From Symptoms to Liability The Distinct Roles of Differential Diagnosis and Causation Assessment

by Ronald E. Gots

The patient has tingling in his leg and some weakness. Was your client’s aerial spraying of a pesticide responsible? Answering this question requires following an investigative path from symptom to culpability and potential liability, and such seemingly routine questions are an integral part of per- sonal injury trial practice. Eventually, they seem so routine that the med- ical and scientific complexities of this analytical exercise are overlooked or taken for granted. Yet, the analysis of such matters is not trivial, nor is it a one-step process. It involves two types of expertise and, often, two dif- ferent experts.

© 2005 DRI. All rights reserved. Some attorneys or their consultants ted to jump from symptoms to external accompanying human interest stories may in toxic tort or pharmaceutical lawsuits cause with neither the expertise, nor the delve into the reasons for the injuries dur- have noticed that physicians are testify- data, to do so. ing the course of a show, they do not affect ing to matters that they know little about, The Federal Judicial Center’s Refer- the care. Rather, the care depends upon a using poorly formulated analytical meth- ence Manual on Scientific Evidence (1994) rapid differential diagnosis, followed by a ods that have no proper place in a court- defined the term “differential diagnosis,” prompt diagnosis that leads to immedi- room. They do so claiming that they have properly, as: ate care. The differential diagnosis of an employed the standard, recognized meth- The method by which a physician unconscious patient may include head odology of clinical medicine—differen- determines what disease process has trauma (either requiring surgery or not), tial diagnosis. caused a patient’s symptoms. The phy- diabetic coma, a stroke, a hypoxic brain Differential diagnostic methodology is sician considers all relevant potential injury following a heart attack, a blow with widely assumed, and now supported by causes of the symptoms and then elim- a baseball bat or poisoning, among many some legal precedent, to arrive at causal inates alternative causes based on a others. Once the MRI is completed and a conclusions in personal injury claims. It physical examination, clinical tests and skull fracture with a subdural hematoma does not. And until counsel understands a thorough case history. is found, the differential diagnostic pro- why and how to explain this to trial judges, Reference Manual on Scientific Evidence, cess is over. The diagnosis is made. The they will be missing important opportu- First Edition (1994) at 214. treatment begins. How the patient got that nities for pretrial management of their The critical component of this defi- injury is neither part of the diagnosis, nor cases and for best serving their clients. nition is that differential diagnosis is a of the treatment. Attorneys who believe that differential quest for a diagnosis: what is wrong with The careful reader will note an excep- diagnosis properly leads to a determina- the patient internally. It is not, inherently, tion to that statement in this example. tion of cause are not alone. Many other a search for the ultimate cause (critical to What if, in the course of the diagnostic lawyers and judges do as well, and deci- liability) of that disease process or disor- work up, the unconscious patient is found sions by widely dispersed state and fed- der. In numerous decisions, however, the to have significantly elevated carboxy- eral courts have given credence and legal courts use cause of symptoms and ultimate hemoglobin (COHgb)—say 50 percent? authority to this erroneous notion. Expe- cause of the disease process interchange- That finding leads to a diagnosis of carbon rienced experts have used and contrib- ably. By doing so, “differential diagnosis” monoxide poisoning that also elucidates uted to this misperception, averring that becomes defined, incorrectly, as the meth- the outside cause of the unconsciousness: their “differential diagnostic methodology odology by which both a diagnosis and poisoning by carbon monoxide. In that is the standard methodology of medicine” the cause of that diagnosis are assessed. case, the diagnosis necessarily leads to a and that “it led [them] to conclude that causal attribution. your client’s product caused the claim- Distinctions—Diagnosis versus For most of the possible diagnoses in ant’s injury.” Identifying Causation unconscious patients, however, the diag- The problems with moving in a one- Before discussing specific cases, an illus- nosis explains the internal reason for the step process from symptoms to the tration of the methodological distinctions unconsciousness, but it does not explain product or agent that caused them are between diagnosing an illness and identi- the external cause of that condition. In the twofold: 1) two separate and disparate fying its cause may be helpful. Medical stu- patient who is unconscious because of a methodologies are rolled into one; and dents are taught to diagnose and to treat. brain hemorrhage, the neurologist finds 2) as a corollary, physicians are permit- Most often they are not taught to uncover and diagnoses that hemorrhage