ChiroView Presents Michael Freeman From: webinfo@ChiroViewPresents.com 8-28-00 Michael D Freeman PhD DC MPH Forensic Trauma Epidemiologist ChiroView Presents Department Head - Trauma Epidemiology Drmfreeman@earthlink.com Archives of Neurology „Chronic Whiplash Syndrome as a Functional Disorder¾ by Henry Berry MD Having recently reviewed the editorial piece detailing Dr. Berry¼s opinions regarding chronic whiplash syndrome ([i]), I was struck by the fragility of Dr. Berry¼s hypothesis that chronic pain complaints following whiplash injury result from a functional disorder. This fragility was due to the fact that Dr. Berry¼s theory revolves around one assumption; that the observation of „parallel identical injuries¾ of 20 demolition derby drivers is a reasonable proxy for the millions of whiplash injuries that occur annually as a result of non- demolition derby crashes, and that the variation of injury response in the 20 drivers represents the variability of injury response in the general population. Dr. Berry¼s theory is that, whereas his unpublished case series of 20 drivers of demolition derby cars demonstrated no chronic symptoms or disability in spite of the fact that they have been exposed to numerous crashes, it logically follows that occupants in real world crashes should not, and therefore cannot sustain a chronically painful injury. The assumption that a functional disorder is the cause of chronic pain complaints following whiplash is largely by default; Dr. Berry¼s perception that there is a pathomechanical vacuum in the genesis of chronic whiplash apparently led him directly to functional disorder without consideration of any alternative theories. When evaluating the validity of novel theories concerning the etiology of a specific condition, it is useful to examine the external validity or generalizability of the methods employed in arriving at the new theory. Dr. Berry¼s unpublished work described a selection of 20 healthy, prepared, risk-taking, and relatively young male drivers who self-selected to participate in a series of potentially dangerous crashes for sport. It is not unreasonable to state that a clinician would find a significant challenge in describing any actual whiplash-injured subjects who fit the same description as the subjects in Dr. Berry¼s cohort, yet he still concluded that demolition derby drivers and whiplash-injured subjects are „largely similar in age, general health, type and speed of collision, blows, strains, and foreseeability or unexpectedness of the accident and vehicle damage.¾ Even an individual who does not frequent demolition derbies would not be expected to believe that the participan! ts are unprepared for the collisions, as are the majority of rear- impact collision-injured occupants. Dr. Berry makes a more significant error that threatens the validity of his conclusions; he compares studies of whiplash-injured individuals, in which the subjects were selected on the basis of their having sustained an injury, to his study in which the subjects were selected not by injury status but by exposure to a potential injury mechanism (multiple demolition derby crashes). If Dr. Berry had compared his group of 20 demolition drivers to a group of 20 males of similar demographics exposed to a low speed rear impact collision, he would have found (based on the findings of the largest epidemiologic study of injury incidence following rear impact collisions to date) that only between two and four of the subjects would have been expected to have any injury at all ([ii]), and that no more than one of the injured would have been expected to have any permanent symptoms ([iii]). Thus, Dr. Berry¼s theory regarding a functional disorder as the basis for complaints of permanent pain following whiplash is based on an expected difference of 5% between his study group and a cohort of real-world crash-exposed occupants. Even if the groups were roughly comparable, a statistical analysis using a binomial probability curve tells us that a significant difference between two groups of 20 would have to be 15% or more, and thus there is no real difference between two groups of 20 that vary by 5%. Simply put, what is illustrated with a cohort of 20 demolition drivers who have not sustained permanent injury is that it is possible to participate in a demolition derby and not sustain permanent injury. It is also possible to be involved in a rear-impact collision for which one is unprepared and not sustain a permanent whiplash injury. Such possibilities at one end of the spectrum do not allow for inferences as to what is occurring at the other end, where susceptible individuals can be and are injured permanently following low speed crashes. If you have comments, send them along to Sig at webinfo@chiroviewpresents.com, and he will pass them on to me. [i] Berry H., „Chronic Whiplash Syndrome as a Functional Disorder¾ Arch Neurol 2000; 57:592-4 mhtml:mid://00000029/ - _ednref2 [ii] Farmer CM, Wells JK, Werner JV. Relationship of head restraint positioning to driver neck injury in rear-end crashes. Insurance Institute for Highway Safety. September, 1998. mhtml:mid://00000029/ - _ednref3 [iii] Freeman MD, Croft AC, Rossignol AM, et al. A methodologic critique of the literature refuting whiplash syndrome. Spine 1999;24(1):86-98 Michael D Freeman PhD DC MPH Forensic Trauma Epidemiologist Department of Public Health and Preventive Medicine Oregon Health Sciences University School of Medicine 2480 Liberty Street NE Suite 180 Salem, Oregon 97303