NJ Robertson, D Azzopardi, AD Edwards

Abstract

There is now a large body of evidence from experimental studies that moderate brain hypothermia applied after hypoxia-ischaemia modifies the process of cell death and ameliorates cerebral injury. Pilot studies examining the feasibility and safety of moderate hypothermia in infants following perinatal hypoxia-ischaemia suggest that there are relatively few adverse systemic effects but that a cautious approach is required; an international randomized trial of head cooling is currently underway. The amplitude integrated EEG has been shown to be useful in selecting infants most likely to be benefit from treatment, however there is no consensus on whether selective head cooling or total body cooling is the preferred method of cooling. Experimental studies suggest that cooling needs to be started within six hours of the insult and continued for at least 72 hours for it to be effective. Until results of the multicentre trials are known there are insufficient data to advise the use of moderate hypothermia in clinical practice.

Introduction

It was suggested many years ago that cooling the brain after birth asphyxia might ameliorate cerebral injury.¹ However, experimental studies at that time failed to support the hypothesis, and it fell from favour.² More recently basic researchers have suggested again that moderate brain hypothermia is one of a number of interventions which might be applied after hypoxia-ischaemia to modify the process of brain cell death and so ameliorate cerebral damage. The possibility that cooling might offer a simple and practical treatment for perinatal hypoxic-ischaemic injury has raised considerable interest among clinicians.

Cerebral hypoxia-ischaemia frequently leads to delayed cell death hours or days later,³ as has been demonstrated in infants suffering birth asphyxia by magnetic resonance spectroscopy (MRS). Asphyxiated infants are frequently found to have normal cerebral energy metabolism soon after resuscitation, but 9 to 24 hours later the concentrations of high energy phosphates in the brain begin to decline.⁴ This delayed impairment of energy metabolism was not contingent upon continued hypoxia-ischaemia, nor was it associated with intracellular acidosis,⁵ but its magnitude was linearly related to the severity of later neurodevelopmental impairment and reduced brain growth.⁶ The effects of hypoxia-ischaemia can thus be delayed perhaps a considerable period.^7,8

These results suggested that it might be possible to intervene before delayed cell death occurred, and thus to ameliorate cerebral damage. Indeed experiments in animals showed that a large number of treatments can reduce brain injury if given soon after hypoxia-ischaemia.⁹ "Neural rescue" treatment is thus a real possibility, and in some cases treatment could still be effective even if delayed for a considerable time after the hypoxic-ischaemic insult.¹⁰

Moderate Cooling as a Neural Rescue Therapy in Mature Animals

Many recent studies have shown that cooling of the brain by 3°C to 4°C after experimental hypoxia-ischaemia reduces the severity of brain injury.¹¹ Greater reductions in temperature do not confer additional benefit and indeed may be less effective as they were associated with systemic toxicity.¹² Consequently most current research is focused upon moderate rather than deep hypothermia.

Because many experiments have used short-term histopathological examination to assess neural cell loss there was concern that hypothermia may not prevent cell death, merely delay it. However, a study of adult gerbils cooled after hypoxia-ischaemia found that they performed better than controls in neuropsychological tests six months later, showing that a persistent beneficial effect of cerebral function could be produced.¹³

Neural Rescue by Moderate Hypothermia in the Developing Brain

Hypoxic-ischaemic damage can also be reduced by cooling the developing brain. In 7-day-old rat pups hypothermia for a period as short as three hours after hypoxia-ischaemia had some neuroprotective effect,¹⁴ and histological differences between treatment and control brains could still be detected six weeks later.¹⁵ In 21-day-old rat pups 72 hours of hypothermia was highly protective. In a study of newborn piglets three hours cooling had only a modest neuroprotective effect and there was no protection at all in more severely injured animals.¹⁶ By contrast, 12 hours of cooling by 4°C in newborn piglets produced a major reduction of both the delayed impairment in cerebral energy metabolism, and histological injury.^17,18

While all these studies examined the effect of whole body cooling, investigations of fetal sheep subjected to total cerebral ischaemia found that a marked protective effect could be produced by a cooling device positioned around the fetal head.¹⁹ In this model cooling was maintained for 72 hours, but significant protection was achieved even if treatment was delayed for five hours after ischaemia.²⁰

These results suggested that optimal benefit is obtained by extending the duration of moderate hypothermia for 12 hours or longer, but that hypothermia might not be effective against very severe hypoxic-ischaemic insults. They also leave unanswered the question of whether whole body or selective head cooling is more effective.

Mechanisms of Hypothermic Neural Rescue

It is possible that the protective effect of hypothermia during hypoxia-ischaemia is through the prevention of the decline in high energy phosphates that can initiate both apoptotic and necrotic cell death.^21,22 The mechanisms by which cooling after hypoxia-ischaemia prevent cell death are less clear. Hypothermia prevents the delayed decline in phosphocreatine and adenosine triphosphate, as well as the simultaneous increase in cerebral lactate concentration, seen eight to 12 hours after hypoxia-ischemia in newborn piglets.^17,23 However it is not clear that preservation of energy metabolism in the delayed phase of injury is the primary mechanism by which hypothermia operates, or whether additional effects are involved.

