Week 3 - Spine Surgery Anesthesia Flashcards
Describe the anatomy of the spinal cord
Extends from medulla (at foramen magnum) to the Filum Terminale (thread like structure made up of connective tissue) –> attaches to coccyx
31 Pairs of spinal nerves that carry motor and sensory information
-lumbar and sacral nerves have long roots that make up the Cauda Equina
Divided into dorsal, lateral, and ventral regions
H shaped central gray region – unmyelinated fibers
What spinal cord nuclei are in the dorsal horn? (5)
- Marginal zone (Rexed’s Laminae I)
- Substantia Gelatinosa (Rexed’s Laminae II)
- Nucleus Proprius (Rexed’s Laminae III, IV)
- Neck of Dorsal Horn (Rexed’s Laminae V)
- Base of Dorsal Horn (Rexed’s Laminae VI)
What spinal cord nuclei are in the intermediate zone? (1)
Clark’s Nucleus, Intermediate Nucleus
Rexed’s Laminae VII
What spinal cord nuclei are in the ventral horn? (2)
Commissural Nucleus (Rexed’s Laminae VIII)
Motor Nuclei (Rexed’s Laminae IX)
What spinal cord nuclei are in the gray matter surrounding the central canal?
Grisea Centralis (Rexed’s Laminae X)
Describe the Pain and Temperature Pathways
Pain and temp fibers from the brain synapse in trigeminal ganglion:
- enter the pons and form trigeminal nerve (CN V)
- these fibers subsequently synapse with second order neurons in the nucleus of the descending tract of CN V
- pain and temp receptors in the skin of the trunk and extremities send signals to spinal cord via dorsal roots of spinal nerves
*pressure and touch also synapse with CN V
Where do the fibers for proprioception of the muscles of the face (facial expressions) and mastication synapse?
They synapse in cell bodies located in midbrain (mesencephalic nucleus)
Where do fibers from the trunk and extremities that carry proprioception, vibration, and touch info synapse?
They synapse with neuron cell bodies in dorsal root ganglion
-first order axons then enter dorsal white column and ascend to medulla
What sensations does the Dorsal Column-Medial Lemniscal System sensory pathway transmit?
- Touch sensations requiring high degree of localization of the stimulus
- Touch sensations requiring transmission of fine gradations of intensity
- Phasic sensations, such as vibratory sensations
- Sensations that signal movement against the skin
- Position sensations from the joints
- Pressure sensations related to fine degrees of judgment of pressure intensity
What sensations does the Anterolateral System sensory pathway transmit?
- Pain
- Thermal sensations, including both warm and cold
- Crude touch and pressure sensations capable only of crude localizing ability on the surface of the body
- Tickle and itch sensations
- Sexual sensations
What do efferent (motor) pathways from the brain transmit?
Information to voluntary muscles of body, smooth muscle and cardiac muscle and various glands
What is the Corticospinal tract?
Motor pathway that supplies voluntary muscles of trunk and extremities
*9 CN supply voluntary muscles of the head and neck
What transmits efferent signals to smooth muscle, cardiac muscle, and some glands (lacrimal, bronchial)?
Autonomic PREganglionic fibers
What two arteries supply blood to the spinal cord?
One Anterior Spinal Artery – supplies blood to lower 2/3 of spinal cord
Two Posterior Spinal Arteries – supplies remaining 1/3
- also branches of the vertebral artery and radicular arteries
- run the length of the spinal cord
What is the Artery of Adamkiewicz? Where does it enter?
Responsible for much of the blood supply to the lower 2/3 of the spinal cord
Enters the cord at approximately T7
What is the difference between direct injury and indirect injury to the spinal cord?
Direct = trauma
Indirect = disease to surrounding bones, tissues, or blood vessels
What are symptoms of a cervical spinal cord injury?
- Unilateral/bilateral extremity weakness/paralysis
- Loss of consciousness
- Complete quadraperisis and respiratory failure
- Potential difficult airway
*associated with head trauma
Describe the cervical spine anatomy
Divided into upper (C1-C2) and lower (C3-C7) segments – upper have less space than lower segments
C1 (Atlas) forms Atlanto-Occipital joint with C2 (Axis)
*injury to atlanto-occipital joint results in high mortality due to anoxia (8-19% of fatal cervical spine injury autopsies)
What are the risks of direct laryngoscopy in airway management of spinal cord injury?
Technically difficult with cervical collar in place
Potential strain and movement of unstable spine (even with collar on)
What airway techniques create the greatest movement of the C-spine?
Chin lift
Jaw thrust
*pre-intubation positioning
What are the advantages and disadvantages of awake fiberoptic intubation for an unstable C-Spine?
Advantages:
- maintain neck in midline position
- good alternative if difficult mask ventilation
- immediate ability to assess and document post intubation neurologic exam
Disadvantages:
- technically challenging
- increased time requirement/cooperative patient
- equipment availability
- trauma (blood/fluids in airway) may compromise visibility/ aspiration risk due to topical med administration
After the initial insult what are spinal cord injury patients at risk for?
Spinal Shock
- associated with lesions above T4
- aggressive fluid management
- hemodynamic monitoring (bradycardia and vasodilation)
- monitor temperature closely (Poikilothermic below level of spinal cord injury)
After acute injury cases of hyperkalemia and cardiac arrest in paraplegic pts following succinylcholine admin – increased extrajunctional receptors
What are some of the common spine surgeries?