with an the causes of an illness. A recognizable MRI. Whether that hemorrhage occurred analogy is found in the difference between because the patient was taking a spe- the TV shows “ER” and “CSI.” In the for- cific medication, or whether it was due to mer, diagnosis and treatment are central. uncontrolled hypertension, or whether it Is there abdominal bleeding? Is there head resulted from a congenital aneurysm or trauma? What do we do for this patient? a blow to the head is neither part of the How do we save his or her life now? While diagnosis, nor of the differential diagnos- tic process. “CSI,” by contrast to “ER,” is all about cause. Usually the diagnosis is simple— Ronald E. Gots, M.D., Ph.D., is the CEO of the International Center for Toxicology and the patient is dead. The question is why Medicine (ICTM) in Rockville, Maryland. For 30 years, Dr. Gots has been an active and by whom? These are causation ques- consultant and expert in major toxic tort and mass tort litigation, working with tions. In “CSI,” they are not answered by national law firms and general counsel throughout the U.S. and other countries. physicians but, rather, by forensic profes- July 2005 25 sionals who gather evidence, put the pieces sion, assume that the resulting diagno- acterization of the type of peripheral neu- together and induce the causal answers. sis is a peripheral neuropathy. This means ropathy. A biopsy of the nerve is generally that something is wrong with one or more one of those diagnostic elements. An Example nerves in the leg. Moreover, it means that An example relevant to defense counsel is the problem is with the nerve(s) them- Causation Assessment the one that opened this article. What was selves, not with the lower back. This ends At this point, this article will examine an the relationship, if any, between the aerial the differential diagnostic exercise. The entirely separate exercise—that of causa- spraying and the patient’s (claimant’s) leg diagnosis of peripheral neuropathy is the tion assessment. Again, we return to the symptoms? Here we have two questions: 1) end of that methodological approach. example that introduced this article— Why does the patient have leg symptoms? Thus, the differential diagnostic exer- whether an exposure to an aerial spray- 2) Did our spraying cause those? The first cise involves six steps: ing (crop-dusting) was the cause of the is the “ER” question: the second, the “CSI” leg symptoms. The elements required to question. Answering each involves a sep- For most diseases, the assess this question differ considerably arate methodology, separate expertise from those required to make the diagno- (rarely combined in one person) and more diagnosis does not bespeak sis and have been discussed extensively than a differential diagnosis. However, the in numerous publications. See R.E. Gots diagnosis comes first. One cannot evalu- a cause. It directs care. and S.W. Pirages, Applying the Principles ate potential causal relationships until one of Science to Daubert Motions in Toxic Tort knows what it is to have caused. • Making a list of possible diagnoses Claims, Wiley Expert Witness Update, 1– The differential diagnostic methodol- (internal disorders) that could explain 88, Aspen Law and Business (2000). These ogy is, therefore, the first step. Its intent is to the presenting symptoms or observa- required elements are also, individually, answer question number one, but only that tions; the subject of numerous Daubert deci- question. It is the standard methodology of • Taking a thorough medical history; sions. They include, among others: clinical practice. The steps of the differen- • Conducting a careful and complete • Can the agent (pesticide) at issue cause tial diagnostic process include identifying physical examination; any type of peripheral neuropathy all reasonable potential internal disorders • Ordering and interpreting the indi- (sometimes called the “general causa- that could account for a patient’s clinical cated tests; tion” question)? presentation. Here, “internal” is the opera- • Ruling out diagnoses that do not fit the • If so, does the patient have the type tive word, for diagnosis focuses on what is history or findings noted above; of peripheral neuropathy that can be going on inside the patient; what is wrong • Arriving at the diagnosis that best fits caused by the exposure, or is this an with him. In our patient with strange sen- the first five elements. entirely different type? sations and weakness in his legs the differ- Before we even get to causation, there • Was there sufficient exposure to account ential diagnosis might include: peripheral is a subpart to the differential diagno- for this condition? neuropathy, a muscle disorder, a vitamin sis in this case—the type of peripheral • Are there equally, or more likely alter- deficiency, potassium deficiency, vascu- neuropathy that exists. In other types nate, competing causes? lar disease, nerve root compression, com- of disorders, such as a malignancy, this • Did the clinical course follow the monly related to degenerative lower spine determination is part of the differential sequence known to be associated with disease, a spinal cord tumor and others. diagnostic exercise, because generally a this potentially causal