PAEDIATRIC ABUSIVE HEAD TRAUMA

• Jurisdiction: Australia

Definition ....................................................................................................... [33C.300]

Epidemiology ................................................................................................. [33C.310]

Anatomy ........................................................................................................ [33C.320]

Mechanisms of injury and their pathological findings................................... [33C.330]

Findings at autopsy ....................................................................................... [33C.340]

Abuse or accident - how to tell them apart.................................................. [33C.350]

Outcomes (morbidity and mortality) .............................................................. [33C.360]

Controversies from a pathologist's perspective............................................ [33C.370]

[33C.300] Definition

The Department of Health and Human Services, USA, defines paediatric abusive head trauma (ABT) as an injury to the skull or intracranial contents of an infant or a young child (< 5 years of age) due to inflicted blunt impact and/or violent shaking. The resulting mechanical forces can lead to outcomes ranging from minor, clinically undetected, small subdural haemorrhages to fatalities. Deaths following abusive head trauma frequently involve subdural and/or subarachnoid haemorrhage and cerebral oedema with compromise of autoregulatory centres for cardiorespiratory function.

[33C.310] Epidemiology

Head injuries are the leading cause of child abuse fatalities, and homicide is the leading cause of injury-related deaths in infants less than 4 years old. The average age is two-four months and it occurs most frequently in male infants (56%). In 2011 a study from Canada showed the median age at presentation was five months, with 75% of children being younger than one year. Lethal abusive head injury is not confined to infants. Children as old as 4 or 5 years can be fatally head injured by abuse, although the great majority are under 2 years of age, and most are under 12 months of age. The Department of Health and Human Services in Washington DC documented that 84.5% of fatally abused children in 2001 died before their sixth birthday. Incidence studies in Queensland, Australia established that occurrence of abusive head trauma was more frequent than that of low-speed runovers, drowning and childhood neoplasms in Australian children during age incidence peaks (Kaltner, 2010).

[33C.320] Anatomy

Development of the nervous system occurs through the interaction of several synchronised processes. Some of these are complete before birth, while others continue into adulthood. The paediatric brain at birth is one tenth of the whole body weight and there is a constant interplay between development and regression, which results in relatively rapid brain growth until two years of age by which time it has achieved 80% of its adult weight. By the age of five years, the brain is approximately 90% of adult size, but significant remodeling of grey and white matter continues into the third decade of life.

Only one fourth of the neuronal cells that exist in the adult are present in the newborn. Myelination is not complete until age 3 and neuronal development finishes at age 12.

The autonomic nervous system is developed at birth, although immature, and the parasympathetic system is intact and fully functional.

There are key anatomic differences that put children at increased risk of inflicted head trauma: high head:body ratio, meaning that in a young baby a large mass is pivoted by the neck; infant cervical vertebral facets are flatter which renders the cord vulnerable to flexion and extension injuries; neck muscles are weak and underdeveloped; and there is head lag until about four months of life. All these anatomical variables mean that the head is poorly supported and thus susceptible to types of forces invoked by shaking and impact, with increased risk of brain and high spinal cord injury.

The bones of the skull also have important differences compared to adult skull bones, which are non-pliable, and fused together as the sutures have ossified. Bones of the child's skull are relatively elastic in nature, thus permitting some bending towards the cranial contents on impact. Due to laxity at the sutures and fontanelles, the cranial bones are less likely to fracture. The combination of these features puts the child's brain at an overall increased risk of trauma.

Due to lack of myelination, paediatric brains have a high water content surrounded by abundant subarachnoid space. These features, together with pliable skull bones, allow transmission of forces into the brain parenchyma which permits production of shearing-type injuries.

Basic anatomy

From outside to inside the layers that may be injured by mechanical forces are the scalp (skin, subcutaneous tissues, aponeurosis of Galen and sub-aponeurotic (potential) space), skull (periosteum, bone matrix, periosteum), dura (dura mater, arachnoid mater and pia mater) then brain.

Folds of dura mater form venous sinuses that drain into the internal jugular veins.