High concentrations of glutamate are found in the synaptic cleft after hypoxia-ischaemia; this has been shown to induce excitotoxic neuronal death.²⁴ In both newborn piglets²⁵ and adult rats²⁶ hypothermia reduced the delayed increase in extracellular glutamate seen after hypoxia-ischaemia, and in a cell culture model of hypoxia-ischaemia mild cooling decreased the impairment in glutamate re-uptake that is important in acute injury.²⁷ Hypothermia also reduced production of nitric oxide, which may be a downstream mediator of the excitotoxic process.^25,28

Ischaemic depolarisation in injured but viable tissue was reduced by cooling,²⁹ and hypothermia prevented the increase in cerebral impedance (which reflects impaired membrane function) during delayed cerebral injury in fetal lambs.¹⁹ Reduced body temperature may also increase catecholamine secretion,³⁰ and as stress also improves neuropathological outcome in developing rats³¹ sympathetic stimulation may be another mechanism of protection. This is a complex area as severe stress or stress at inappropriate times may have a deleterious effect on the brain. For example, in a piglet model of hypoxia ischaemia, the authors speculated that the stress of shivering interfered with the previously shown neuroprotective effect of hypothermia.³²

The number of apoptotic cells seen in the newborn piglet brain after hypoxia-ischaemia is reduced by hypothermia, without any difference in the number of necrotic cells suggesting that it may specifically inhibit the apoptotic pathway.¹⁸ However, comparable hypothermia delays rather than prevents apoptosis in cell culture systems,³³ and an attractive hypothesis is therefore that hypothermia delays commitment to apoptosis for long enough to enable endogenous protective mechanisms, including the production of growth factors,^34,35 to be induced. If this is correct is suggests that hypothermia may have a role in prolonging the therapeutic window during which other therapies might be applied.

It remains to be seen whether these or other mechanisms are the most important in the developing brain. However it is likely that the success of hypothermic treatment depends on it affecting several of the many mechanisms of damage which are activated by hypoxia-ischaemia.

Clinical Studies of Hypothermic Neural Rescue

Trials of cooling in adult patients suffering head trauma found that hypothermia was associated with quicker recovery and less severe brain injury than conventional neuro-intensive care, despite the fact that in one study the median time of establishing effective cooling was 10 hours after injury.^36,37 These important results support observational data showing a significant relation between better survival after stroke and lower body temperature in the 24 hours following onset of symptoms.³⁸

Three groups have recently published data from preliminary trials of hypothermic neural rescue after suspected birth asphyxia. A preliminary phase II study of selective head cooling with a reduction in rectal temperature to about 35°C showed no serious side effects were reported. A pilot study of whole body cooling to 33°C to 34°C equally found not major side effects. However a third study which considered both head and whole body cooling found evidence of circulatory problems in some infants. These data suggest that a cautious approach is required, and an international randomised trial of head cooling is currently underway. Establishing this trial led investigators to consider a series of important issues, such as entry criteria for any trial of neuroprotection in the new born, the relative value of head and whole body cooling, and the ethics of trials in this group of patients.

Entry Criteria

The correlation between simple measures of intrauterine asphyxia and neurodevelopmental outcome is poor, and assignment of prognosis by clinical criteria in the first few hours after birth is difficult.³⁹ If simple variables are used to enrol subjects there is a danger that large numbers of infants without an adverse prognosis may be entered into trials. At best this will add experimental noise and increase the numbers needed for a statistically valid outcome; at worse it may subject a significant number of normal infants to potentially hazardous intervention.

Several techniques provide a useful guide to neurodevelopmental outcome but there is no method which will allow completely certain determination of prognosis within a few hours of birth.³⁹ However recent studies have reported the value of continuous EEG monitoring in the first few hours of life, using single channel amplitude integrated EEG recordings (aEEG, often termed the "cerebral function monitor").^40-42 These techniques have a high predictive value for neurological impairment.^40,42 Electrophysiological techniques could be employed effectively in non-specialist centres.⁴³ This method has been used successfully to select infants for neuroprotective trials.

Selective Head Cooling or Whole Body Hypothermia

There is some doubt among researchers whether the whole body needs to be cooled to achieve neural rescue. Recent data from computer models of heat distribution and transfer within the brain have suggested that deep brain structures cannot be cooled by surface cooling of the head alone, largely because of the high rate of cerebral blood flow.⁴⁴ Using direct measurement of deep intracerebral temperature in a piglet model, the cooling cap was shown to be effective at selectively head cooling.⁴⁵ It is possible, however, that in the larger head of a term infant this method may not cool the deepest parts of the brain effectively. The opportunity of cooling the head alone and thus avoiding systemic toxicity is appealing, but further work is needed in this area.