agent? (E.g., was The diagnostic work up—the method- biopsy of the tumor is integral to the work there evidence of severe toxicity?) ology of clinical practice—is designed up and provides the answer to this ques- • Was the temporal relationship the one to eliminate those possibilities that are tion. In the case of a peripheral neuropa- expected if the agent at issue were incorrect and to elucidate the proper diag- thy, we need to know what kind it is. The causal? nosis. In this case, that would include tak- answer to this question gets us closer to Only after these questions are explored ing a history from the patient, performing potential cause, but is still insufficient. The and answered affirmatively can the expert a complete examination—particularly a complexities of this question are beyond properly offer a causal conclusion. It good neurological examination in this the scope of this article—suffice it to say should be obvious to the reader that the case—and conducting appropriate tests, that entire textbooks cover the subject of diagnosis (resulting from the differential various blood tests, x-rays of the lower evaluating peripheral neuropathies. A few diagnostic methodology) is only one of back and electrical testing of nerve and of numerous considerations include: is many steps required in causation assess- muscle function. It is likely, but not inev- this one nerve or many; axonal or demy- ment. The others not only require differ- itable, that a specialist (most likely a neu- elinating, metabolic, vascular, chronic or ent data, including an appreciation of the rologist in this case) would participate in acute. Complex algorithms describe the dose received by the claimant and a com- the diagnostic activity. For sake of discus- diagnostic pathways that lead to the char- prehensive knowledge of the toxicology 26 For The Defense of the chemical(s) at issue, but they also Because physicians are trained to diag- to cause using that differential diagnostic require specialized and different exper- nose and treat, they are little involved approach. That, of course, assumes (incor- tise. Most physicians, even neurologists, with causation assessment. How and why rectly, in the author’s opinion), that phy- who diagnose peripheral neuropathies the patient developed Hodgkin’s disease sicians are the community “relevant” to have limited toxicological backgrounds. or prostate cancer is not important to determination of cause. Other courts have The words “organophosphates, pyre- the practice of oncology. How the lupus recognized certain methodological needs thrins, carbamates, piperonyl butoxide, or the scleroderma came about is irrele- beyond differential diagnosis to conduct volatile organic chemicals” are rarely spo- vant to the rheumatologist. And why the a causation analysis, but rarely have they ken in medical schools or in postgraduate patient developed MS is of little impor- enunciated the clear and distinct differ- medical training programs. tance to the practice of neurology. Cause ences that actually characterize the two. There are a few situations in which is only important to the actual practice the diagnosis does suggest or establish of medicine when it affects treatment or Examples of Case Law on the Issue a cause. Clear cut toxicological illnesses future prevention. Certainly, the allergist The examples to follow are selected from come to mind. The patient described ear- may try to figure out what triggers his or nearly a hundred such decisions, but are lier, brought comatose to an emergency her patient’s asthma attacks to treat with illustrative of the confusion that pervades department, found through blood test- immunotherapy (allergy shots), or to sug- this issue. ing to have a COHgb level of 60 percent, gest avoidance. One wants to know what In Moore v. Ashland Chemical, one of will be diagnosed properly as suffering the poison was to find the right antidote. the better discussions of the role of dif- from carbon monoxide toxicity. Here, the Culturing the specific bacterium in an ferential diagnosis took place. The Fifth diagnosis explains both the reason for the infectious disease permits selection of the Circuit Court of Appeals overturned the coma and its cause. A rattlesnake bite or correct antibiotic. The discussion in this trial court’s decision to exclude a medical poisoning by aspirin might be similarly article, however, is not focused on those expert who had reached a causation con- diagnosed and causally attributed. Cer- disorders in which the diagnosis clearly clusion. Moore v. Ashland Chemical Co., tain allergic responses—anaphylaxis due connotes the cause. Rather, it is those dis- Inc., 126 F.3d 679 (5th Cir. 1997). There, the to peanut allergy or a bee sting—are sit- orders, commonly seen in personal injury, appellate court had concluded that Dau- uations in which the diagnosis may be particularly toxic tort and pharmaceuti- bert factors were inappropriate for deter- readily and inextricably tied to the cause. cal liability claims, in which the diagnosis mining the reliability of testimony from a Another notable category of disorders in tells us little about cause and where a sep- clinical medical expert. It suggested that which diagnosis may suggest the cause arate causal analysis is