HOW EVOKED POTENTIALS BEHAVE , WHEN BRAIN SUFFERS FROM ISCHEMIA ❓

🕶Cerebral ischemia slows neurotransmission and neuronal energy metabolism, resulting in decreased amplitude and increased latency of specific peaks. 

🕶For SSEPs, a 50% reduction in amplitude and/or a 10% increase in latency [changes in the central conduction times, namely, the interpeak latencies between the N14 and N20 peaks] of SSEP signals from the baseline are generally accepted to be a significant change 

🕶A 50% reduction on SEP amplitude has been shown to occur when cerebral blood flow decreases below 14 mL/100 g/min 

🕶MEP have less well-defined warning criteria as compared to SSEPs; however, increased stimulus thresholds and/or decreased MEP amplitudes in relation to dramatic events (i.e., clip application) are indicative of pending neurologic insult. 

🕶For BAEP, an increase in latency of more than 1 msec, particularly in wave V, is considered to be clinically significant. 

🕶Unlike EEG monitoring the evoked potential tests can detect subcortical functional status by way of perforating branches such as the anterior choroidal and medial striate arteries 

Reference: Anesthesiology Research and Practice Volume 2014, Article ID 595837, Controversies in the Anesthetic Management of Intraoperative Rupture of Intracranial Aneurysm, Tumul Chowdhury, Andrea Petropolis,Marshall Wilkinson, Bernhard Schaller  Nora Sandu and Ronald B. Cappellani

CAUSES OF DETERIORATION OF #GCS IN #NEUROSURGICAL #ICU

☑️IN A PATIENT WITH ANEURYSMAL-SAH, AFTER CLIPPING OR COILING 

❓Re-bleed 

❓Acute hydrocephalus 

❓Cerebral vasospasm 

❓Seizure

☑️ IN A PATIENT WITH TRAUMATIC BRAIN INJURY (#TBI)

❓Re-accumulation of haematoma 

❓ Haemorrhage into contusion 

❓ Oedema 

❓ Seizure

☑️IN A PATIENT, WHO UNDERWENT #CRANIOTOMY

❓ Haematoma (sub-dural/intracerebral etc.) 

❓ Oedema 

❓ Seizure 

❓ Pneumocephalus/ Air Encephalocoele

#NeuroAnesthesia , #NeuroCriticalCare , #NeuroIntensiveCare , #NeuroICU , #Anaesthesiology , #Anesthesiologist , #Anesthesia , #CriticalCare

BRIEF FUNCTIONAL ANATOMY OF THE BRAIN

✔️Precentral gyrus contains the primary motor cortex

✔️Premotor cortex lies immediately anterior to Primary motor cortex ( Brodmann’s area 6 on the lateral surface of the frontal lobe.)

✔️Premotor cortex is active in response to EXTERNAL visual or somatic sensory cues (e.g. reaching for an object in full view, or identifying an object by touch alone). Also has role in bilateral postural fixation (e.g. stabilization of the hips during walking).

✔️The supplementary motor area occupies a neighbouring part of Brodmann’s area 6 on the medial surface of the frontal lobe.

✔️The supplementary motor area seems to respond to INTERNAL cues, especially intentions to make voluntary movements (even if the movement is not carried out).

✔️Lesions of the supplementary motor cortex are associated with contralateral inability to initiate movements (akinesia).

✔️The inferior frontal gyrus of the dominant hemisphere (usually the left) contains the (Broca’s) motor speech area (Brodmann’s areas 44 and 45)

✔️The somatic sensory cortex, occupies the entire postcentral gyrus (Brodmann’s areas 3, 1 and 2, rostral to caudal).

✔️ Caudal to the somatosensory cortex is the sensory association cortex, which is divided into a superior and an inferior parietal lobule by an intraparietal sulcus.

✔️ The superior lobule is thought to be responsible for conscious awareness of the contralateral half of the body. Lesions of this part of the cortex may result in neglect of the contralateral side of the body.

