Neurological Complications of Heart Surgery

Figure 85.1. Cerebral magnetic resonance, diffusion sequences, axial plane. Ischemic lesions in borderline areas in both brain hemispheres following heart surgery.

85.4.3 Epiaortic Ultrasound and Neurosonological Study

Epiaortic ultrasound is a highly sensitive technique that can be used to determine the severity of atherosclerosis of the ascending aorta, which is one of the main risk factors for the development of neurological complications. It has been demonstrated to be superior to manual palpation and transesophageal ultrasound and is used for intra-operative diagnosis of aortic atherosclerosis.

A carotid color Duplex scan is a necessary tool for both the screening of at-risk patients and diagnosing a post-operative arterial occlusion. Carotid stenosis is a risk factor for the development of neurological abnormalities as it is both a source of emboli and because it contributes to cerebral hypoperfusion in low flow situations. Guidelines of the American Heart Association propose performing a carotid eco-Doppler for patients with stenosis of the anterior descending artery, peripheral vascular disease, smokers, a history of transient ischemic attack (TIA) or previous stroke, carotid murmur or aged older than 65 years.

Transcranial Duplex monitoring during cardiopulmonary bypass has shown the existence of microemboli (documented in the form of HITS – High-Intensity Transient Signals) and could be useful for monitoring cerebral perfusion during surgery.

85.4.4 Biochemical Markers of CNS Complications

The quantification of various substances released following a hypoxic-ischemic cerebral lesion could be a marker for brain damage. Although the concentration of several substances in both serum and the cerebrospinal fluid has been studied (brain-specific CPK, adenylate cyclase, neurospecific enolase, myelin basic protein, S-100 β, etc.), their clinical utility has not been demonstrated. An association has recently been found between pre-operative serum concentrations of anti-NMDA anti-receptor antibodies and the subsequent development of neurological abnormalities, although this requires further confirmation.

85.5 Treatment of CNS Complicacions

The therapeutic direction for heart surgery complications in the CNS can be focused on three aspects: preventive treatment, neuroprotective treatment during surgery and treatment of neurological complications.

85.5.1 Preventive Treatment

A pre-operative assessment reduces the risk of neurological damage by enabling pre-existing modifiable risk factors to be identified and treated. An optimum control of blood pressure, glycemia and heart and kidney function prior to surgery reduces the risk of suffering a subsequent neurological complication. Because of their etiopathogenic importance, carotid stenosis and aortic atheromatosis require special consideration.

Carotid stenosis. Carotid endarterectomy performed prior to, or together with, myocardial revascularization is usually recommended in patients with symptomatic carotid stenosis or in asymptomatic patients with carotid stenosis (unilateral or bilateral) greater than or equal to 80%. This intervention reduces mortality and the risk of developing ischemic stroke after heart surgery.

Aortic atheromatosis. When the aorta presents a thickness above 3 mm, it is essential to choose the area to perform the clamping, aortic cannulation and proximal anastomosis of the aortocoronary grafts carefully, or even consider revascularisation surgery without extracorporeal circulation. No benefit of pre- or intra-operative endarterectomy has been demonstrated.

85.5.2 Neuroprotective Treatment

Protection of the CNS during heart surgery includes pharmacological and non-pharmacological measures.

Non-pharmacological measures

In procedures with extracorporeal circulation, temperature, acid-base balance and blood pressure are related to the risk of development of CNS abnormalities:

