Friday, 24 February 2012

SBS: Biomechanics of Retinal Hemorrhages

February 23, 2012 
John  D. Lloyd, Ph.D., M.Erg.S., CPE, CBIS
Board Certified Ergonomist & Certified Brain Injury Specialist

The occurrence of retinal hemorrhages has been proclaimed as one of the three cardinal features of the triad in the clinical presentation of an infant injured by repetitive rotational acceleration/deceleration from abusive shaking, where the retinal findings were asserted to be the unique primary result of vitreous traction on the retina. However, such claims are unconfirmed and   biomechanically implausible. First, retinal hemorrhages are clearly not unique to infants abused by alleged shaking, since such hemorrhages have been reported to occur as a result of increased intracranial pressure of any etiology and are often found in cases of impact injury. Furthermore, efforts to confirm the vitreous traction hypothesis as a valid cause of extensive retinal hemorrhages, retinal schisis, or folds have failed.

Anatomy and Vasculature of the Eye
In the vitreous traction hypothesis, it is asserted that traction is generated between the vitreous humor and the retina inside the eye (Figure 1 ), causing hemorrhage from ruptured capillaries in the retina. However, the relative densities of the vitreous humor and the retinal both approximate that of water (1.0 kg/l). Moreover the point of rotation of the infant head during ascribed non-accidental trauma is the lower cervical spine, therefore the radius of curvature of the retina and the vitreous traction are, for all intents and purposes, equal. Given equal radius of curvature and equal densities, it is highly improbable that a mechanical differential would be generated, particularly at the levels of rotational acceleration/deceleration that might be generated during an abusive shaking. Hence, the concept of vitreous traction cannot be supported by principles of physics.
It is now generally understood and accepted by medical professionals that retinal hemorrhages are caused by any etiology, which increases intracranial pressure of the brain. The eye is the only externally visible element of the central nervous system, where the optic nerve and central retinal vasculature are sheathed within the dura (Figure 1). Since we know that arterial pressure exceeds venous pressure, the venous return from the central retinal vein will be impeded prior to obstruction of blood flow in the associated artery, thereby increasing pressures within capillaries in the retina, which consequently burst producing of retinal hemorrhage. Furthermore, since the underlying etiology is increased intracranial pressure, bilateral observations would be typical, where the extensiveness of hemorrhagic findings would be proportional to the intracranial pressure.
In summary, retinal hemorrhages are no longer generally accepted by medical doctors to be caused by vigorous shaking of an infant.