The Role of Neuroimaging in Neonatal Brain Injuries due to Hypoxic-Ischaemic Encephalopathy
Brain injuries arising from hypoxic-ischaemic encephalopathy (HIE) can be devastating and are one of the commonest causes of morbidity and mortality in new-born infants. HIE occurs in around 1.5 in every 1,000 live births. An inadequate supply of blood and oxygen to the brain leads to focal or diffuse brain injury, and the resulting damage may manifest as cerebral palsy, epilepsy, developmental delay or other neurological deficits. The type of brain injury is dependent on the severity and duration of the hypoxia, and the maturity of the infant’s brain. Similarly, prognosis depends on the severity of the injury and the gestational age of the infant. In full-term infants, myelinated fibres are more metabolically active and thus more susceptible to HIE, but preterm infants generally have a poorer prognosis. Treatment options for HIE are limited but early diagnosis is vital to determine the appropriate management and assess prognosis. The window of opportunity for intervention that may reduce the severity of the injury is very short, at around 2–6 hours. However, assessing the neurological status of a new-born baby can be extremely challenging, due to the severity of their condition and the drugs used to stabilise it. Therefore, imaging techniques are vital tools in the management of infants with suspected HIE.
Initial investigation of suspected neonatal HIE cases is usually carried out with cranial ultrasound (cUS). This technique has the advantages of being cheap and portable, so that it is not necessary for the infant to leave the intensive care unit. It can detect conditions such as intracranial haemorrhage and hydrocephalus with great sensitivity. However, it is less good at detecting lesions in the cortex, cerebellum or brainstem. Images produced by ultrasonography are also more open to interpretation than other imaging methods, so that results may depend to a great extent on the operator. This is due to the oedema associated with injury in neonatal brains, which makes image assessment difficult.
Computed tomography (CT) is better than cUS for detecting cortical injuries and there is less interobserver variability in the interpretation of the images it produces. It can also be used to screen for intracranial haemorrhage in very sick babies without the need for sedation. One major drawback of CT is that it exposes the infant to radiation. Its usage has therefore diminished since the development of magnetic resonance imaging (MRI), which is both more sensitive and more specific in diagnosing neonatal HIE. However, one advantage over MRI is the shorter time taken to produce images, so this technique is still occasionally used where time constraints mean that MRI is not appropriate.
The advent of MRI meant that detailed images of all brain structures could be obtained. This allowed a scoring system for the appearance of abnormalities after HIE to be introduced. Nowadays, MRI is the standard tool used to determine the type and extent of neonatal brain injuries in infants with HIE and is also used to predict clinical outcome. Furthermore, it can exclude causes of brain injury other than HIE (also known as HIE-mimics), such as congenital malformations, neoplasms and cerebral infarctions and haemorrhages. Drawbacks of MRI include lack of availability in some hospitals, the time taken to produce images and the need for sedation.
The emergence of more advanced techniques has led to new discoveries. Conventional MRI has shown that small subdural haemorrhages are more common than previously thought after HIE. Damage of this nature may be an important contributory factor in the development of seizures in infants with relatively mild brain injuries. Detailed visualisation of the vascular system can be performed using techniques such as MR angiography and MR venography, and use of the latter has led to an increase in the detection of sinovenous thrombosis in infants with HIE.
Newer MRI techniques, such as diffusion-weighted imaging (DWI) and magnetic resonance spectroscopy (MRS) are even better at identifying acute brain injury. In particular, DWI can detect hypoxic brain injuries before the changes are evident on conventional MRI. However, DWI may also give false negative results if performed within the first 24h following the injury. Furthermore, the changes evident from this technique can normally only be seen for 10–12 days. After this, affected tissues take on a more normal appearance and it can be difficult to visualise damaged areas. Additionally, DWI cannot detect the severity of brain injuries due to HIE, nor can it predict the clinical outcome. Conversely, the degree of injury can be evaluated if MRS is performed within 24h of birth. Adverse outcomes can also be predicted. However, due to differences in brain chemistry, which may affect results, this technique is only recommended for use in full-term infants.
Assessment of both the severity of injury and the prognosis in neonates with suspected HIE remains challenging. Imaging of the brain can provide information on the timing, location and severity of the injury. Imaging techniques have an important role in the early diagnosis of brain injury in neonates, as timely intervention can help to reduce the severity of injury and give the best possible prognosis for this potentially devastating condition. All of the available imaging methods have limitations; however, a combination of cUS at admission to hospital and MRI performed during the first week can be used to distinguish between injuries caused before birth and those occurring perinatally.
About Dr Kieran Hogarth
Dr Hogarth is a consultant at the Royal Berkshire Hospital, where he is the lead radiologist for neuroimaging. His practice includes adult and paediatric neuroradiology.
He is able to provide expert opinion on:
- paediatric neuroimaging, which includes CT and MR scans of the head and spine;
- adult brain and spine imaging;
- adult craniofacial imaging;
- adult orbital imaging;
- adult head and neck imaging and ENT.
Bano, S., Chaudhary, V., & Garga, U. C. (2017). Neonatal Hypoxic-ischemic Encephalopathy: A Radiological Review. Journal of pediatric neurosciences, 12(1), 1–6. https://doi.org/10.4103/1817-1745.205646
Groenendaal, F., & de Vries, L. S. (2017). Fifty years of brain imaging in neonatal encephalopathy following perinatal asphyxia. Pediatric research, 81(1-2), 150–155. https://doi.org/10.1038/pr.2016.195