Concepts of Neuroanesthesia
Gianluca Bini, DVM MRCVS DACVAA
Key Points:
Most commonly neuroanesthesia is performed on patients with spinal cord or brain disorders.
A neurological exam and collecting a thorough history (e.g., abnormal behavior, history of seizures, etc.) can completely change the anesthetic plan.
An increase in intracranial pressure may lead to CBF disruption and encephalic herniation.
The main anesthetic goal for patients with intracranial disease is to avoid an increase
intracranial pressure.
If intracranial pressure increases a Cushing reflex can be observed, this generally manifests as an increase in blood pressure and decrease in heart rate.
The main anesthetic goals for patients with spinal disorders are adequate pain management as well as monitoring for potential complications such as venous sinus hemorrhage, hypoventilation and regurgitation.
In veterinary medicine, neuroanesthesia is mainly used to define anesthesia for patients with a disorder of the central nervous system. Most commonly we anesthetize veterinary patients for neurologic procedures involving the spinal cord or the brain. In the first case most of our focus is around pain management and how to manage the potential post-surgical deterioration of the patient (e.g., hypoventilation post ventral slot). On the other hand, dealing with a patient with intracranial disease is a very different and more complex scenario.
The most common intracranial diseases in our small animal patients are brain tumors, traumatic brain injuries and seizure disorders. The real problem for patient undergoing anesthesia with the first two is the potential for an increase in intracranial pressure, and with these we can also include those patients that have tumors originating outside the skull (e.g., nasal tumors) but that when growing they ultimately invade the skull.
While intracranial pressure will likely increase in case of neoplasia, it is mainly reported only in moderate to severe forms of traumatic brain injury, and that is caused by the cerebral edema or mass effect from the hemorrhage 1 . Patients with seizures (unless caused by an intracranial mass) are less of an anesthetic concern compared to those with an increase in intracranial pressure, although debates on anesthetic drugs affecting the seizure threshold are common.
Neurological history and signs
When examining a patient with intracranial disease it is important to gather a very thorough history. We need to ask the owner if they have ever noticed abnormal behavior, if there is an history of seizures, and what is the incidence of the seizures (e.g., if there is an increasing frequency or if the frequency of the seizures has been stable) as well as if they had any previous diagnostics (e.g., if their pet has ever had an MRI or a CT), this may help identify a cause for those seizures or the abnormal behavior. Such information can change the anesthetic management, for example if we know there is an intracranial mass, we treat these patients differently than a patient we know it does not have one.
Before anesthetizing these patients, we also need to investigate their neurological status. Repetitive pupillary light reflex and size examination can help monitoring the progression of the neurological disorder. If the pupil size increases, or it become mydriatic, or the patient has anisocoria or Horner’s syndrome, doesn't necessarily mean the patient has a brain tumor, but it definitely warrants a neurological exam and advanced imaging.
Some signs may aid in identifying patients with increased intracranial pressure, commonly these patients show: vomiting, pupillary dilation Cushing’s reflex and deteriorating neurological status. If a patient suffered traumatic brain injury hypovolemic shock is commonly present, and volume resuscitation should be performed to target a mean arterial pressure of 80-100 mmHg.
Intracranial pressure
Inside the skull there is a fixed space, which contains the brain, cerebrospinal fluid (CSF) and the intracranial blood. If the volume of one or more of these increases the volume of the others have to decrease or there will be an increase in pressure inside the skull. This is often referred as the Monro- Kellie hypothesis.
The cerebral perfusion pressure is the net driving pressure leading the blood flow to the brain and is derived from the subtraction of the intracranial pressure from the mean arterial pressure. 1 When the intracranial pressure increases the cerebral perfusion pressure may be decreased, hence the body will increase the mean arterial pressure to counteract it and try to maintain a normal cerebral perfusion pressure. When the baroceptors located in the aortic arch and carotid sinus sense this systemic blood pressure increase, they will slow down the heart rate. This systemic hypertension with associated bradycardia is referred as Cushing’s reflex. 1 Eventually the increase in intracranial pressure will be so high that it may lead to CBF disruption and encephalic herniation, which can be fatal for the animal.
Anesthetic considerations for patients with intracranial disease
The main goal under anesthesia is to try and minimize any potential increase in intracranial pressure. The cerebral blood flow is the only factor that we can easily influence, decreasing it may prevent an increase in intracranial pressure and prevent exacerbation of the patient neurological status.
Cerebral blood flow is affected by the arterial partial pressure of carbon dioxide, the arterial partial pressure of oxygen, the cerebral metabolic rate and ultimately the mean arterial pressure. The rate of change in cerebral blood flow per mmHg arterial partial pressure of carbon dioxide change also plays a role, some drugs allow the anesthetist to get a more effective reduction of cerebral blood flow when manipulating the end-tidal carbon dioxide.
Anesthetic considerations for patients with spinal disorders
As mentioned in the introduction, when anesthetizing patients with spinal disorders the anesthetist should opt for a multimodal analgesic approach in order to effectively manage peri-operative pain. Neurological deterioration is another important consideration in all patients undergoing neurosurgery, but the most challenging for the anesthetist are definitely those undergoing cervical spine surgery, this is due to the close proximity of the phrenic and the recurrent laryngeal nerve. 2 Post- operative complications such as life threatening hypoventilation and pneumonia may occur. 3-4
Hemorrhage from the venous sinus is also a relatively common complication in spinal surgery, 2 in severe cases a transfusion may be necessary.
References
1. Miller RD. Miller’s Anesthesia. 8th Edition, Churchil Livingstone, San Diego, 2015.
2. Gordon-Evans W. Ventral Slot and Fenestration. In: Griffon DJ, Hamaide A, eds. Complications in
Small Animal Surgery. Wiley Blackwell. 2016;590–595. doi:10.1002/9781119421344.ch84
3. Beal MW, Paglia DT, Griffin GM, Hughes D, King LG. Ventilatory failure, ventilator management,
and outcome in dogs with cervical spinal disorders: 14 cases (1991-1999). J Am Vet Med Assoc. 2001;218:1598–1602. doi:10.2460/javma.2001.218.1598
4. Java MA, Drobatz KJ, Gilley RS, Long SN, Kushner LI, King LG. Incidence of and risk factors for postoperative pneumonia in dogs anesthetized for diagnosis or treatment of intervertebral disk disease. J Am Vet Med Assoc. 2009;235:281–287. doi:10.2460/javma.235.3.281
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