Temperature of the cardiopulmonary bypass. Systemic hypothermia reduces cerebral metabolism (7% for each Celsius degree) and has a protective effect against transitory ischemic attacks. Moderate hypothermia is normally used in both closed-chamber procedures that require extracorporeal circulation (32°C) and open procedures (28°C). The beneficial effects of hypothermia disappear when reheating of the system begins. Hyperthermia, with temperatures over 41°C, increases cerebral metabolism, denatures proteins, and facilitates gas embolism, therefore this process should always be performed very gradually.
Acid-base equilibrium. Hypothermia alters the acid-base balance and as a consequence affects CBF. The main techniques to regulate the acid-base balance during hypothermia are alpha-stat and pH-stat. In alpha-stat, measurement of the pH and CO₂ is performed directly in blood at 37°C, whereas in pH-stat this is performed using values adjusted for the patient’s current temperature. The use of pH-stat requires the addition of CO₂ to the circulation during the hypothermic phase of cardiopulmonary bypass, thereby increasing the risk of embolization. Acid-base regulation by means of the alpha-stat technique is recommended as it preserves cerebral self-regulation and reduces the risk of embolization and subsequent neuropsychological abnormalities.
Blood pressure. The majority of patients tolerate mean blood pressures to between 50 and 70 mmHg during extracorporeal circulation well. However, elderly and hypertensive patients will require higher values as their cerebral self-regulation curve is shifted to the right. Hypotension should be avoided during the procedure as this causes cerebral hypoperfusion, less clearance of circulating microemboli and a higher risk of cerebral ischemic lesions and the development of neuropsychological abnormalities.
Surgical position. Placing the patient in the Trendelemburg position seems to reduce the risk of gas embolization, especially during open chamber procedures.
Intraoperative monitoring. Transesophageal echocardiography enables the detection of permeable oval foramen, thrombi in the left atrium in patients with atrial fibrillation, air retention in open chamber procedures, mural thrombi in the left ventricle, thrombosis in the valve prosthesis, valvular vegetations, dissection of the proximal aorta and atherosclerosis of the aorta to be detected, thus allowing the risk of neurological complications to be reduced. Epiaortic ultrasound can also help to locate the most ideal place to perform aortic clamping, thereby reducing possible disruption of the atheroma.
Kind of surgery. Coronary revascularization surgery, which is performed without extracorporeal circulation, is less invasive and reduces the complications of cardiopulmonary bypass. This technique requires a careful positioning and very close cardiac monitoring. The heart rate needs to be controlled pharmacologically (under 75 beats/minute) to reduce the movement of the heart in the surgical field and reduce myocardial consumption during the procedure. A reduction in ischemic stroke has been observed in high-risk patients treated with this kind of surgery, although its influence on the risk of neuropsychological complications is debatable.

Pharmacological Measures

Strategies for pharmacological protection attempt to interfere with the metabolic cascade that occurs during and after cerebral ischemia in order to prevent or delay cell death. NMDA receptor antagonists, calcium and sodium channel blockers, antioxidants, complement inhibitors, magnesium sulfate, leukocyte adhesion inhibitors, barbiturates, amongst other agents, have been studied. Although several drugs have shown neuroprotective effects in experimental models of both focal and global ischemia, none has demonstrated clinical efficacy in humans. Patients treated with aprotinin, a non-specific serine-protease inhibitor, to reduce bleeding during heart surgery seem to present a lower incidence of infarction in the post-operative period. The possible neuroprotective effect of citicoline is currently being assessed in a double-blind placebo-controlled clinical trial in patients with cerebral infarction. Citicoline could be of great use in patients undergoing heart surgery given its excellent safety profile.

85.5.3 Treatment of Complications

Ischemic Stroke

Patients who present a cerebral infarction during the peri-operative period are not candidates for endovenous thrombolytic treatment as their use is specifically contraindicated up to 14 days later due to their high risk of hemorrhage. Despite this limitation, intra-arterial treatment and mechanical thrombolysis are two possible therapies that could be applied in selected patients. Intra-arterial thrombolytic treatment during the first 4.5 hours of symptom onset has been administered to patients with ischemic stroke 2-9 days post-surgery, with high recanalization rates and no significant side effects being documented. Mechanical extraction of the embolus (MERCI system) has been successfully applied to selected patients with a therapeutic window of up to eight hours.

As in all cases of cerebral ischemia, oxygenation, blood pressure, glycemia and body temperature should be monitored and corrected, just as for any other patient who has undergone an ischemic stroke, but taking the underlying heart disease and the surgery performed into account. Hypoxia in patients with saturations below 92% should be treated. Low blood pressure should be avoided and treated posturally, with fluid therapy and, if necessary, low doses of phenylephrine. High blood pressure during the acute phase of stroke is usually reactive and tends to reduce spontaneously over the next few days, therefore blood pressure values need not be “normalized”. In principle, the general treatment criteria established for acute phase ischemic stroke, which are covered in another chapter of this book, should be followed. Isotonic fluids can be administered to prevent dehydration, although a volume overload that may worsen the cerebral edema and heart failure should be avoided. Hyperglycemia is an independent predictor of worse neurological evolution, therefore it should be controlled early and glucose-containing solutions should be avoided. Hyperthermia is also an independent predictor of worse neurological evolution, therefore it should be treated vigorously using physical measures and antipyretics (paracetamol).