✔️ The inferior lobule in the dominant (usually left) hemisphere is associated with language functions.

✔️ The cortex adjacent to the parieto-occipital sulcus and the calcarine sulcus, which lies on the medial surface of occipital lobe, constitutes the primary visual cortex (Brodmann’s area 17).

✔️ The superior surface of the temporal lobe contains the primary auditory cortex (Brodmann’s areas 41 and 42), responsible for the conscious perception of sound. Unilateral lesions of the primary auditory cortex will therefore cause partial deafness in both ears.

✔️ The auditory association cortex, known as Wernicke’s area corresponds to Brodmann’s area 22 in the dominant hemisphere. It allows for understanding of the spoken word, and connects with other language areas of the brain.

✔️ The temporal lobe curls inward to form the hippocampus which lies in the floor of the inferior horn of the lateral ventricle, and forms part of the limbic system. Its functions relate to short-term memory and the emotional aspects of behaviour.

✔️ Lying close to the anterior end of the hippocampus is the amygdala. Its a mass of subcortical grey matter that also forms part of the limbic system. It is associated with the conscious appreciation of smells.

✔️ The insula is one of the cortical centres for pain, and is also involved in involuntary activities such as the control of viscera by the autonomic nervous system.

✔️ The basal ganglia includes

• striatum (caudate nucleus, putamen of the lentiform nucleus and the nucleus accumbens) 

• pallidum (globus pallidus of the lentiform nucleus) 

• subthalamic nucleus 

• compact part of the substantia nigra 

✔️ Four basic circuits are known to occur from the cerebral cortex, through the basal ganglia and back to the cortex, by a direct or indirect route: 

-a motor loop is concerned with learned movements;

-a cognitive loop with motor intentions; 

-a limbic loop with emotional aspects of movement and 

-an oculomotor loop with voluntary saccades.

✔️ Limbic system is composed of the the parahippocampal and cingulate gyri, the septal area, the hippocampal formation and the amygdala. Declarative memory (i.e. of new facts and events) may occur as modifications of synapses within the hippocampus.

✔️ Diencephalon comprises, from superior to inferior, the epithalamus, thalamus, subthalamus and hypothalamus.

✔️ The thalamus is the largest component of the diencephalon and is composed of numerous nuclei.

✔️ The epithalamus consists principally of the pineal gland that secretes melatonin and is involved with circadian rhythm and regulation of the onset of puberty and the habenular nuclei.

✔️ The subthalamus contains the subthalamic nucleus which connects to the globus pallidus and substantia nigra and is involved with the control of movement.

✔️ The hypothalamus has important connections with the limbic system, a controlling influence on autonomic nervous system activity and a role in neuroendocrine function.

✔️ The cerebellum coordinates movement by maintenance of equilibrium, posture and muscle tone at an unconscious level. Also concerned with muscular coordination, including trajectory, speed and force of movements.

✔️ The tectum (roof) of the midbrain is formed by four colliculi –the paired superior colliculi are associated with the visual system, and the inferior colliculi with the auditory system.

Reference: The brain: functional divisions, Leo Donnelly, Neurosurgery, Neuroradiology & Neurocritical Care in Anaesthesia

CSF PRESSURES AND VOLUMES

SURGICAL APPROACHES FOR VARIOUS ANEURYSMS

🌀FRONTO TEMPORAL (PTERIONAL ) CRANIOTOMY 

🔺Allows access to most aneurysms on lateral and anterior Circle of Willis vessels.

