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ABSTRACT
Injury from gunshot can occur in tissue
remote from the trajectory of a missile.
This injury has been ascribed to
remote displacement of tissue
induced by motion of the cavities
generated by bullet penetration,
or, equivalently,
to pressure waves associated with tissue displacement.
Nonuniformity of the acoustic properties of tissue
can deform wavefronts and intensely concentrate energy into
localized regions.
We report a case in which
localized severe injury to the spinal column
remote from the bullet trajectory may have been
caused by the focusing of missile wound pressure waves
by the bony tissue of the thoracic vertebrae.
A young woman was shot in
the left side of the chest.
She became unable to stand,
and was subsequently noted to have a Brown-Séquard syndrome[1]-[3].
with weakness of the left leg and impaired pain and temperature sensation
below T8 on the right.
Multiple radiological studies showed that no fragments of the bullet
had entered the spinal canal.
The focusing effect of thoracic vertebrae on pressure waves
impinging on the spine from different directions
has been calculated
using the theory of geometrical acoustics.
The bony tissue lateral to the spinal canal
forms a bi-concave lens which
owing to the large sound velocity of bone
strongly focuses pressure waves into the canal.
This effect occurs over a large range of incidence angles,
so it is likely to occur any time
missile-generated pressure waves impact on the spine.
Because of this focusing effect,
individuals receiving lateral impulsive impact by gunshot
are subject to neurological threat even when the missile does not enter
the spinal column.
brad@galcit.caltech.edu