I have skimmed this thread up until this point, but in response to the question about Neuro exam under anesthesia... This (below) is a teaching thing that I put together for the residents a while back. I gave them a piece of paper in the OR that just had the questions- this is the “answer key”. Enjoy...
More than you ever wanted to know about the eye:
→ Lash reflex: what is a proxy for?
Basically an approximation of the corneal reflex… But, “lash reflex” has limitations when compared to corneal reflex. Non-standardized stimulus, both in method of application and in intensity of stimulus… However, easier to perform than a corneal reflex, and tends to dissapear/reappear at approximately the same depth of anesthesia
→ If you wanted to properly test a corneal reflex, how should you do it?
To test the corneal reflex, use a wisp of cotton or a drop of sterile water to touch the cornea. Intact reflex = eyes blink consensually. Impaired reflex = just one eye blinks. Absent reflex = no blinking.
→ What neurologic pathways are we testing?
Corneal reflex (and by proxy lash reflex): Afferent limb is ophthalmic branch of CN V (V1) → synapses in nucleus of CN V in the pons → efferent limb of reflex is CN VII (facial nerve), also originating from it’s nucleus in the pons → CN VII synapses on orbicularis oculi, causing constriction and eyelid closure
→ What information does it give us?
Assuming no muscle relaxation on board, loss of corneal (and by proxy, maybe also the lash reflex) tells us that our anesthetic agents have caused dysfunction of the motor and sensory nuclei of the eyes and the face. These nuclei, in the pons, are adjacent to the arousal centers in the midbrain/pons/hypothalamus, and so we infer that there is also likely anesthetic effect on those arousal centers
→ What is the oculocephalic reflex?
Eyes turn opposite way that head is turned (aka doll’s eyes). Horizontal motion of head activates vestibular system → afferent limb is CN VIII → synapses on it’s nucleus in the pons → sends projections to nuclei of CN III (oculomotor), IV (trochlear), and VI (abducens) → Synapses back on extrinsic eye musculature
→ Does an awake patient have an an oculocephalic reflex?
No- can be voluntarily suppressed in an awake state.
→ Why shouldn’t we see it in patients under anesthesia?
Disappearance of oculocephalic reflex under anesthesia reflects anesthetic-induced dysfunction of brainstem. Similarly to corneal reflex, the nuclei for CN III, IV, and VI live near arousal centers in pons and medulla.
→ What muscle dilates the pupil? How is it innervated, and what neurotransmitter mediates it’s contraction?
Radial muscle of the iris. Innervated by sympathetic nervous system (specifically fibers originating from superior cervical sympathetic ganglion, which travel with carotid artery to enter the skull base, and then join the long ciliary nerve). Neuromuscular transmission is mediated by alpha -1 receptors; topical phenylephrine drops will dilate the eye, for example
→ What muscle constricts the pupil? How is it innervated, and what neurotransmitter mediates it’s contraction?
Sphincter muscle of the iris. Innervated by the parasympathetic nervous system (specifically fibers originating in the Edinger-Westphal nucleus, which travel with the oculomotor nerve before synapsing in the ciliary ganglion and then continuing in the short ciliary nerve). Neuromuscular transmission is mediated by muscarinic (M3) receptors; anti-muscarinic agents like atropine, for example, will block these receptors and cause flaccid paralysis of the sphincter muscle of the iris (leaving the radial muscles unnoposed, causing mydriasis)
→ Do circulating catecholamines and vasopressors with alpha-1 agonist activity cause mydriasis? Why or why not?
No. Normal circulating level of catecholamines (including usual dosing of vasopressors) will not reach high enough levels to activate radial muscle of iris. Extreme catecholamine excess, like with pheochromocytoma, can cause mydriasis.
→ Do neuromuscular blocking agents affect pupil size? Why or why not?
No. Neuromuscular blockers do not cross the intact blood/brain barrier. Also, sphincter and radial muscles of the iris are smooth muscle (not dependent on nicotinic neuromuscular transmission); recall that sphincter muscle contraction is mediated by muscarinic receptors, and radial muscle contraction is mediated by alpha-1 receptors
→ What happens to baseline pupillary size upon induction of general anesthesia? Why?