needed. physicians who use “differential diagnosis” is infectious diseases, e.g., Tuberculosis Cause is, of course, a central legal con- methodologies to reach causal conclusions is caused by the TB bacillus. What is it cern. It is the cause that links the disor- are not subject to the scientific exclusions about such disorders that permits this der to the responsible party. The scientific enunciated in Daubert. linkage between diagnosis and cause? In methodology of causation assessment The Fifth Circuit granted rehearing to the case of poisonings, it is either the lab- was the focus in the Daubert decision. consider this case en banc and to clarify oratory evidence (blood determinations Daubert v. Merrell Dow Pharmaceuticals, the standards district courts should apply of toxins or their products) or the imme- Inc., 509 U.S.579 (1993). in determining whether to admit expert diate, specific and profound nature of the In Daubert, there was no differential testimony. On rehearing, the court held response—or both. It is the latter that per- diagnosis. The child’s birth defect was that expert testimony must demonstrate mits the bee sting/anaphylaxis connec- visible to a layperson. The only question that findings and conclusions are based tion. In these cases, it is not merely the was what caused it and how the claim- on scientific methodology. 151 F.3d 269 temporal relationship that makes the con- ants’ experts decided that it was the drug (5th Cir. 1998). They concluded that the nection, although that is a factor. The same Bendectin taken by the mother during her testifying physician had not satisfied the cannot be applied to vague outcomes with pregnancy. In concluding that their meth- elements of proper causation methodol- uncertain causes such as non-specific odology was flawed, the U.S. Supreme ogy. He had no support for the general symptoms connected to mysterious or Court enunciated some of the required causation issue (that the industrial sol- simply speculative indoor environmen- components for a properly constructed, vent, toluene, could cause reactive airways tal agents. Nor can it be applied, absent scientifically based causal conclusion. dysfunction syndrome (RADS)). He had a thorough causation assessment, to dis- When it comes to physicians, some courts no exposure data. The fact that symptoms orders like asthma, RADS or neuropsy- have determined that since Daubert speaks developed shortly after exposure was not chological dysfunction, which may have of standard methodology within the rel- a sufficient basis to assert causation. many causes. evant community, and since differential Thus, we have two competing deci- For most diseases, the diagnosis does diagnosis is the methodology of clini- sions, both flawed in their reasoning, but not bespeak a cause. It directs care. cal medicine, that physicians can speak one closer to the mark than the other. In July 2005 27 the first instance, the court determined ential diagnosis and some understanding Here, the court reached a conclusion that that a differential diagnostic methodol- that causal assessment involves more. The general causation is a separate methodol- ogy led physicians to causal conclusions court’s comments are revealing. ogy, falling under “toxicological method- and, since that methodology was specific …he primarily applied a methodol- ology.” While that is true, they incorrectly to medicine, it satisfied Daubert. In other ogy of differential diagnosis. Thus, he concluded that all of specific causation is words, merely claiming that “I performed determined from Ms. Cavallo’s med- satisfied through a differential diagnostic, a differential diagnosis” was sufficient to ical history, from her description of the medical methodology. Differential diag- insulate a physician from fulfilling any of spill incident, from his initial exami- nosis is, in fact, one part of specific cau- the other essential causation methodolog- nation of her, and from the timing of sation (which this court earlier said), but ical elements. the spill in relation to her development the other parts—dose, timing, ruling out The en banc decision added a causation of symptoms, that her exposure to the alternate causes—are not. These are part element—general causation—to the phy- of causation analysis and might better sician’s methodological requirement say- Diagnoses do not involve be placed, using this court’s terminology, ing that, at least, he had to have some basis under “toxicological methodology.” for claiming that the agent was an estab- products; they involve The medical expert’s testimony was lished potential cause of the disorder. And, excluded in this case because he had failed they noted further, that a specific causa- disorders or diseases. to assess “general causation.” The remain- tion element—dose—was also important der of his reasoning, a combination of and not part of differential diagnosis. petroleum hydrocarbons could have differential diagnosis and elements of In Heller v. Shaw Industries, following a caused her chronic illness. specific causation, was accepted. Daubert hearing, the trial court excluded 892 F.Supp. at 771. Another recent case involving ortho- the testimony of the claimant’s medical In accepting this testimony, the court pedic spinal screws