- Spinal Fusion: most common surgery for back pain – fuse vertebrae together
- Laminectomy: removal of parts of the bone, bone spurs, or ligaments in the back – relieves pressure on spinal nerves and can ease pain or weakness (can make spine less stable - may lead to fusion down the line)
- Foraminotomy: bone cut away at the sides of the vertebrae to widen the space where nerves exit – relieves pressure on the nerves and ease pain (may make spine less stable)
- Diskectomy: removal of all or part of the disk
What is scoliosis?
Abnormal curving of the spine
Congenital (present at birth) or Neuromuscular (caused by nervous system problems that affect muscles)
Treatment depends on cause, extent, where the curve is and if you are still growing
What are the potential risks of prone positioning?
Facial swelling
Blindness:
- central retinal artery occlusion (10%)
- anterior or posterior ischemic neuropathy (most common)
- cortical blindness (not associated with pressure on the globe – describe in prone pts in pins (nothing near the eyes)
What are the mechanisms of retinal ischemia?
External Compression –> Retinal ischemia or Anterior chamber ischemia or Extraocular muscles ischemia –>
Reperfusion –>
- Swelling increased –> Further increases in compartment pressure –> Propotosis or Extraocular muscle damage or Chemosis or Corneal injury
- Retinal hypoperfusion –> Retinal reperfusion injury –> Retinal cell loss
What is neurophysiologic monitoring used for? What are the types?
Used to identify nerves and define brain regions and spinal cord regions that may be at risk for damage/ischemia
-intraop monitoring is used routinely in spine cases where instrumentation is used
Types: EEG, SSEP, MEP, BAEP, EMG
*SSEP and MEP most common in spine surgery
How do SSEP’s (Somatosensory Evoked Potentials) differ from EEG?
- EEG is a random, continuous signal that arises from the ongoing activity of the outer layers of the cortex whereas an evoked potential is the brains response to a repetitive stimulus along a specific nerve pathway
- EEG signals range from 10-220 millivolts – Evoked potentials are smaller in amplitude (1, 5, 20 microvolts), requiring precise electrode positioning and special techniques
Signs of ischemia are noted by ____ latency and _____ amplitude in SSEPs.
Signs of ischemia are noted by INCREASED latency and DECREASED amplitude in SSEPs.
How do volatile agents (Iso, Sevo, Des, Nitrous oxide) affect the latency and amplitude of SSEPs?
They INCREASE latency and DECREASE amplitude
Iso, Sevo, Des decrease synaptic transmission»_space; axonal conduction
*marked decreased cortical SSEP with >0.5 MAC
How does Fentanyl affect the latency and amplitude of SSEPs?
Slight increase in latency
Slight decrease in amplitude
How does Propofol affect the latency and amplitude of SSEPs?
Increases latency
No change in amplitude
How does Ketamine affect the latency and amplitude of SSEPs?
Increased latency
Increased amplitude
How does Etomidate affect the latency and amplitude of SSEPs?
Increased latency
Increase amplitude
How does Dexmedetomidine affect the latency and amplitude of SSEPs?
No change to either latency or amplitude
How do paralytics affect SSEPs?
Paralytics do not affect SSEPs
Where in the brain does SSEPs of the median nerve detect ischemia?
In the middle cerebral artery
Where in the brain does SSEPs of the posterior tibial nerve detect ischemia?
In the anterior cerebral artery
When are MEPs (motor evoked potentials) used? Why are they used?
MEPs are used in high risk circumstances due to the theoretical limitations of SSEPs in monitoring motor function
Assess function of the motor cortex and descending tracts – stimulated by electrical or magnetic stimulation
Peripheral response of MEP is recorded by measuring the compound muscle action potential
*use of soft bite block to prevent dental/tongue injury
How do volatile agents (Iso, Sevo, Des) affect MEPs?
Marked inhibition of neuronal activation (suppress the response)
Multi-pulse stimulation allows use of ~0.5 MAC
Less inhibition with Des
How does Nitrous Oxide affect MEPs?
Less inhibition than other volatile agents
Multi-pulse stimulation allows use of 50-60%
How does Propofol affect MEPs?
Marked inhibition of neuronal activation
Multi-pulse stimulation allows 50-100 mcg/kg/min with N2O or opioid
How does Ketamine and Etomidate affect MEPs?
Minimal suppression
- Ketamine 1-4 mg/kg/h with N2O, Propofol, Remi
- Etomidate 10-30 mcg/kg/min N2O, Propofol, Remi
How do Opioids affect MEPs?
Less inhibition than volatile agents
Used to supplement other agents
How do Muscle relaxants affect MEPs?
Infusion to maintain constant subparalytic weakness of T1 at 20-50% of baseline
What could be the cause of an abnormal MEP signal response?
Anesthesia: has there been a change in anesthetic? med bolus, muscle relaxant? any physiologic changes?
Surgical Team: depends on surgery
- CEA - may need to place shunt to improve blood flow – ischemia
- Spine - retraction, distractin of spine - cord ischemia
What are the anesthetic considerations for spine surgery?
- Standard ASA monitoring for all cases
- One PIV for single level – Two for >/= 3 levels +/- instrumentation – especially in prone cases
- Art line is indicated based on pt history and extent of surgery
- All posterior cases will be prone – Positioning is very important
- Type and Screen
- Will there be a wakeup test during the procedure?