Ethical Considerations

It is possible that a "successful" neural rescue therapy might alter death to survival with severe impairment. Although data from animals and from adult head injury studies suggest that hypothermia has little effect on the most severe form of injury,^37,46 clinicians may feel that appropriate criteria need to be built into clinical trials to prevent this type of result. Unfortunately it is not clear what exit criteria could be included to allow this to be achieved. It is possible that the EEG may help identify those infants who are unlikely to benefit from further intervention: an isoelectric EEG or a burst-suppression pattern almost always indicates a very severe injury.⁴⁰

Current Difficulties with Hypothermic Neural Rescue Therapy

There are several unsolved issues which need consideration before planning trials of hypothermic neural rescue therapy:

First, the dose and administration regime remains inadequately defined. Animal studies have provided compelling evidence that hypothermia has a protective action, but systematic data which demonstrate an optimal dose or duration are incomplete. The maximum permitted time delay before therapy is ineffectual is still unclear, although studies are in progress to define this interval. A delay of 10 hours did not prevent benefit in adults cooled after head trauma,³⁷ but some animal studies suggest that a delay of six hours significantly reduces the cerebroprotective effect. Equally, it is unclear how long to continue with cooling. Protection has been seen in studies of developing animals with cooling periods varying between three and 72 hours.^14,20 While the possible adverse effects may be more significant over longer time periods many researchers feel that longer cooling must be better. Indeed, evidence suggesting that after birth asphyxia cerebral energy metabolism remains abnormal for many months, together with studies of adult rats demonstrating that cells die by apoptosis for a similar period following hypoxia-ischemia may suggest that even 72 hours is an excessively brief period.^7,47 More work is needed in this area.

Second, it is currently difficult to monitor the dose of hypothermia being administered, especially to the deep brain structures. In studies of animals the deep brain cools significantly less that cortex especially when selective head cooling is employed,^48,49 and common drugs such as barbiturates can selectively increase deep brain temperature.⁵⁰ MRI studies have shown that infants with basal ganglia injury suffer far worse neurological impairment that those with cortical damage alone.⁵¹ Therefore measurement of deep brain temperature will be important, and methods that record only cortical temperature may be inadequate. It has been suggested that a major reason for the failure of neural rescue therapies in adult stroke has been the poor permeation of drugs into the brain,⁵² and it will be important to know that any therapeutic cooling regime adequately penetrates deep brain structures. Without these data interpretation and comparison of trial results will be difficult.

Third, hypothermia may have adverse effects. Profound hypothermia to less than 30°C has been shown to affect a large number of physiological functions. It decreases perfusion and oxygenation by: impairing myocardial contractility; reducing cardiac output; and making myocardial muscle more prone to dysrhythmia;^53,54 as well as causing peripheral vasoconstriction; increasing blood viscosity;⁵⁵ and shifting the oxygen dissociation curve of blood to the left.⁵⁶ This deranged circulation can lead to renal failure, pulmonary oedema, metabolic acidosis and inadequate cerebral blood flow.⁵³ Cooling also impairs clotting,⁵⁷ depresses the immune system,⁵⁸ disrupts serum potassium homeostasis,⁵⁹ alters acid base balance⁶⁰ and is associated with hypoglycaemia.⁶¹ Hypothermia may cause gastrointestinal lesions in the developing animal, although the data are somewhat contradictory.^62,63 These and other changes can significantly worsen the outcome of experimental subjects,¹² and early observational studies of newborn infants showed a number of these adverse events, of which pulmonary haemorrhage (probably due to raised left atrial pressure) was the most severe.⁶⁴

However, although moderate hypothermia to around 32°C has been less well studied, it would appear that adverse effects are less severe. Indeed, newborn animals physiologically induce moderate hypothermia in response to hypoxia.⁶⁵ Recent studies of newborn piglets subjected to moderate whole body cooling for 12 hours (as a treatment for cerebral hypoxia-ischaemia) or 3 hours (for whole body hypoxia) showed little evidence of circulatory or metabolic disruption during cooling¹⁷ or systemic pathology at autopsy.^16,66

Nevertheless systematic studies of the potentially toxic effects of moderate hypothermia maintained for longer periods are required, as the risk of adverse effects from moderate hypothermia seems likely to be increased if hypothermic therapy is prolonged to maximise its neuroprotective effect. There is likely to be a complex trade-off between optimising protection and minimising side effects which will involve decisions not only about the length and depth of hypothermia but also about the appropriate balance between selective head and whole body cooling.

Conclusion

Cooling the brain to around 32°C for between 12 and 72 hours beginning after resuscitation significantly reduces cerebral damage and long term sequelae in experimental models. This moderate hypothermia may be associated with relatively few adverse systemic effects, and although the mechanisms of cerebral protection are incompletely understood, interest is growing in the possibility of clinical neuroprotection by reduction in brain temperature, although there are presently insufficient data to advise the use of moderate hypothermia in clincial practice.

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