Myers’ primate studies

When Dr. Ron Myers arrived at the Puerto Rican primate facility, he first reproduced the experiments of Dawes. He found that these experiments produced neuronal necrosis in the brainstem and cerebellum. This was not the pattern of cortical and basal ganglia necrosis seen in human infants with cerebral palsy. By chance, he discovered the circumstances that produced cerebral palsy like necrosis in newborn monkeys. His 1969 report is an epiphany¹. Monkey 796 had suffered from maternal acidosis that caused a moderate fetal acidosis that was still present at birth. This newborn was then subjected to the same experimental routine of immediate postpartum asphyxia as his peers, only his response was very different. He had a prolonged post-asphyxial neurological deficit including seizures; he had flame shape retinal hemorrhages; and he eventually developed symptoms consistent with cerebral palsy. He was sacrificed at 6 months of age and the brain showed cortical atrophy and gliosis, and basal ganglia gliosis similar to that seen in humans with cerebral palsy. Dr. Myer reasoned that it was the prenatal acidosis that made the difference. In 1972 he published a summary of his subsequent primate studies entitled “Two patterns of perinatal brain damage and their conditions of occurrence” reprising a 1970 conference². The first pattern was associated with the Dawes’ model of complete asphyxia in term monkeys delivered with an occlusive bag over the face, and then clamping of the umbilical cord. These monkeys had a standard pattern of physiologic response much like that described by earlier physiologists. Starting at 12 minutes of complete asphyxia, there was evidence of necrosis of brain stem nuclei. There was a rank order of lesions, meaning a progression that did not vary in pattern, but varied only in the extent that each animal had progressed along the order of lesions. The longer the asphyxia, the further the brain damage progressed. The order corresponded approximately to the experimentally determined order of blood flow intensity. Nuclei began to succumb after 13 to 16 minutes of asphyxia, in the following order: inferior colliculi, superior olives, sensory nuclei of the trigeminal nerve, gracile and cuneate nuclei, nuclear and spinal vestibular nuclei, and last posterior and ventral thalamic nuclei. Longer periods of asphyxia demonstrated wide spread thalamic and brainstem lesions and even extending to the intermediate and anterior grey matter of the spinal cord. The earliest lesions appeared confined to neurons, but more severe lesions involved all cell types. Mononuclear cells infiltrated the damaged areas 24 to 60 hours after the injury, with subsequent accumulation of cellular debris. By the 5^th to 12^th day, the glial cells demonstrated activation (increased filaments) and eventually produced a glial scar.

Dr. Myer emphasized that fetal monkeys had to have no prior compromise in order to demonstrate this pattern of injury. This pattern of brain lesion is rare in human perinatal asphyxia, but is well documented. One study selected 11 infants with neonatal seizures who had, reassuring fetal heart rate tracings until the development of a sudden persistent bradycardia³. They all had radiologic evidence of injury to subcortical brain nuclei with sparing of the cortical white and gray matter as well as sparing of other organ injury.

The second pattern of brain injury followed prolonged partial asphyxia, defined as interference with placental gas exchange sufficient to cause fetal hypoxia, and a mixed respiratory and metabolic acidosis. This occurred when fetal partial pressure of oxygen fell to or below 13-14 mm Hg. Experimentally this could be produced by various manipulations including maternal hypotension from fluorothane⁴, oxytocin hyperstimulation of the uterus, carbon monoxide administered to the mother⁵, partial placental separation, and most consistently by mechanical compression of the aorta below the level of the renal arteries⁶. If the mean maternal blood pressure perfusing the uterus fell below 40 mg Hg of pressure, partial fetal asphyxia occurred with fetal death in 0.5 to 1 hour. Fetal partial asphyxia could also be caused by maternal infusions of catecholamines that led to a 60-80% increase in blood pressure and visceral vasoconstriction. Maternal stress during handling could also cause partial fetal asphyxia, presumably from endogenous catecholamine release⁷.

Partial asphyxia did produce type II dips in the fetal heart rate, often called late decelerations because of the delay after a uterine contraction. He noted that the heart rate decelerations began with a pH in the 7.10-7.15 range, a level at which there was no fetal brain damage, but that the lower the pH the more likely brain injury and the longer the partial asphyxia, the more likely the brain injury. Myers suggested the measures needed to determine the risk of neurologic injury would need to measure tissue blood flow, tissue pH, hemoglobin concentration as well as oxygen levels in blood. In later studies he would emphasize the importance of tissue lactate, a function in part of tissue glucose availability.

The brain lesion in this second pattern resulted from brain swelling due to the transfer of fluid into brain cells. The swelling created a feedback in which swelling decreased perfusion leading to more ischemia and more swelling. The pattern initially showed regional variations with the posterior circulation being the most spared, but eventually there was pan cortical necrosis including hemispheres and basal ganglia. This pattern of injury more closely resembled human cerebral palsy, and some of the surviving monkeys demonstrated cerebral palsy. Many others died despite resuscitation.

