Brain Stem Anesthesia: Mechanism, Clinical Presentation, and Prevention

Introduction:

Regional anesthesia has many benefits over general anesthesia regarding safety, efficacy, and patient satisfaction. Local anesthesia is widely used worldwide for regular ophthalmic surgical interventions such as cataracts and glaucoma. Regional anesthesia techniques vary across countries, ranging from topical anesthesia to akinetic blocks such as the retrobulbar, peribulbar, and sub-tenon's block. A retrobulbar block is used to achieve both profound akinesias of the eye and anesthesia of the surgical site during the procedure. Generally, two to four milliliters of anesthetic solution is injected between the extraocular muscles. Peribulbar blocks are a relatively safer alternative in which the needle is pushed along the floor of the orbit, maintaining it tangential to the globe. High volumes of local anesthetic agents are used. The sub-tendon block was introduced as a relatively safer alternative in early 1990. The patient is instructed to look upward and outward. The conjunctiva and tenon's capsule are held with non-toothed forceps five to ten millimeters away from the limbus, generally in the inferonasal quadrant. To reveal the white sclera, a cut is made through these layers with Westcott scissors. Next, a sub-tenon cannula is carefully inserted along the globe's curvature.

Life-threatening risks include retrobulbar hemorrhage, perforations and ischemia of the globe, central retinal artery occlusion, damage to the optic nerve, squint, myotoxicity, intra-arterial injection, brainstem anesthesia (BSA), and cardiopulmonary arrest are possible due to the anatomy of the eye.

What Is the Anatomy of the Brain Stem Anesthesia?

• The brainstem is the dorsal portion of the brain that accomplishes certain functions. The midbrain, pons, and medulla are major subdivisions from rostral to caudal. The reticular activating system is housed in the pons, and the cardiac and respiratory centers are housed in the medulla.

• The brainstem incorporates the most cranial nerve nuclei and serves as a channel for multiple ascending and descending pathways. The reticular formation constitutes a central core of the brainstem, which takes part in various functions such as movement control, pain modulation, autonomic reflexes, alertness, and consciousness.

• Cardiovascular control is another crucial integrative function. Control of respiratory rate and pain sensitivity are the other functions of brain stem controls.

• It also produces monoamine neurotransmitters (norepinephrine, dopamine, and serotonin) that influence behavior, cognition, and mood. Vascular innervation comes from the posterior cerebral artery and the vertebral basilar artery.

What Is the Mechanism of Brain Stem Anesthesia?

The local anesthetic can enter the central nervous system through three different pathways.

• The first mechanism is through LA systemic absorption.

• The second occurs through the direct intra-arterial injection of local anesthetic and backward flow into the cerebral circulation.

• The third mechanism involves perforating the Dural optic nerve sheath and injecting LA into the subdural space, which causes it to accumulate around the brainstem.

• The third mechanism has been recognized as the most probable option when the central nervous system's adverse symptomatology takes place after retrobulbar LA application.

What Are the Clinical Presentation of the Brain Stem Anesthesia?

Derangement of the brainstem's processes is likely associated with multisystemic signs and symptoms because it limits multiple major physiological systems-failure to recognize and handle this complication as soon as possible and effectively may prove fatal.

Onset: The onset of signs and symptoms varies, ranging from a few seconds to 40 minutes after retrobulbar block. Even though there are huge discrepancies, the beginning has been noted to be within 5 to 10 minutes after the retrobulbar block, with a total recovery of consciousness within an hour. Because of the injection into the subdural space and subsequent diffusion into the subarachnoid space near the affected area of the brainstem, the onset of signs and symptoms of brainstem anesthesia can be infrequent.

Signs and Symptoms: The signs and symptoms of brainstem anesthesia are determined by the metabolic derangement of its physiological functions. The collapse of both cardiovascular and respiratory failure is frequently associated.

• The cardiovascular system's symptoms are bluish skin discoloration, decreased blood pressure, and reduced heart rate, but increased blood pressure and heart rate (tachypnea) have also been reported.

• Most patients experience decreased blood pressure and heart rate due to cardiorespiratory collapse, but a vasovagal response must be ruled out.

• A relatively short increase in blood pressure and an increase in heart rate have been observed in some cases, but there is no accurate description of these events.

• It is hypothesized that the local anesthetic agent tracks just below the dura of the optic nerve sheath and enters the cerebrospinal fluid at the cranial end of the neuraxins, causing blockade of the parasympathetic and sympathetic hyperactivity, resulting in hypertension.

• Moreover, blockage of the glossopharyngeal nerve results in a decreased carotid sinus reflex and heart rate.

What Is the Differential Diagnosis of the Brain Stem Anesthesia?

• Clinical features appear quickly and are usually linked to the rate of injection. Moreover, anaphylactic reactions, acute myocardial infarction, massive pulmonary embolism, stroke, and seizure must be ruled out as causative factors of cardiorespiratory arrest.

• It can be challenging to distinguish clinical signs and symptoms of oculocardiac reflex from brainstem anesthesia. After retracting extraocular muscle or pressure around the orbit, the oculocardiac reflex manifests intraoperatively as intense decreased heart rate, defects in the conduction, or cardiac arrest. Still, these events are predicted afterward during surgery.