was dismissed by the expert who supported her claim that off- accepted an admixture of differential trial court for a number of reasons. Law- gassing from carpeting was a cause of the diagnostic methodologies—medical his- rence v. Synthes, et al., (No. 1623 EDA 2003 claimant’s respiratory difficulties saying: tory and examination—with causation Pa.Super.). Among them was the lack of “I concluded that the carpeting in her methodologies—description of the spill, reliability of plaintiff’s expert physician’s house was the major factor in her illness.” and timing of the spill. In the same ruling, testimony. The motion in limine and sum- Heller v. Shaw Industries, Inc., 167 F.3d 146 the court distinguished between specific mary judgment were reviewed and the (3rd Cir. 1999). He based this testimony and general causation saying: decision was upheld. The appellate court on a “differential diagnosis.” The process of differential diagnosis is properly noted: The Third Circuit Court of Appeals undoubtedly important to the question Differential diagnosis is the determina- agreed with the trial court’s exclusion of of “specific causation.”… But it is also tion of which of two or more diseases plaintiff expert’s testimony. However, and important to recognize that a fundamen- with similar symptoms is the one from distressingly, the court’s reasoning was tal assumption underlying this method which the patient is suffering, by a sys- not that it was the lack of scientific sup- is that the final, suspected “cause” re- tematic comparison and a contrast- port for the general causation conclu- maining after this process of elimina- ing of the clinical findings…. In other sion that rendered the physician’s opinion tion must actually be capable of causing words, differential diagnosis is used to inadmissible; rather, it was his flawed dif- the injury. That is the expert must “rule determine what the condition is and ferential diagnosis. in” other possible causes. And, of course, how to treat it. Causal assessment, on Apparently, the appeals court in this expert opinion on this issue of “general the other hand, is the methodology case believed that a correctly performed causation” must be derived from a scien- used to determine how the condition differential diagnosis was the proper way tifically valid methodology. arose and to determine responsibil- to arrive at a causal conclusion. Id. ity or liability.… The differential diag- In Cavallo v. Star Enterprise, 892 F.Supp. The court goes further in a related foot- nosis methodology does not lead to a 756 (E.D. Va. 1995), residents of a com- note discussion. causal determination. munity near a fuel tank farm alleged a …Ms. Cavallo contends that Dr. Bel- The claimant’s experts in Mattis v. Car- variety of illnesses from fugitive emis- lanti applied both 1) the toxicological lon Electrical Products, 114 F.Supp.2d 888 sions and several small releases into their methodology previously described to (D.S.D. 2000), incorporated the cause of community. In discussing the claimant’s determine whether the AvJet spill could the RADS within their diagnosis, assert- medical expert’s differential diagnostic have caused her injuries and 2) a differ- ing not only that the patient had RADS, assessment, the court arrived at a con- ential diagnosis to rule out other pos- but providing a causal attribution. “RADS clusion with reasoning that reflects both sible causes. due to toluene exposure,” was the offered confusion about the meaning of differ- Id. diagnosis. The court found, incorrectly, 28 For The Defense that the determination of cause was part a physician begins by “ruling in” all sci- ples indicating that talc was found in of that “differential diagnostic” activity. entifically plausible causes of the plain- Westberry’s sinuses, nor did he have The opinions of Dr. Hansen and Mr. tiff’s injury. The physician then “rules studies showing that talc, at any thresh- Wabeke, and indeed the theory of out” the least plausible causes of injury old level, causes sinus disease. Instead, RADS itself, are based upon a technique until the most likely cause remains. The Dr. Isenhower merely relied on differen- called differential diagnosis. “Differen- final result of a differential diagnosis is tial diagnosis—supported in part by the tial diagnosis, or differential etiology, is the expert’s conclusion that a defendant’s temporal relationship between West- a standard scientific technique of iden- product caused (or did not cause) the berry’s exposure to talc and the prob- tifying the cause of a medical problem plaintiff’s injury. [Emphasis added.] lems he experienced with his sinuses…. by eliminating the likely causes until 252 F.3d at 989. GGAB maintains that neither a differen- the most probable one is isolated.” While the subsequent discussion delin- tial diagnosis nor a temporal relation- Id. at 892. (Aff’d, 295 F.3d 856 (8th Cir. eated the elements of causation assess- ship between exposure and onset or 2002).) ment, it is clear from that discussion and worsening of symptoms is sufficient to “Differential diagnosis” is not equiv- from the above statement that the court establish the reliability of Dr. Isenhow- alent to “differential etiology.” “Etiology” failed to understand the definitional dis- er’s opinion. We