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EFFCTS SF SYERRESSURE
U THE EAR -AR KIEW
Frederic G. Hirsch, M.D1.
Technical Progress Report
Contract No. DA-49-146-XZ -372
work, an aspect of ietgiosdealing with
fro Bombs, VM
EFFECTS OF OVERPRESSURE
ON THE EAR- AREVIEWFrederic G. Hirsch, M.D.
Technical Progress Report on Contract No. DA-49-146-XZ-372 This work, an aspect of investigations dealing with the Biological Effects of Blast from Bombs, was supported by the Deferse Atomic Support Agency of the Department of Defense.
(Reproduction in whole or in part is permitted for any purpose of the United States Government. ) Lovelace Foundation for Medical Fducation and Research Albuquerque, New Mexico November 1966 Ip
FOREWORDThis report, which is primarily a review of published material con- cerning the effects of blast on the human and the mammalian ear, also incorporates some previouily unpublished data concerning the impact of age on the vulnerability of the tympanic membrane to blast overpressures, and some further material on intraspecies scaling.
An attempt has been made to correlate such quantitative data as can be found in the literature, but this has not been found possible of accom- plishment in a completely satisfactory way. By rescaling some previously published material in view of available data, and incorporating some un- published material a value for the threshold of eardrum rupture in man is estimated to be 5 psi, and the "short"-rising overpressure required for rupture of human eardrums is 15 psi.
This information will be of use to those whose task it is to assess hazards and identify populations at risk, as well as those responsible for providing safe working environments and similar problems in the field of environmental health.
UA " i i t KIid
ABSTRACTInformation regarding blast effects on the ear has been reviewed in an attempt to gather quantitative information available for animals and man for help in the establishment c! relationships betweer :-rious levels of overpressure and the incidence of eardrum failure, the degree of damage to the middle and inner ear and other identifiable sequelae referable to cochlear or vestibular functions.
ii ACKNOW LEDGMENTS
The author wishes to acknowledge the considerable assistance in the
preparation of this report which was rendered by the following people:
to Dr. Donald R. Richmond of the Lovelace Foundation, for permission to include some of his unpublished data; to Mr. Roy H. Reider, of the Los Alamos Scientific Laboratory, for providing data on some recent industrial explosions; to Mr. I. Gerald Bowen, of the Lovelace Foundation, and Mr. Luke J. Vortman of the Sandia Corporation, fcr their help in pressure-distance scaling calculations; to Dr. Thomas L. Chiffelle, of the Lovelace Foundation, for assistance in preparing and interpreting histological preparations of tympanic membranes; to - r. Thomas Shipman, of the Los Alamos Scientific Laboratory, and Dr. Donald E. Kilgore Jr., of the Lovelace Clinic, for making clinical material available; to Mr.
Robert A. Smith, of the Lovelace Foundation, for the preparation of illustrative material; to Mr. Walter Scheurle, of the Lovelace Foundation, for the translation of material; to Mrs. Laura M. Carrasco and Mrs. Ruth P.
Lloyd for secretarial and editorial assistance; to the Defen.e Atomic Support Agency for contract support; to Colonel Gerrit L. Hekhuis, USAF. MC, Defense Atomic Support Agency, Department of Defense and Lt.
Colonel Edmund L. Fountain, USA, VC, Defense Atomic Support Agency, Department of Defense, for their interest and support; and finally to Dr. Clayton S. White, President-Director of the Lovelace Foundation, whose very great personal interest and valuable suggestions contributed very much to whatever merit resides in this report.
For many years physicians have been aware that the ears of humans and other mammals have an especial vulnerability to injury when exposed to overpressures generated by explosions and the muzzle blast of guns. By the year 1900 there appeared in the medical literature thirteen articles on the subject, beginning with the report of Greeni in 1872. 1 The most comprehensive treatment was that of Castex in 1893.' Since that time, well over one hundred papers have been published on blast injury of the ear, their number increasing during periods when a major armed conflict was being waged and diminishing during peaceful times; although, whenever major explosions have occurred, renewed interest has followed. 3,4.5 A review of that which has been written concerning blast injury of the ear reveals that, with some important exceptions, the reports deal mostly with clinical considerations, and that quantitative data relating overpressures to incidence or degree of injury are lacking. These exceptions date back quite a few years, however, to the work of Zalewski in 1906.6 This study, which will be dealt with more fully later on, was for almost forty years the only one of its kind until the work of Zuckerman 7, 8 and hi colleagues during World War II was done. Also about this time, Perlman9 I published his s4 dies which have contributed so much to an understanding of the response of the otic structures to blast waves.
In the recent past, the Comparative Environmental Biology Department of the Lovelace Foundation for Medical Education and Research has had an opportunity to obtain some quantitative data on a number of mammalian species during the full-scale weapons tests, its shock tube studies, and some of its other activities. 12-2I Recently, studies of the vulnerability of the ear to blast injury have been resumed in England by Golden and Clare. 22 It would appear useful and timely to prepare a review of what is known about otic blast trauma in view of recent advances in explosives technology, the development of new types of ordnance which have higher muzzle velocities and consequently higher muzzle blast overpressures, and for purposes of Civil Defense planning. This communication represents such an undertaking.
