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September 30, 1974

Laplace's Law, Pressure Neuropathy, Tabes Dorsalis, Tic Douloureux, Facial Spasm

Author Affiliations

Vanderbilt University School of Medicine Nashville, Tenn

JAMA. 1974;229(14):1864. doi:10.1001/jama.1974.03230520012008

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