Hemispheric hypodensity (HH) or ‘black brain’ on CT scan of the head following severe head injury in infants and toddlers.
It is characterised by:
- loss of grey-white differentiation of the affected hemisphere
- loss of definition of basal ganglia (check)
- Involving all three vascular territories despite the vessels being patent.
- HH can be unilateral or bilateral
HH is commonly seen following a non-accidental injury (NAI) to the head. It is particularly noted in the cerebral hemisphere with the overlying subdural haematoma. However, it can occur with accidental injuries too. Based on the HH itself, the mechanism (i.e. accidental vs non-accidental injury) cannot be inferred.
Of the patients seen in hospital for non-accidental injury (NAI), 25-50% have either unilateral of bilateral HH.
By the time a patient with bilateral HH dies, he/she would have accrued other brain abnormalities; as a consequence in these patients pathological changes specific to HH are difficult to ascertain. However, not all patients unilateral HH die of the injury. Later pathologioca examination of the brainsof these patients upon their later demise show:
- hypoxic-ischaemic patter
- +/- traumatic axonal injury
HH might be due to mis-match of metabolic demand and substrate delivery. This is because of hypodensity findings i) multiple foci, ii) patuent blood verssels and iii) instances of unilateral occurence.
The HH can be seen on CT of the head within hours of the injury or develop over a few days. Late finding in CT scan of head is atrophy of the cortical ribbon.
In mortality in patents with NAI and HH is double of those with NAI but no HH (e.g.