disagree. speaks more to the ultimate cause, that is, tinction between differential diagnosis and 178 F.3d at 262. the cause of the disease process. Diagno- causation assessment. The suggestion that Here, the court noted some of the neces- sis speaks to the disease process: what is differential diagnosis has anything to do sary elements of causation when it repeated wrong with the patient. Thus, in this case, with a product is illustrative. Diagnoses do the defense’s arguments. While rejecting the differential diagnosis would establish not involve products; they involve disor- those, it accepted the identification of a the existence of RADS (Reactive Airways ders or diseases. Here the differential diag- temporal relationship as a component of Dysfunction Syndrome). This diagnosis is noses and the ultimate diagnosis which “differential diagnosis.” This should rarely the disorder that the patient has, not what arose from that differential diagnostic pro- be the case, except in acute poisoning mat- caused it, and is the end of the differential cess were clear: intracranial hemorrhage. ters. Rather, temporal relationships are a diagnostic analysis. That diagnosis was the end of the differen- regular component of “causation assess- Etiology, or cause of the RADS, is the tial diagnostic exercise. The next question ment.” Finally, the court ruled that differ- next question. This involves a new pro- “What caused the ICH?” begins the new ential diagnosis is the proper methodology cess necessitating answers to other types analytical exercise: the causation analysis. for assessing causation. In fact, assessing of questions: whether the agent at issue is The Fourth Circuit case, Westberry v. causation requires knowledge of those ele- capable of producing RADS, i.e., whether Gislaved Gummi AB, 178 F.3d 257 (4th Cir. ments enumerated in the defense’s argu- it has irritant properties; approximately 1999), is frequently cited to support the ments and, sometimes, others as well. what level of exposure occurred (dose); use of differential diagnosis as the meth- In Cutlip v. Norfolk Southern, 2003 WL what was the timing between symptom odologically proper way of assessing cau- 1861015, 2003 Ohio App LEXIS 1785 (Ohio onset and exposure (clinical course); were sation. In the underlying case, the claim App. April 11, 2003), the claimant’s med- there alternate potential causes of the was that the inhalation of talc from work- ical experts testified that the claimant’s RADS considered and ruled out? The place exposure caused a sinus disorder in respiratory disorders arose from exposure evaluation of these critical elements is not the claimant. A verdict was delivered for to diesel fumes in his worksite. In doing part of diagnosis, but is central to causa- the claimant. The appeal argued, among so, they used “differential diagnosis” as tion analysis methodology. other things, that the claimant’s physician’s their methodological argument, which was In Glastetter v. Novartis Pharmaceuti- methodology in arriving at a causal con- accepted by the trial court. The appellate cals, the trial court dismissed the claimant’s clusion was defective. The appeals court court agreed, stating the following: expert testimony that the drug Parlodel disagreed with this argument saying: In this case both physicians testified caused the claimant’s intracranial hemor- GGAB contends that Dr. Isenhower’s tes- that they personally examined appellee, rhage (ICH). 252 F.3d 986 (8th Cir. 2001). timony was inadmissible because it was they reviewed his medical records, they Affirming that decision, the appellate court not based on reliable scientific method- took a history, they ordered tests and discussed, properly, certain elements of ology. This is so, it argues, because Dr. they reviewed the results of those tests. causation assessment, but they incorpo- Isenhower had no epidemiological stud- They both also considered other pos- rated, improperly, cause into the concept of ies, no peer-reviewed published studies, sible causes (such as the chest wound a differential diagnosis. They said: no animal studies and laboratory data and smoking) and ruled those out as Each of Glastetter’s experts conducted to support a conclusion that the inha- possible causes of appellee’s asthma. a “differential diagnosis,” which con- lation of talc caused Westberry’s sinus Based on the law discussed above, we cluded that Parlodel caused her ICH. disease. Further, GGAB continues, Dr. conclude that Drs. Khan and Kelly In performing a differential diagnosis, Isenhower did not have any tissue sam- arrived at their conclusions following July 2005 29 a thorough differential diagnosis, and causation, it not only failed to recog- ticing physicians. In clinical medicine, their testimony is therefore reliable nize that Dr. Linz applied a method of “differential diagnosis” describes “the under Daubert and Evid R. 702. differential diagnosis in reaching his process of determining which of several Cutlip at *7. conclusion, it seemed actually to reject diseases is causing the patient’s symp- The first sentence of the above state- this method…. toms.” Reference Guide on Medical Tes- ment correctly delineates the process of Id. at 261. timony, supra, at 443. Expert witnesses differential