🔺Internal Carotid : Paraclinoid / Superior hypophyseal artery

🔺Internal Carotid : Ophthalmic artery

🔺Posterior Commununicating Artery

🔺Anterior Choroidal artery

🔺Internal Carotid Bifurcation 

🔺Middle Cerebral Artery

🔺Anterior Commununicating Artery

🌀PTERIONAL / SUBTEMPORAL APPROACH 

🔺Upper Basilar Artery

🔺Superior Cerebellar Artery

🌀SUBOCCIPITAL APPROACH 

🔺Vertebral Artery

🔺Posterior Inferior Cerebellar Artery

🌀COMBINED SUBTEMPORAL AND SUBOCCIPITAL APPROACH 

🔺Basilar trunk

🔺Vertebrobasilar Junction

MEASUREMENT OF CEREBRAL BLOOD FLOW

✔️Can be measured by Fick Principle

✔️This states that the uptake/ release of a substance e.g. O2 (Vo2) by an organ is the product of the blood flow (Q) through that organ and the arteriovenous difference in content (Cao2-Cvo2)

✔️This is applied using Kety-Schmidt technique where 10% Nitrous oxide is inhaled for 10-15 minutes, and the jugular venous concentration is measured and assumed to be the same as the brain concentration

✔️Once CBF is determined, additional values like CMRO2 and vascular resistance may be derived. 

✔️N2O offers significant advantages over other agents used for the measurement of CBF in that it is safe, stable, cheap, readily available and has a partition coefficient unaffected by varying levels of lipid and water and hence is unlikely to change with age or cerebral oedema.

✔️CBF calculated by this technique represents the mean blood flow from the area of the brain draining into the particular jugular venous bulb being sampled: i.e. the ipsilateral cerebral hemisphere. Therefore, the Kety–Schmidt method of CBF measurement is unable to discriminate between grey and white matter and is insensitive to regional changes in flow. 

Ref: Textbook of Neuroanaesthesia and Critical Care, Basil F Matta

ANTERIOR AND POSTERIOR APPROACHES TO CERVICAL SPINE / SAFE Anesthesiologist Series [Surgical Aspects For Empowering Anesthesiologist]

ANTERIOR APPROACH TO CERVICAL SPINE

➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖

✔️ADVANTAGES

Easy access to disc

Little pain

Evacuate disc

Fusion by plate or graft

✖️DISADVANTAGES 

Multiple levels difficult 

May require fusion if unstable

Loss of mobility 

Risk of injury to adjacent structures

POSTERIOR APPROACH TO CERVICAL SPINE

➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖

✔️ADVANTAGES

Access to multiple levels

Direct visualisation of root

Fusion is usually not required

✖️DISADVANTAGES 

Risk of instability 

Poor access to disc space

Poor access to osteophytes

Higher risk of cord injury

Painful procedure

Ref: Bailey & Love’s Short Practice of Surgery, 24/e

💹EEG IN ACUTE INTOXICATIONS

💠Barbiturates 

Usually they produce fast activity (15-35/sec ) In the acute overdose, these type of fast activity may still be present, with some deceleration (10-16/sec). The inability of the cortex to produce, barbiturate induced fast activity has been described as a sign of cerebral impairment 

💠Benzodiazepines 

〰Fast activity (15-25/sec) range

💠Tricyclic Antidepressants

〰Widespread and poorly reactive 8-10/sec activity with paroxysmal discharges

💠Lithium Carbonate

〰Marked slowing, paroxysmal bursts, and triphasic waves

💠Neuroleptic drugs ( phenothiazines, butyrophenones)

〰Diffuse slow activity; often with burst like appearance or intermingled with paroxysmal discharges. Fast frequencies are absent

💠Opioids:

〰Relatively little repercussion in EEG; if produces profound coma → diffuse slowing

💠Carbon monoxide

〰Massive slowing (1-4/sec)

💠Ethyl alcohol

〰Effect on EEG is mild

💠Methyl alcohol

〰EEG slowing is correlated with acidosis rather than blood and CSF methanol levels

💠Organophosphorous compounds

〰Initially, fast EEG activity may be supplanted by slow activity 

Ref: EEG and intensive care medicine : RB Hansen, E Niedermeyer; Prog neurol Surg, vol 12, pp 105-145 ( Karger, Basel 1987)

Nonthrombolytic therapy for patients who are not candidates for rTPA after ischemic stroke

TARGET: 

✔️optimization of CBF

✔️prevention of secondary brain injury, infarct extension & hemorrhagic conversion 

✔️avoid post-stroke complications (e.g., pulmonary embolus and aspiration pneumonia)

✔️ early mobilization and rehabilitation

✔️ attention to psychiatric ( e.g. Depression) and social consequences of stroke and assistance with daily activities.

