This chapter reviews epidemiological, clinical, and pathological aspects of benign external hydrocephalus, a medical condition that is a risk factor for development of subdural haematoma, and that frequently is mistaken for abusive head trauma (AHT). For infants, there are striking epidemiological similarities regarding gender and age between external hydrocephalus, subdural haematoma (SDH), and AHT/SBS. There is a marked male preponderance, in most infants the symptom debut occurs during the first 6 months, and prematurity appears to be more frequent. External hydrocephalus is known to predispose for development of SDH. Most infants with external hydrocephalus are born with a close-to-normal head circumference (HC) that starts to grow abnormally fast during the first postnatal months; most of these infants reach HC values compatible with hydrocephalus at the age of 2 to 3 months, the peak age at which AHT/SBS most often is diagnosed. Both in infantile SDH and AHT/SBS, the subdural fluid collections appear to be chronic, not acute as one would expect after a traumatic event. There are reasons to assume that external hydrocephalus often has been and will be misdiagnosed as AHT/SBS.

The radiological characteristics of abusive head trauma (AHT) appear to be vaguely defined. A literature search during the period 2008-21 identified 63 articles presenting 172 illustrations with subdural hematomas described as representative of AHT. We evaluated these for signs of benign external hydrocephalus (BEH) or expansive acute subdural haematoma (ASDH). Signs of BEH were widened interhemispheric distance, preserved subarachnoid space and preserved cortical relief despite an overlying SDH, and absence of ventricle compression or midline shift. Signs of an ASDH were hyperattenuating SDH combined with compression of the subarachnoid space, the cortical relief, and ventricles, as well as midline shift. Radiological findings suggesting BEH were detected in 59 illustrations (34.3%). A weaker suspicion of BEH was raised in 32 images. An expansive ASDH was detected in 24 images, and 57 illustrations showed neither signs of BEH nor expansive ASDH or were inconclusive. Males were overrepresented in all groups. Mean age was 4.2 months in the BEH-like group and 17.6 months in the ASDH-like group. BEH complicated by SDH may have been misdiagnosed as SBS/AHT in many of these articles.