How Is Brain-Stem Anesthesia Managed?

• It is critical to begin adequate treatment as soon as possible. Then, the patient is revived using Advanced Trauma Life Support (ATLS) principles, which include retaining the airway and analyzing breathing and circulation.

• Oxygen supplemental therapy if the patient is conscious, intravenous fluids, pharmacological circulatory support with vasopressors (agents causing constriction of blood vessels), vasodilators (agents causing dilation of blood vessels), vagolytics (drugs that inhibit the action of the vagus nerve on organs), adrenergic blocking agents, and seizure suppression of seizures, if evident, are all supportive measures.

• It is also essential to ensure the patient is still conscious. Therefore, endotracheal intubation should be employed to secure the airway in patients who present with apnea and become unconscious, and mechanical ventilation should be started as soon as possible.

• The vital signs of the patient must be carefully monitored and adequately assessed. Visual acuity must be determined in patients with ocular signs and symptoms. Other analyses, such as a complete blood count, renal analysis, and electrolytes, as well as consecutive cardiac enzymes and imaging studies, such as a chest radiograph and a computed tomography scan, should be considered if necessary.

• Surgery can usually be performed on stable patients with adequate anesthesia. However, when anesthesia is insufficient, it is favorable to postpone surgery.

• In the event of prolonged apnea or seizure while the patient is intubated, general anesthesia should be taken into account, and surgery should be accomplished if the hemodynamic conditions are stable.

What Are the Ways to Prevent Brain-Stem Anesthesia?

These precautions include maintaining a neutral gaze throughout injection, using a smaller needle, using a local anesthetic agent slowly and selectively, and using relatively safer and simpler alternatives to needle block, such as sub-tenon block.

The Gaze of the Globe during Injection:

• It has been shown that maintaining the globe in the primary gaze with a straight-head fixation keeps the optic nerve far away from the direction of the needle. Intrasheath injection can be prevented if extra precautions are taken to avoid deep penetration of the orbit. This procedure, however, has been found to lower the incidence but avoid the risk of spreading to the central nervous system.

Blunt vs. Sharp Needle:

• Sharp-tipped needles have been strongly advised because they are less likely to pierce the optic nerve sheath. However, blunt needles have been used to spread local anesthetics to the central nervous system.

• Furthermore, blunt needles may cause more pain and trauma, the risk of tissue damage may be more significant, and any theoretical advantage is unproven.

• It is suggested that ophthalmologists and anesthetists use sharp, same-sized needles and syringes consistently to become acquainted with the specific resistance of the tissues that can be anticipated with the standard placement of the retrobulbar injection.

Length of the Needle:

• Needles be 31 millimeters (1.25 inches) in length at maximum. Adequate anesthesia has been obtained with a 12 mm long needle and a higher volume of local anesthetic agents.

The volume of Local Anesthetic Agent:

• It has also been suggested that the amount of drug or volume of local anesthetic used to be reduced. It is proved that blood levels of local anesthetics were comparable in those with apnea and those with a normal response.

• Based on studies, the use of hyaluronidase causes an increase in the central spread of local anesthetics. However, a significant spread was observed even in the absence of hyaluronidase.

• As a result, hyaluronidase is generally thought to be beneficial. Still, at most 7.5 to 15 IU/ml, and the volume of intraconal injection should be at most three to four ml.

Selection of Local Anesthetic Agent:

• Long-acting local anesthetics are proven to be more cardiotoxic than short-acting anesthetics. In addition, the central nervous system can influence local anesthetic cardiotoxicity, and Bupivacaine was identified as more cardiotoxic than Lidocaine.

• Many authorities include a highly effective long-acting local anesthetic agent during the retrobulbar block to extend the duration of anesthesia and achieve postoperative analgesia.

Use of Alternative Technique or Blocks:

• Usually, multiple alternatives to retrobulbar block have been proposed, including peribulbar, Sub tenon's block, and even topical anesthesia.

• Topical anesthesia has several advantages: it is inexpensive, simple to inject and offers comprehensive visual rehabilitation and surface anesthesia.

• This technique is not appropriate for all patients receiving cataract surgery and trabeculectomy. In addition, although the sub-tenon block technique is simple, effective, and safe, it comes with complications.

• Although brainstem anesthesia is uncommon, it can be significantly reduced using a low volume of local anesthetic agents and a short, flexible cannula.

Conclusion:

Brainstem anesthesia is an uncommon but possibly fatal complication of the regional orbital block. Brainstem anesthesia is more common when a block is performed using a long needle in a rotated eye, and the injection is given posteriorly into the apex. A shorter hand should be used, maintaining it tangential to the globe, and the needle's tip should never reach the posterior part of the apex of the orbit. While the sub-tendon block is generally safe, it could be made safer using a shorter and more flexible cannula with a low volume of local anesthetic agents. Brainstem anesthesia is a potentially fatal risk that should avoid, but it does occur, and clinicians must identify and treat it appropriately.