Under anesthesia, sympathetic input to the radial muscle of the iris is lost (sympathetic activity continues in other areas of the autonomic nervous system- for example, cardiovascular reflexes remain intact. The reason for this selective ablation of sympathetic input to the eye isn’t totally understood; one theory is that the sympathetic pathways which mediate pupillary dilation are located in the upper mesencephalon, which may be more sensitive to anesthetic-induced suppression than the lower brainstem areas where other sympathetic reflexes are mediated). Because sympathetic tone to the radial muscles of the iris are lost, and because circulating catecholamines don’t reach high enough levels to cause radial muscle contraction, the sphincter muscle of the iris is now relatively unopposed. Therefore, the pupils will tend to become smaller (on average 1-3mm) after induction of anesthesia. In general any changes in pupillary size under anesthesia are due to changes in parasympathetically mediated sphincter muscle tone
→ You are providing anesthesia for a radical neck dissection, and the surgeon is concerned that they may have damaged the cervical sympathetic chain. They ask you to check for Horner’s syndrome. What do you tell them?
Because the sympathetic innervation to the eye is suppressed under anesthesia, the anisocoria with ipsilateral miosis that is typical of Horner’s syndrome will not be able to be detected while under anesthesia
→ You examine the pupils of a patient who is under anesthesia, being maintained on 0.8 MAC of isoflurane. Do you expect the pupils to constrict in response to light?
Yes. Volatile anesthetics and propofol both depress the magnitude of the pupillary light reflex, but values > 1 MAC are required to see any significant change. At values of 1 MAC or less, the pupillary light reflex should remain unchanged (other than starting from a slightly smaller pupil size)
→ What will happen to the pupillary size of a patient under anesthesia who receives a strong painful stimulus? What is the term for this, and how does it happen?
Pupillary Reflex Dilation (PRD) refers to dilation of the pupils in response to a painful stimulus. PRD is actually amplified about 3-fold in patients under general anesthesia. How does this happen given that the sympathetic input to the eye is suppressed? As it turns out, even though general anesthesia suppresses direct sympathetic input to the eye, it will actually enhance the activity of sympathetic inhibitory interneurons which synapse onto the Edinger-Westphal nucleus (recall that this is the origin of the parasympathetic innervation to the eye). Activation of nociceptors → Increased firing of inhibitory interneurons onto Edinger-Westphal nucleus → decreased parasympathetic outflow to the eye → sphincter muscle relaxation → pupillary dilation
→ What is the effect of opioids on pupillary size in anesthetized patients? What is the mechanism?
1) As with awake patients, opioids will cause a decrease in the baseline pupillary size of an anesthetized patient. 2) Opioids may depress the pupillary light reflex. 3) Most interestingly, opioids will also blunt PRD in response to a painful stimulus in a dose-dependent manner. There are 2 proposed sites of action, both of which inhibit the PRD reflex arc: firstly, opioids will diminish the intensity of ascending pain signals via their action in the spinal cord; secondly, opioids directly inhibit the firing of the sympathetic inhibitory interneurons that suppress parasympathetic outflow from the EW nucleus (they directly inhibit the inhibitor of parasympathetic tone). It is possible to suppress PRD entirely
→ At the conclusion of a case, before turning off your anesthetic, you examine the patient’s eyes and note that the pupils appear pinpoint. Does this reassure you that you have given enough opioid to ensure analgesia? Why or why not?
No. Baseline miosis may be from opioid effect, or simply from GABA-ergic sedative hypnotic agents (propofol, volatile agent)
→ You have placed a thoracic epidural for an exploratory laparotomy, and you want to know if you have dosed it adequately before you wake your patient up. How can you use pupillary size to evaluate the adequacy of anti-nociception in an anesthetized patient?
Assuming that you have not given an excessive amount of opioid, you can monitor for the presence and amplitude of PRD in response to a painful stimulus over the dermatomes which you are seeking to anesthetize. For example, you could place electrodes on the skin and use the TOF monitor to deliver a strong tetanic stimulus while monitoring the pupils. Lack of PRD may suggest adequate antinociception over the dermatome(s) being stimulated. One could also use this same approach to assess the adequacy of analgesia using opioids prior to awakening a patient.
→ This all sounds great. Can I go ahead and get started using changes in pupillary size to assess analgesia?
Probably shouldn’t (or at the very least, proceed with caution). Changes in pupillary size may be small and hard to quantify accurately without specialized equipment. Handheld infrared pupillometers are available for this purpose; however, we do not currently have them available at XXX hospital. Furthermore, once PRD in response to a painful stimulus is elicited, it can take several minutes for the pupillary size to return to baseline (even though the painful stimulus is removed). For this reason, it is recommended to assess PRD no more frequently than every 5 minutes. And of course, there is always the risk of corneal abrasion during pupillary examination… If your patient wakes up with an adequate degree of surgical analgesia but a painful eye, they will not be happy with you
- A Neurologic Examination for Anesthesiologists | Anesthesiology | American Society of Anesthesiologists
- Portable Infrared Pupillometry: A Review : Anesthesia & Analgesia
- https:// deleted to avoid doxxing myself : )