Anticoagulation therapy with unfractionated or low molecular weight heparin has not been shown to be beneficial for the acute treatment of atherothrombotic or hypoperfusion-related stroke and increases the risk of systemic complications such as hemopericardium. Anticoagulation therapy should be considered in the case of stroke with a demonstrated and persistent cardioembolic source by weighing up the risk/benefit ratio. The use of platelet anti-aggregants (acetylsalicyclic acid) following coronary revascularisation surgery has been shown to be safe and effective as it improves the prognosis of the infarction.

Hemorrhagic Stroke

Attempts should be made to normalize the abnormal coagulation status and the general treatment protocol for acute phase stroke should be followed.

Confusion Syndrome and Post-anoxic Encephalopathy

Confusion syndrome and post-anoxic encephalopathy should be treated vigorously and the underlying causes that may trigger them, especially reversible metabolic abnormalities, corrected. Post-anoxic myoclonic encephalopathy may be treated with piracetam at high doses (up to 24 g/day); valproic acid can be associated according to the response.

Atypical neuroleptics should preferably be used for patients with acute confusion syndrome; benzodiazepines should be avoided if possible.

Epileptic Seizures

The cause should be treated whenever possible, and symptomatic treatment with antiepileptic drugs (carbamazepine, oxcarbazepine, gabapentin, levetiracetam, etc.) should be administered. Endovenous levetiracetam or valproic acid according to protocol, instead of phenytoin, is recommended in the case of status epilepticus.

Neuropsychological Dysfunction

Cognitive rehabilitation is recommended and, depending on the intensity and kind of deficits detected, specific treatment by specialized personnel should be considered.

85.6 Complications of the Peripheral Nervous System

Post-operative complications that affect the CNS are highly relevant from the clinical point of view because of their prognosis and risk of disabling sequelae. In contrast with the severity of these complications, little attention has been paid to neurological abnormalities that affect the peripheral nervous system, thus meaning that they are frequently underdiagnosed. However, it is estimated that 2-15% of patients who have undergone heart surgery present post-operative brachial plexopathies and 1-30% neuropathies affecting the phrenic or intercostal nerves.

85.6.1 Brachial Plexopathy

This may occur as a consequence of traction of the brachial plexus, its compression between the clavicle and the first rib with sternal retraction, and during dissection of the internal mammary artery. It manifests clinically as pain, motor weakness, paresthesias and decreased osteotendinous reflexes in the upper limb. Paresthesias are more frequent in the fourth and little fingers and may also present upon compression of the cubital nerve in the elbow. Weakness in the extensor muscle of the second finger of the hand and the small abductor muscle of the thumb differentiates plexopathy from cubital neuropathy.

85.6.2 Phrenic Nerve Neuropathy

The incidence of phrenic nerve neuropathy has been estimated at between 1% and 30% of patients, although its frequency has decreased significantly in the last few years. Lesion of the phrenic nerve occurs upon application of ice to cool the heart, a procedure that is used increasingly infrequently. Patients who present bilateral dysfunction of the phrenic nerve require prolonged mechanical ventilation. Most make a full recovery during the first year.

85.6.3 Intercostal Neuropathy

Dissection of the internal thoracic artery may be associated with a lesion of the anterior intercostal nerves. Intercostal neuropathy evolves clinically with paresthesia and “burning pain” in the sternal region and the left anterolateral region of the thorax. Symptoms are usually partially or fully reversible, although patients may require up to two years to recover.

85.6.4 Femoral Neuropathy

The production of a femoral neuropathy following insertion of an intra-aortic balloon as a result of local trauma, vascular occlusion or formation of pseudoaneurysms has been reported.

85.6.5 Optic Neuropathy

Visual impairment due to ischemic optic neuropathy is rare and occurs as a result of embolism in approximately 0.1% of heart surgery patients.

85.7 Diagnosis and Treatment of Complications of the PNS

Diagnosis is based on clinical data, and symptoms usually remit once the acute phase is ended and the general condition improved. Most lesions occur because of reversible disruption of the myelin sheath (neurapraxia) and usually have a good prognosis, therefore early rehabilitation is recommended. If symptoms do not improve within three weeks of evolution, electroneurography and an electromyogram should be performed to assess damage to axons and to locate the site of the lesion. Administration of vitamin B12 may be useful in both cases. In case of dysesthesias or painful hyperalgesia, treatment with carbamazepine, oxcarbazepine or gabapentin, amongst other drugs, is indicated.

General References

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