Dr. Waney Squire
The diagnosis “shaken baby syndrome” (SBS) has been widely accepted for over 30 years, but recent evidence from biomechanical and clinical observational studies questions the validity of the syndrome.
The diagnosis of SBS is based on the clinical triad of encephalopathy, retinal hemorrhage (RH), and subdural hemorrhage (SDH) in infants, usually under six months of age, who may die unexpectedly or survive with greater or lesser degrees of neurological damage . The term non-accidental head injury (NAHI) has been preferred as it has no implications for mechanism of injury. Other features often associated include a sole carer at the time of collapse and a clinical history that is incompatible with the severity of the injuries. The diagnosis of inflicted injury becomes less problematic if there is objective evidence of violence, such as bruises, fractures, or burns, but objective evidence of trauma has not always been necessary in making the diagnosis. Central to the assessment of these cases is whether the triad of findings can be regarded as diagnostic of abuse with any degree of certainty. This review examines the evidence base for each element of the triad and the current biomechanical evidence regarding mechanisms of infant head injury and its pathological investigation.
Retinal Hemorrhages (RHs)
RHs have been regarded as an important indicator of inflicted injury, but many other causes of retinal bleeding are recognized in infants, for example after normal birth, raised intracranial pressure, blood dyscrasias, hemoglobinopathies, extracorporeal membrane oxygenation, cataract surgery, and accidental trauma . Postmortem indirect ophthalmoscopy has shown RHs to be more common after natural disease and accidental injury than after inflicted injury.
Lucid intervals are more frequently seen in infants less than two years of age , reflecting the very different responses of the infant brain to injury due to the specific intracranial pathophysiology before the skull bones fuse.
Subdural Hemorrhage (SDH)
SDH is perhaps the most important and consistent component of the triad. In the acutely sick infant, it is frequently the first clinical sign, identified on brain scan, to raise the question of abuse. There are no specific imaging patterns that can distinguish inflicted from accidental intracranial injury. Autopsy and imaging studies show that infant SDH is usually a thin bilateral film and not a thick, unilateral space occupying clot as seen in traumatic SDH in older children and adults. This raises the question of whether the two forms have the same etiology and anatomical source.
Causes of Subdural Hemorrhage
The commonest cause of SDH in infants is said to be trauma although a recent study has shown a significant
Shaken Baby Syndrome incidence (26%) of birth-related SDH . Other causes in infants include benign enlargement of
the extracerebral spaces (BEECS), clotting disorders, hemorrhagic disease of the newborn, rare metabolic diseases, vascular malformations, and neurosurgical procedures.
Proposed traumatic causes of infant SDH are inflicted injury such as shaking and/or impact and accidental injuries such as falls. Impact includes blunt impact of an object on the head and that resulting from a fall or striking the moving head on a rigid surface. The biomechanical aspects of these injuries are discussed below. The vast majority of cases described as SBS have evidence of impact . While the pathologist may be able to determine features indicative of impact, it is not, of course, possible to distinguish accidental from non-accidental injuries by pathology.
Low-level falls have the potential, albeit only rarely, to cause SDH in infants and young children. Absolute height is not as important a criterion for injury as the exact nature of the fall for a particular infant, in a particular circumstance. The effects of twisting, rotation, or crushing of the structures of the neck are crucial in terms of outcome. Biomechanical studies show that falls even from low levels of 3–4 ft can generate far greater forces in the head than shaking. There are a number of case series demonstrating that infants and children may suffer intracranial damage including retinal and intracranial hemorrhage after falls from levels as low as 3 ft. While most babies may suffer little from an apparently trivial fall, this is clearly not always the case.
It is likely that the forces required to cause intracranial injury will also damage the weak infant neck. In road traffic accidents, infants who suffer single severe hyperextension forces have cervical fractures, dislocations, spinal cord injury, and torn nerve roots, not SDH.
Differential Diagnosis of SBS
The most common causes of the triad are impact, birth-related SDH, BEECS, coagulopathies, apnoea, asphyxia and choking, acute life-threatening events, (ALTEs), osteogenesis imperfecta, osteopenia of prematurity, and metabolic diseases.
Biomechanics is the application of principles of physics to biological systems and has been the mainstay of research into motor vehicle safety for six decades. It was just such research into noncontact head injury from rear-end shunts that stimulated Guthkelch to formulate his hypothesis for SBS in 1971. Ommaya had caused concussion, SDH,
and white matter shearing injury (diffuse axonal injury) in primates by whiplash. Guthkelch suggested that the rotational forces of shaking would cause tearing of bridging veins and bilateral subdural bleeding, although Ommaya himself warned that “It is improbable that the high speed and severity of the single whiplash produced in our animal model could be achieved by a single manual shake or even a short series of manual shaking of an infant in one episode”.
More recent studies using “crash test dummies” indicate that impact generates far more force than shaking and that impact is required to produce SDH . Cory and Jones generated forces that exceeded the injury threshold for concussion, but not for SDH or axonal injury. Their adult shaker volunteers fatigued after 10 seconds. While they concluded that “It cannot be categorically stated, from a biomechanical perspective, that pure shaking cannot cause fatal head injuries in an infant ”, they noted that in their experiments there were chin and occipital contacts at the extremes of the shaking motion that could have caused impact. These authors expressed their concerns regarding the difficulties in extrapolating to human infants the findings in both dummy and animal models.
Biomechanical studies have shown that falls and impact to the head produce significant rotational forces when the impacting forces are not aligned through the center of gravity of the head, due to hinging of the head on the neck. Shaking is not necessary to cause rotational acceleration.
Other Contributing Experts:
Dr Irene Scheimberg
Dr Pat Lantz
Dr Chris Van Ee
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