP with evidence of herniation.[30][31][32][33][34]

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.[35]

Autonomic dysreflexia can also cause high blood pressure with reflexive bradycardia; however, these patients are not comatose, and their breathing is unaffected.[36]

Acute spinal injury with neurogenic shock, if cervical, can cause bradycardia, but with associated hypotension.[37]

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.[29] 

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.[3] Brain herniation following TBI has a poor prognosis, and herniation with signs of brainstem compression is "uncommonly reversible".[38] Patients in a coma after gunshot wounds with clinical evidence of brainstem impairment had a 100% mortality.[39]

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.[40] TBI with skull base fractures can also present later with hypopituitarism.[41]

• Neurosurgeon
• Neurologist
• Intensivist
• Respiratory therapy
• Physical medicine and rehabilitation

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.[7][42]

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:

• Increased blood pressure with a widened pulse pressure
• Bradycardia 
• Irregular breathing pattern

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.