(1) Airway management, hemodynamic monitoring, and treatment of increased ICP 

(2) aggressive antihypertensive therapy may exacerbate ischemia by decreasing CBF. A generally accepted cutoff for the administration of antihypertensive therapies is SBP >220 mm Hg, DBP >120 mm Hg, or MAP >130 mm Hg. 

(3) In the absence of hemorrhage on a CT scan, antiplatelet therapy is initiated in the form of aspirin starting with 325 mg by mouth followed by 81 to 160 mg daily.

(4) Multiple studies have failed to show a benefit to heparin administration ; but anticoagulation is usually initiated when atrial fibrillation is present.

(5) Hyperglycemia worsens neurologic outcome. So , euglycemia (80 to 110 mg/dL) is beneficial if it can be achieved without substantially increasing the risk of hypoglycemia.

(6) Seizures are treated with phenytoin, loading dose 15 mg/kg i.v. over 20 minutes followed by 5 to 7 mg/kg/day, or fosphenytoin, loading dose PE 15 to 20 mg/kg i.v., and then 4 to 6 PE mg/kg/day.

(7) DVT prophylaxis is provided using pneumatic compression or low-molecular-weight heparin e.g. enoxaparin, 0.5 mg/kg subcutaneously twice a day.

(8) After the evaluation of airway reflexes and adequacy of swallowing, nutrition is provided via a suitable route.

(9) A few studies suggest that positioning the head end of the bed at 15° for patients who have normal ICP improves CBF and neurologic function.

(10) Rehabilitation and psychiatric evaluation. e.g. Treatment of depression and other psychiatric comorbidities facilitates rehabilitation and improves functional status.

Ref: Seth Manoach, Jean G. Charchaflieh, Handbook of Neuroanesthesia, 4th Edition, Lippincott Williams & Wilkins

ANAESTHETIC IMPLICATIONS IN EPILEPSY SURGERIES

✔️If Awake Craniotomy/ wake up testing is planned, the procedure details should be explained to the patient, including what he/she is expected to hear and feel, inside the O.R.

✔️Get a brief description of the seizures and any prodromal symptoms 

✔️Continue anti epileptics through the morning of surgery

✔️Antiepileptics can reduce duration of action of NMBAs

✔️Usually a preoperative WADA test or fMRI would have been done, to check, whether the side of proposed surgery has any cerebral dominance or speech function 

✔️Abnormal LFTs can be expected with long term Valproate or Carbamazepine therapy

✔️WHAT THE COMMON AEDs DO?

✔️Phenytoin and Phenobarbitone: Reduce Hematocrit

Carbamazepine, Valproate,Ethosuximide and Primidone : Reduce platelet count

Carbamazepine and Primidone: Reduce WBC count

✔️If a difficult intubation is anticipated ( for e.g. After fixation of stereotactic frame ), an awake fibreoptic intubation can be considered 

✔️Moderate Sedation, if required can be achieved with Propofol 25-75 ug/kg/min plus Remifentanil 0.02-0.05 ug/kg/min OR Dexmedetomidine 1ug/kg/min over 10 minutes followed by 0.2-0.7 ug/kg/min

✔️Anticipated Surgical duration: 3 hours

✔️EBL: 250-500 ml ; blood transfusions are seldom needed

✔️Pain score 2-4

✔️If an Asleep-Awake-Asleep technique is used:

GA  continued for positioning, craniotomy and till the exposure of the surgical area. Then patient is allowed to awaken, to monitor neurological function during brain stimulation ( LMA removed OR  if ETT, it is removed over a tube exchanger). When seizure area, is fully delineated, GA is reinstituted.