If you have a personal injury claim that for whatever reason cannot be settled fairly with the insurer for the wrongdoer, and you and your attorney agree the case is worth pursuing further, then it will end up in “litigation”- a term for the entire process that occurs after a lawsuit is filed against the wrongdoer.
A key part of litigation is “discovery.” That's where the parties find out what evidence the other has through requests for documents, depositions of witnesses, and the like. There are a number of specific Washington court rules (“CR's”) governing discovery, including CR 35. In a nutshell, CR 35 provides that when the physical or mental condition of a party is “in controversy”, the court may order that the party submit to an evaluation by a doctor or psychologist, if there is good cause for the court to do so. We're discussing Washington's rule here, but it's worth noting that Oregon's ORCP 44 is quite similar.
I'll discuss this rule in the context of personal injury cases (although the rule applies to all types of civil cases). The party whose bodily or mental condition is in controversy is invariably the injured plaintiff. As a practical matter, although the court does not have to grant the defense's request for a CR 35 exam (recall the rule says “may”), it almost always will if the injured person is claiming symptoms from the injury event that are ongoing during the litigation. If the symptoms fully resolved, no exam would likely be granted, because there's not really anything an exam would reveal. In that case, the defense would likely have to settle for their hired doctor reviewing the injured party's medical records.
The rationale put forth for allowing the exam is that the injured plaintiff will usually have one or more doctors, arguably aligned with them, who are coming to testify at trial and support the relationship between what the wrongdoer did and the medical condition the plaintiff suffers from, and therefore it's only fair for the defense to be able to hire a doctor to examine the injured person, review the medical records, and come to their own conclusions, with an eye towards being a witness for the defense in the case.
As an aside, defendants and their insurers/lawyers will usually refer to these exams as “independent medical exams” or “IMEs.” To say that “independent” stretches the truth is to be kind. There is an entire industry of medical professionals who make handsome livings testifying for defendants in injury and disability cases. Their bread is buttered, so to speak, by the defense, and they are almost always going to be slanted severely in that direction. We call them DMEs- defense medical exams. It's a different, larger topic for a different day.
The CR 35 exam usually has to be performed by a doctor in the right specialty for the injury: a neurologist and/or neuropsychologist for a brain injury/concussion, an orthopedist for a knee or shoulder injury, a neurosurgeon for a herniated spinal disc, and so on.
Most of the time, because we know the court will grant a reasonable exam request by the defense (and possibly hit our client with money sanctions for frivolous opposition to a reasonable request in court), we can agree with the defense attorney on certain conditions of the examination, to make it as fair and comfortable for our clients as possible. If the conditions can't be agreed upon by the parties, the one seeking the exam can file a motion with the court, and the judge will decide whether to allow the exam and under what conditions it can be held. Our clients who must attend a CR 35 exam will always go to it well prepared by us to do their truthful best, and also (except in some exams requiring some proprietary psychological testing), accompanied by a representative from our office who will usually be able to observe and audio record the exam.