Coma and Brain Death

Question 1

What is a coma?

Answer 1

Sleep-like, unarousable, unresponsive state

Only brain stem reflexes are clinically testable, as cortical function is absent

Question 2

A solitary, unilateral cerebral lesion does not produce coma unless what?

Answer 2

It adversely affects the opposite hemisphere via brain edema or herniation

Question 3

Coma may be produced by what kind of brain stem lesion?

Answer 3

One that disrupts the reticular formation in the tegmental brain stem that projects to the thalamic and subcortical nuclei and is important for wakefulness and arousal

Question 4

Strictly speaking, motor responses to command or withdrawal to painful stimuli do not occur in coma - why?

Answer 4

Because an appropriate, localizing response to a noxious stimulus requires some cortical function

Question 5

What is decorticate posturing?

Answer 5

Flexion of the upper limbs with extension of the lower limbs associated with a lesion at the level fo the cerebral cortex or hemisphere

Question 6

What is decerebrate posturing?

Answer 6

Extension of the upper and lower limbs, associated with a lesion at the level of the midbrain (red nucleus)

Question 7

What is Cheyne-Stokes respiration?

Answer 7

Distinct pattern of alternating tachypnea and apnea

Question 8

In a comatose patient, what causes Cheyne-Stokes?

Answer 8

Bilateral cortical involvement due to metabolic encephalopathy, a unialteral lesion with severe brain edema, or from bilateral structural lesions in the cortex

Question 9

Who else can have Cheyne-Stokes breathing?

Answer 9

Elderly subjects while sleeping

CHF due to slowed circulation time that delays feedback to the carotid chemoreceptors influencing the RR

Question 10

Rarely, central neurogenic hyperventilation may result from a lesion or edema where?

Answer 10

Low midbrain or upper pons

Question 11

What is an ataxic respiration pattern and what causes it?

Answer 11

Variable breaths at an irregular rate from a lesion or edema in the medulla -> ominous sign, impending respiratory arrest

Question 12

Often in coma from metabolic causes, what happens to the pupillary light reflex?

Answer 12

It is preserved despite loss of other brain stem or CN reflexes -> sympathetic (pupillodilator) fibers travel down the entire brain stem and parasympathetic (pupilloconstrictor, CCN3) fibers are a circuit at the midbrain level

Question 13

What causes large fixed pupils?

Answer 13

Tectal (dorsal) midbrain lesion selectively involving the parasympathetic fibers (unopposed sympathetic fibers)

Question 14

Cause of a blown fixed pupil?

Answer 14

Unresponse consensually or directly to light, due to compression of CN III ipsilaterally from a swollen temporal lobe (uncal herniation)

Question 15

What causes small, pinpoint pupils?

Answer 15

Pontine lesion selectively involving the sympathetic fibers (unopposed parasympathetics)

In the absence of this lesion, may also be caused by high dose narcotics, cholinergic eyedrops used to treat glaucoma, etc.

Question 16

What is the oculocephalic reflex?

Answer 16

Doll’s eyes reflex -> brainstem mediated reflex in which the eyes move in the direction opposite to the lateral turn of the head

Question 17

What is the oculovestibular (cold caloric) reflex?

Answer 17

Brain stem mediated reflex -> irrigate ears with cold water, reduces vestibular activity, causing the eyes to normally move slowly toward the cold ear

Conscious patient would also have nystagmus with the eyes beating toward the opposite ear

Question 18

Emergency treatment of the comatose patient?

Answer 18

1. ABCs
2. Immediately rule out hypoglycemia or empirically give 50% dextrose IV
3. Suspect structural lesion if asymmetric neuro signs -> brain CT or MRI
4. Suspect toximetabolic causes if symmetric -> evaluate for electrolyte abnormalities, hypothermia, hepatic or renal failure, CO poisoning, drug intoxication or OD

Question 19

Which cause of increased ICP is UNAFFECTED by steroids?

Answer 19

Brain edema from ischemic infarct or hemorrhage

Question 20

What must be done to declare brain death?

Answer 20

• Apparent cause should be known and of sufficient severity to account for the irreversible coma
• No neuro improvement despite adequate treatment of any reversible cause
• Observe for 6 hours if these things are true, if younger, wait longer
• Bedside neuro exam shows no hint or suggestion of cerebral function in a comatose patient unresponsive to painful stimuli -> no decorticate or decerebrate posturing, seizures, swallowing, yawning, vocalizations; note that some SC mediated movements may still persist
• All CN and brain stem reflexes must be absent without any spontaneous respirations -> pupils do not react, oculocephalic, oculovestibular, and gag reflexes are absent
• Verify apnea - no breathing despite reaching a pCO2 of 60 or greater

Question 21

Confirmatory tests of brain death are not required but are used when exam is equivocal or impossible to do. What are they?

Answer 21

• Flat line or isoelectric EEG after 30 minutes of recording with a special protocol
• Cerebral angiography with no intracranial flow over 10 minutes
• Radioisotope brain scan - demonstrates absence of cerebral blood flow over 1 0minutes, can be done at the bedside (preferred)