diagnosis. The second sen- The statement by Dr. Linz noted above and courts, however, frequently use the tence, including ruling out other causes and cited by the appellate court actually term “to describe the process by which of the disease, does not. Once again, the incorporated both methodologies: differ- causes of the patient’s condition are court intermingled “differential diagno- ential diagnosis and causation assessment. identified, particularly causes external to sis” with “causation assessment.” The end of the differential diagnostic the patient.” Id. At 443–44. On the whole, The Hardyman case involved a claim- the plaintiff’s counsel and their experts ant with carpal tunnel syndrome (CTS), Once again, the court have used the term the latter way. the alleged cause of which was a specific Id. at *24 n.224. set of occupational activities. In a Sixth intermingled “differential The court went on to reject this lat- Circuit decision overturning the trial ter use of the term in which differential court’s exclusion of the claimant’s expert, diagnosis” with “causation diagnosis and causation assessment were there was extensive discussion of both employed interchangeably. differential diagnosis and of cause. At the assessment.” trial level, the claimant’s expert himself Conclusion clearly separated the elements of each, but exercise was the diagnosis (carpal tunnel Courts’ failures to understand the impor- the appellate court merged them. Dr. Linz, syndrome). Whether or not that diagnosis tant differences between differential diag- the claimant’s medical expert, said: was accurate could be a matter of dispute nosis and causation assessment are largely [O]nce we go beyond the issue of gen- between medical experts, but the method based upon confusion about, and mis- eral causation and get away from evalu- of arriving at that diagnosis, history, phys- construction of, the meaning of the word ating and criticizing the literature whose ical examination, laboratory testing, is “cause.” In case after case, the courts allow only purpose really is to establish how not. In contrast to the misunderstand- physicians to testify to “cause” using a dif- well-established this work is [sic] cau- ing of the appellate court, however, the ferential diagnostic approach, noting that sation in general, then we get to specific diagnosis—and only the diagnosis—was this approach is the standard methodol- causation. And the approach to specific the outcome of the differential diagnosis. ogy of clinical medicine. The error is in causation is really quite difficult. We are All of the other components (which the failing to ask: cause of what? If, by “cause,” looking at: 1) whether or not the person claimant’s expert acknowledged as diffi- the courts were referring to the cause of had carpal tunnel syndrome. It is abso- cult) were elements of causation analy- the patient’s symptoms or internal dis- lutely clear in this case that he did; and sis. This included the general causation order, i.e., the diagnosis, they would be 2) does he have documented medical question: Can such activities cause this correct. Uncovering that through a differ- conditions which might be significant disorder, CTS? The causation analysis ential diagnostic approach is the standard contributors to his condition. That was also included the specific causation ques- methodology of clinical medicine. But evaluated by me and I felt that he did tions: Have other causes been considered in the context of a tort claim, there are not. The third is whether his work activ- and ruled out? Was the work activity suf- at least two causes: the internal cause or ities are significantly more hand inten- ficient? Was it more significantly intensive diagnosis, and the external cause, the sive than his nonwork activities, and in than non-work-related activities? subject of the liability claim. It is the fail- addition are his work activities signifi- Finally, in a recent decision, the U.S. ure to make this critical distinction that cantly more hand intensive than other District Court for the Southern District has characterized often flawed decisions. jobs [sic]…. of New York, enunciated the difference In some cases there may even be a third Hardyman v. Norfolk & Western Railway, between the customary medical use of the causal question—whose chemical, e.g., 243 F.3d 255, 262 (6th Cir. 2000). term “differential diagnosis” and the inter- which asbestos or silica or lead. The appellate court misconstrued this pretation of that by the courts (In re Rezu- Differential diagnosis asks: what is correct analytical approach as a “differen- lin Products Liability Litigation, 2005 WL wrong with the patient? Causation asks: tial diagnosis” when it commented upon 583751, MDL No. 1348, Master file 00 Civ. how and why did the condition arise? For that testimony: 2843 (LAK) (S.D.N.Y March 14, 2005). In counsel to challenge appropriately, physi- Although the district court acknowl- this multidistrict decision, the court said: cians’ ill-founded causal attributions, they edged that differential diagnosis is an Courts have come to use the term “differ- must understand the difference and be acceptable method of determining ential diagnosis” differently from prac- able to explain that to the court. 30 For The Defense