Benign external hydrocephalus
The individual components of the triad can also be seen in other conditions. Benign external hydrocephalus is one such condition, and is the result of an imbalance between the production and elimination of cerebrospinal fluid. In cases of benign external hydrocephalus, cerebrospinal fluid accumulates between the brain and the inside of the cranium, the intracranial pressure increases, the subarachnoid space expands and the circumference of the head increases. The bridging veins, which pass through this space from the cortex to the inside of the cranium, are stretched and may begin to haemorrhage slightly, causing subdural accumulations of blood. These are not usually acute, but tend to resemble chronic accumulations of blood of different ages. New blood products, as seen in acute haematoma, usually make up a very small proportion of the total fluid volume in subdural blood accumulations caused by benign external hydrocephalus.
Subdural haematomas may thus occur as a spontaneous complication of benign external hydrocephalus (10, 11). This can be a diagnostic pitfall with respect to the diagnosis of shaken baby syndrome (10). Consequently, subdural haematoma cannot be pathognomonic for shaken baby syndrome.
Epilepsy with unconsciousness and seizures may be a startling initial symptom of benign external hydrocephalus (12, 13). A reasonable response by parents who witness the sudden onset of unconsciousness and respiratory arrest will be to try to shake life into the child – an act that may be misinterpreted and further strengthen suspicion.
Together with colleagues, I have recently published a study showing that approximately 25 children with benign external hydrocephalus are born in Norway every year (14). The head circumference of these children is normal at birth, but increases too rapidly in the first months post-partum. There is also a marked preponderance of boys (86 %) among these children. A similar age and gender distribution is found in most major articles on shaken baby syndrome, such as those by Adamsbaum, Hobbs and Vinchon, who together found that 70 % of cases were in boys (3, 8, 15), with an average age of four months. Could the epidemiological similarity between benign external hydrocephalus and shaken baby syndrome with respect to age and gender be the result of benign external hydrocephalus with subdural haematoma as a complication being misdiagnosed as shaken baby syndrome?
Traumatic shaking and benign external hydrocephalus can, according to the literature, both give rise to subdural haematomas. One might expect there to be differences in radiological images associated with the two conditions. Traumatic shaking is usually assumed to have occurred immediately prior to the child becoming acutely ill, and it is the person who was alone with the child at the time who comes under suspicion. If the haematoma is caused by an acute action, then one should expect to see an acute-looking haematoma with white, coagulated blood (on CT), and not a chronic haematoma or simply bloody fluid, as often seen in such cases. Likewise, in an acute haematoma one should expect to find compression of the ventricles and subdural space as well as a midline shift to the opposite side of the brain if the bleeding is unilateral.
By contrast, if the subdural blood accumulation is a complication of benign external hydrocephalus and due to gradual leakage of blood in an already expanded subarachnoid space, the subdural haematoma should have an appearance consistent with precisely that: a chronic subdural haematoma, perhaps with some smaller clots with an acute-like appearance.
So, is there a clear radiological difference between the CT and MRI images that are presented as cases of benign external hydrocephalus and shaken baby syndrome respectively? There is in part, but in my opinion only to a small degree. I found 40 articles published in the last ten years with illustrations said to show examples of shaken baby syndrome. The articles contained a total of 68 MRI or CT images which, according to the authors, showed examples of shaken baby syndrome without signs of external violence. The large majority of these (78 %) had radiological characteristics that are more consistent with benign external hydrocephalus (16) than with a head injury inflicted by acute violence. Only 22 % were most consistent with a traumatic acute subdural haematoma. The radiological similarity between alleged shaken baby syndrome and benign external hydrocephalus is also evident in illustrations said to represent benign external hydrocephalus (11, 16) and shaken baby syndrome (8, 17). To me these pictures are alarmingly similar.
Why these infants also show extensive retinal haemorrhages has not been established for certain, but high intracranial pressure can cause such bleeding – so-called Terson's syndrome (18). Elevated intracranial pressure is transmitted to the retina via the fluid-filled optic nerve sheath (4), which in infants is very short, causing haemorrhages. Not even extensive retinal haemorrhages can be considered an unambiguous sign of shaken baby syndrome (4, 10, 19, 20).