In 1975 Myers published a summary of his work in Advances in Neurology expanding and to some extent reinterpreting his ideas. The article was titled “Four Patterns of Perinatal Brain Damage and Their Conditions of Occurrence in Primates”⁸. The first pattern of neuronal necrosis from acute total asphyxia remained as in his previous description. The second pattern, as before, was brain swelling due to partial asphyxia, but the pattern was now confined to cases with hypoxia induced acidosis. The necrosis was limited to the neocortex, and could be global or involving only paracentral or posterior parietal regions. Frequently there was herniation of the cerebral tonsils due to the brain swelling. This pattern occurred following a mixed respiratory and metabolic acidosis when oxygen content in fetal blood diminished below 0.8 to 1.5 vol % for periods longer than 30-40 minutes. If the content fell below 0.5% vol %, the fetus rapidly decompensated and went into shock. Myer states that the “severity of asphyxia required to produce brain damage in the term fetus is only slightly less than that which leads to death”. The third pattern was cortical white matter injury and hemorrhage that was produced by hypoxia without acidosis. This could occur with gradual asphyxia with partial oxygen pressures as low as 8-10 mm Hg but without elevated partial pressure of carbon dioxide and without blood acidosis. This pattern of injury occurred if the circulation of the animal was maintained while carbon monoxide or cyanide was given to inhibit brain oxidation. The fourth pattern of predominately basal ganglionic necrosis occurred when complete asphyxia was superimposed on partial asphyxia. Such infant monkeys could also show some of the patterns of complete acute asphyxia and of acidotic hypoxia.

In a later summary of his work, Myers noted. “ The few animals that survived asphyxia to show the long term clinical abnormalities and the specific patterns of brain injury simulating human cerebral palsy almost all experienced a marked hypoxia followed by a brief period of anoxia. The anoxia or near anoxia they experienced generally developed as a consequence of a circulatory failure that appeared late during the exposure to marked hypoxia⁹.” His major conclusions were that only severe hypoxia produced brain damage, with a change in vital signs as oxygen reduced from 10-12 volume % of oxygen to below 3-4 volumes %. At this point the effects were reversible. Brain damage occurred at 0.8-1.5 volume %, and death at 0.5 volume %. All of the monkeys at the lower levels of oxygen had bradycardia and decreasing oxygen levels. The duration of such asphyxia was generally 25-30 minutes. He could not predict the degree of injury in a given case. In studies of adult monkeys, he found that the level of blood lactate was more predictive of brain injury than the blood levels of oxygen or pH.

In summary, Myers demonstrated that to produce the equivalent brain lesions of cerebral palsy required partial but quite severe hypoxia for 25 to 30 minutes causing cardiovascular collapse, adding ischemia to brain hypoxia. The transition to irreversible injury or death was very quick. I recall from a conversation with Dr. Myers that he emphasized that his experiments were difficult to do because the monkeys would often die given the same degree of compromise that caused brain injury. (I was interested in seeing if the visceral organs or their microscope slides from the stillborn monkeys were available. They were not) His original epiphany case (cited above) demonstrated that if the fetus is already acidotic, not necessarily severely, and then suffers a terminal episode of anoxia, severe brain injury occurs. Myers suggested that the clinically prudent course was to treat any obvious source in the mother’s condition, and then deliver the infant with type II dips.

This last advice approximates current clinical care. This approach must increase the number of Cesarean deliveries and it is possible that the very conditions of delivery, without first correcting any fetal acidosis, might precipitate brain injury. I will discuss some of the refinements in our understanding of fetal asphyxia garnered since Myers’ studies, and consider two special types of human brain injury not addressed by Myers, then we will address the anatomic/physiologic causes of fetal hypoxia/asphyxia, anticipating that the first part of Myers’ advice is the crucial one, namely “to treat any obvious source in the mother’s condition”.

References:

1. Myers RE. Atrophic cortical sclerosis associated with status marmoratus in a perinatally damaged monkey. Neurology 1969;19:1177-88.
2. Myers RE. Two patterns of perinatal brain damage and their conditions of occurrence. Am J Obstet Gynecol 1972;112:246-76.
3. Pasternak JF, Gorey MT. The syndrome of acute near-total intrauterine asphyxia in the term infant. Pediatr Neurol 1998;18:391-8.
4. Brann AW, Jr., Myers RE. Central nervous system findings in the newborn monkey following severe in utero partial asphyxia. Neurology 1975;25:327-38.
5. Ginsberg MD, Myers RE. Fetal brain injury after maternal carbon monoxide intoxication. Clinical and neuropathologic aspects. Neurology 1976;26:15-23.
6. Myers RE, Mueller-Heubach E, Adamsons K. Predictability of the state of fetal oxygenation from a quantitative analysis of the components of late deceleration. Am J Obstet Gynecol 1973;115:1083-94.
7. Myers RE. Maternal psychological stress and fetal asphyxia: a study in the monkey. Am J Obstet Gynecol 1975;122:47-59.
8. Myers RE. Four patterns of perinatal brain damage and their conditions of occurrence in primates. Adv Neurol 1975;10:223-34.
9. Myers RE, de Courten-Myers GM, Wagner KR. Effect of Hypoxia on Fetal Brain. In: Beard R, Nathanielsz PW, eds. Fetal Physiology and Medicine, Second Revised Edition. New York: Marcel Dekker, Inc.; 1984:419-58.