To the Editor.—
In reference to the recent letters of Donald B. Douglas, MD (227:439, 1974), and Peter J. Jannetta, MD (228:1637, 1974), we would like to present a theoretical explanation for the selective susceptibility of large-diameter axons to pressure-induced neuropathies.1-3 Although this is not a new observation, the theoretical explanation has been lacking. We suggest that this is due to Laplace's Law4 that states that in the special situation of a thin-walled cylinder (like an axon), T=P×R where T is the "tension" (stress or force per unit area) in the wall of the cylinder parallel to a circumference; P, the transmural pressure; and R, the radius of the cylinder. For example, consider two cylinders whose transmural pressures are identical but whose radii differ: the larger cylinder will have more tension (stress) in its wall. If this stress is too great, the cylinder's wall, in our case an
Strain RE, Strain RS, Olson WH. Laplace's Law, Pressure Neuropathy, Tabes Dorsalis, Tic Douloureux, Facial Spasm. JAMA. 1974;229(14):1864. doi:10.1001/jama.1974.03230520012008
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