The arachnoid layer protrudes upwards as arachnoid granulations into the dural venous sinuses. The arachnoid layer adheres below it to the pia mater via the arachnoid trabeculae, which is a mesh of connective tissue. The subarachnoid space is filled with cerebrospinal fluid. The pia mater is adhered to the surface of the brain.

The inner dural border zone region (inner meningeal dura) possesses a vascularised layer where loose intercellular junctions exist. This zone plays an important role in the resorption of cerebrospinal fluid as the arachnoid granulations are maturing. Haemorrhage in this location conforms to the classic morphology of subdural bleeding when venous blood from the superior sagittal sinus refluxes into the dura, and thereafter into the subdural compartment.

Cerebral blood flow (CBF) depends on cerebral perfusion pressure; cerebral vascular resistance; viscosity; venous pressure; and intracranial pressure. The normal CBF is around 50 ml per 100 g of brain tissue per minute, and is a higher percentage of the cardiac output in small infants. It is also highest in grey matter (100 ml per 100 g). Impairment of CBF due to raised intra cranial pressure (ICP) is clinically seen as distension of scalp and retinal veins, pulsation of the fontanelle and a loud systolic cranial bruit when the ICP exceeds the diastolic blood pressure.

[33C.330] Mechanisms of injury and their pathological findings

There are a number of mechanisms that lead to head injury in infants. These include:

• linear impact (acceleration or deceleration);
• rotation with or without impact (shaken baby syndrome);
• whiplash (cervicomedullary syndrome);
• head compression; and
• penetrating head injury.

The biomechanical classification of head injuries (Faris, 2004) groups injuries according to similar biomechanical genesis. Biomechanical forces acting on the head can be dynamic (which refers to movement of the head or an object or both) or static (which involves forces exerted on an immobile head). Dynamic head loading forces are categorised as either contact (ie, the head coming into forceful contact with an object or a surface) or non-contact (forces loading through the neck and transmitted to the brain via the tissues).

Dynamic head injuries occur when force is rapidly loaded to the head at less than 200 msec. It can be caused by impulsive loading that causes the head to move, either by direct impact to the head, which is free to move, or by an action to the body that causes the head to move. Impulsive loading will impart inertial movement of the brain within the cranial cavity. Static injuries occur over longer time periods-usually greater than 200 msec- and cause crushing head injury.

Primary injuries are those caused directly by the mechanical insult and secondary injuries result as part of the pathophysiological progression following the primary injury. However, there is not necessarily a distinct boundary between primary injury and secondary injury.

Linear impact

Injury statistics have found oblique or tangential impact to be the most common accident situation. These will give rise to both linear and rotational forces, of which the brain is most sensitive to rotational motion. Radial or linear impacts occur when there are forces applied at 90 degrees to the plane of the surface, thus having no tangential component or force. Purely radial impact without rotation will cause primarily linear acceleration of the head, which results in higher contact forces and larger linear accelerations being applied to the skull, which increase the risk of skull fractures, epidural haematoma and contusions. Cerebral contusion at the site of impact in the presence of skull fracture is likely induced by the direct impression of the skull against the underlying brain tissue and thus caused by the contact force and linear acceleration.

A study by Mertz et al (1997) estimated a 40% risk of skull fractures for a peak acceleration of 180G and an 85% risk of fractures for 250G (The expected chest acceleration into the airbag during a car crash at 48km/h would be 60G and the acceleration limit during a bicycle crash with helmet would be 150G-200G). Far more common is oblique impact, which gives rise to both linear and rotational head kinematics. The brain is more sensitive to the strain of rotational loading than linear forces. A study by Kleiven (2013) illustrated the differences between radial and oblique impacts by simulating perpendicular impacts through the centre of gravity of the head and 45° oblique impacts. Substantially higher strain levels in the brain were obtained for oblique impact than perpendicular, when impacted on to the same padding using identical impact velocity.

Rotational injury, with or without impact

In the medical literature, "shaken baby syndrome"...