PHYSICAL, ANATOMICAL AND PHYSIOLOGICAL CONSIDERATIONS2.0 The vulnerability of the ear to blast overpressures is readily appreciated when one considers that a blast wave is physically the same phenomenon as a sound wave. It is generated when air molecules are rapidly and violently compressed such as occurs when a solid or liquid substance is suddenly changed into a gas. In an explosion, this change in physical state occurs in times of I x 10-5 seconds or less, depending on the chemical nature of the substance. The resultant compression can be of many hundreds or thousands of pounds to the square inch at the source. Near the source, the molecules are forced outward for some distance before commerning to -i u-illate. Once oscillation has occurred, a wave is propagated radially. As it is propagated through the air, the peak overpressure and its velocity fall off at a rate which is faster than an inverse square of the dis tance relationship, so that at distances of several hundreds of feet the overpressure will have diminished to a magnitude of several atmcspheres. One definition of a shock pulse can be given as a sound wave of great initial condensation and great initial velocity. I Itfollows, therefore, that an organ system which is built especially for the reception oi sound will receive blast overpressures equally well.
The ear has evolved as an organ system for the transduction of sound waves into nerve impulses which are electrical in nature. It has developed an ability to respond to a limited band of frequencies which for the human may be regarded a3 lying between 20 Hz and 20, 000 Hz. It has developed a high order of sensitivity. It responds to signals which have an energy level as low as 10-16 Watts/cm 2, or whose pressure is about one five billionths of an atmosphere, or whose force causes an excursion of the eardrum - a distance whiLh is less than the diameter of a single hydrogen molecule. 9 - 11 23 The drumhead cannot respond faithfully to pulses which have periods of less than 0. 3 milliseconds, but it attempts to do so by making a single large excursion which corresponds to that occasioned by a sound wave of high audible frequency.9- 1 It is this excursion which mediates the trauma to the ear.
The mammalian ear is divided anatomically and physiologically into the external, middle, and inner ears. Each of these plays a different role in the overall function of the auditory apparatus.
The external ear is a sound-gathering device which has two main functions: the first is that of amplification; and the second is as a directionsensing device. From the standpoint of blast trauma, both functions are affected albeit in different ways.
Golden and Clare 2 2 have found in their studies that, when a shock pulse is generated in the air at a distance from an ear, there is an increase of about 0. 2 percent in the overpressure at the site of the tympanic membrane over that which is measurd in the air at the same distance from the source FIGURE 1 shows their experimental arrangement and a typical of the pulse.
When an ear has been significantly injured in such a way that a hearing loss results, there also occurs an interference with stereognosis, which if the ability of an animal to locate the origin of sounds in its environPart of this ability depends on differential loudness in one ear as ment.
opposed to the other, and the rest de-ends on the reception of sounds at slightly different times in each ear, so that a phase difference between the two ears is occasioned. This phase difference is mediated and interpreted by the nervous system in a complex way, the details of which are not germane to the present consideration. Suffice it to say that when there is injury to an ear, both loudness and phase are affected, since an altered impedance has been introduced into one side of the system.
Before leaving the consideration of the external ear, something should be said about the influence of ceruminous, or wax, plugs on the response of an ear to blast waves. These are not always present in the external auditory canal, but their incidence is quite high. Most often a plug which has a diameter sufficient to occlude the cross section of the canal will not be sufficiently long to fill the length of it, so that an air space exists between it and the eardrum. Less frequently, one end of a plug will be impacted against the drum.
When a ceruminous plug of the first sort is present, it will diminish the gain function of the canal and will act as an attenuator of a blast wave because, being both compressible and displaceable, it absorbs some of the energy of the shock pulse. That plugs of this sort are effective in reducing trauma to the ear by blast has been remarked by several of the 9 authors who have written on the subject.,24.26 Sometimes, however, the presence of a ceruminous plug will behave in a manner which makes an ear injury worse. If the deep end of the plug is very close to the eardrum, or actually is in contact with it, and if the displacement of it by the shock pulse is sufficient, then the plug behaves as a ramrod causing not only an extensive rupture of the drum but also a displacement of the ear ossicles. Colledge 2 4 has remarked about this, and experience with dogs at the Nevada Test Site was confirmatory. 12 In the latter case, a plug was placed in one ear of each experimental animal. It was made of cotton and liquid rubber which solidified after exposu-e to the air. These plugs completely filled the external auditory canals. The relatively "slow"-rising, but very "long"-duration overpressure, measured near the exposure locations inside underground shelters, apparently acted for sufficient time to push the plugs into the middle ear causing obliteration of the eardrums and dislocation of the ossicies.
Experience with exposure of animals to high-explosive detonations
4 on the one hand, and overpressures generated in shock tubes on the other, has shown that for relatively "low" overpressures there is little displacement of a ceruminous plug when the duration of the overpressure is "short" as in the case of some high-explosive detonations. When the duration of an overpressure of similar magnitude is relatively "long," as in the case of some of the shock tube-generated pulses, the plugs are frequently displaced more deeply into the canal. So whether or not a ceruminous plug in the external auditory canal protects the drum or makes matters worse depends on a variety of things: the diration of the overpressure, whether there is a space between the plug and the eardrum, and the character of the plug; i. e., its density, mass, and composition.
The eardrum, or tympanic membrane, separates the external ear from the middle ear. It is this structure which has engaged most of the attention of the clinicians who have concerned themselves with blast injuries of the ear. This is due to the fact that it is frequently ruptured.
It is not, however, the most important lesion associated with blast trauma.
Indeed, as will be subsequently developed, it is probably better for the drum to give way to the overpressure than for it to remain intact.