THE TWO most important factors of which a plaintiff must have knowledge before the limitation period can commence (see s 14 of the Limitation Act 1980) are the knowledge that he or she has suffered a significant injury (which means one worth suing for if liability is admitted) and knowledge that the injury was attributable in whole or in part to the act or omission which is alleged to constitute negligence.The 'act or omission which is alleged to constitute negligence' must clearly be understood to mean 'the act or omission which is alleged to constitute negligence in the particulars or statement of claim'.

But how specific an interpretation should be given to the words 'the act or omission alleged to be negligent'? And what if there are a number of different acts or omissions alleged to be negligent?In a number of cases at first instance it had been held (prior to the Court of Appeal decision in Nash v Eli Lilly [1993] 1 WLR 782, the Opren judgment), that for the limitation period to start running in the context of the medical injury the patient must know not merely in a generalised way that he or she had been caused injury by the treatment he or she had received but, far more specifically, must know th at the injury was reasonably capable of being attributed to the precise act or omission that constituted his or her allegation of negligence.

It can immediately be appreciated that in many cases the patient will not know that until receiving an expert report.In the case of Khan v Ainslie [1993] 4 Med LR 319 the defendant optician had precipitated acute angle glaucoma by the administration of mydriatic fluid.

For years the plaintiff had continued to believe, despite expert advice to the contrary, that it had been negligent to administer the fluid; it was only when he later received a further expert report that he learned that his real complaint (the complaint that his claim was later based on) should be about the failure to recognise the glaucoma and refer him to hospital immediately.

The judge held that the plaintiff only knew his injury was attributable to that act or omission when he received the later report.It would obviously be improper to overdo the specificity requirement in a case where multiple allegations are made.

For example, in a drug case one of the many allegations often made is that the defendants failed to pay heed to certain research work.

It would be unreasonable to contend that the plaintiff needed to know that the defendants had failed to pay heed to that research work before the limitation period could begin.

How hard it will be to isolate the relevant 'act or omission' must depend on the facts of the particular case and the form of the allegations made in the statement of claim.In Nash v Eli Lilly the principle of specificity was accepted by the court (ie for plaintiffs to be held to have the relevant knowledge so as to trigger the limitation period they must know that they had suffered a significant injury which was capable of being attributed to the (specific) act or omission alleged in the pleading to constitute negligence).

However, the court, reasonably enough, was not prepared to accept that the plaintiff had to know the precise terms in which it would be alleged that the negligence lay: 'What is required is knowledge of the essence of the act or omission to which the injury is attributable.'Applying this test, the court rejected the submission of plaintiffs' counsel that the judge had been wrong to hold that the relevant 'act or omission' of which the plaintiff had to have knowledge was 'providing for the use of patients a drug which was unsafe in that it was capable of causing persistent photosensitivity in those patients and/or in failing to discover that such was the case so as properly to protect such patients'.Counsel submitted unsuccessfully that a further degree of specificity was required, namely that the plaintiff had to have knowledge that the marketing of an unsafe drug was due to lack of care in testing it or in informing the medical profession about it or in responding to adverse drug reaction reports.The nearest the court came to defining what is meant by 'knowledge' was: 'Knowledge is a condition of mind which imports a degree of certainty and the degree of certainty appropriate for this purpose is that which, for the particular plaintiff, may reasonably be regarded as sufficient to justify embarking upon the preliminaries to the making of a claim for compensation such as the taking of legal or other advice.'Whether or not a state of mind for this purpose is properly to be treated by the court as knowledge seems to us to depend, in the first place, upon the nature of the information which the plaintiff has received, the extent to which he pays attention to the information as affecting him, and his capacity to understand it.

There is a second stage at which the information, when received and understood, is evaluated.

It may be rejected as unbelievable.

It may be regarded as unreliable or uncertain.

The court must assess the intelligence of the plaintiff; consider and assess his assertions as to how he regarded such information as he had; and determine whether he had knowledge of the facts by reason of his understanding of the information.'It is important to note that the court envisaged the possibility that a particular plaintiff might have been taking the view that, although he or she had received information on which knowledge could be based, he or she needed expert confirmation before his or her belief could attain that degree of firmness to amount to knowledge.If, however, 'the plaintiff held a firm belief which was of sufficient certainty to justify the taking of the preliminary steps for proceedings by obtaining advice about making a claim for compensation, then such belief is knowledge and the limitation period would begin to run'.

It is also to be noted that the court said that once knowledge had been gained it could not then be lost, eg as a result of reassurances from doctors.The court said that it had difficulty in perceiving how in any case where a claimant had sought advice and taken proceedings it could rightly be held that he or she had not then had the relevant knowledge.

This is reasonable, provided that merely consulting solicitors is not deemed to be due to the relevant knowledge having been gained.

That would clearly be unjust, because many a patient asks for legal assistance without having any real idea about the attributability of any injury that he or she may have suffered.In Broadley v Guy Clapham and Co [1993] 4 Med LR 328, 39-year-old Maureen Broadley underwent an operation on her knee on 13 August 1980, which was promptly followed by foot-drop.

She consulted a solicitor in June 1983, and saw an independent orthopaedic surgeon on 6 July 1983, who told her that the operation might have been negligently performed.

The solicitor never obtained an expert report or issued a writ.Nothing was done about suing the solicitor until the writ in the present action was issued on 17 August 1990, alleging that the plaintiff had had a good claim for negligence in the original operation in that the left lateral popliteal nerve had been negligently divided and/or damaged.

The solicitor-defendant pleaded limitation in respect of the claim against him, a plea that would succeed if the plaintiff's cause of action against him had accrued more than six years before the issue of the writ, ie before 18 August 1984.The solicitor's negligence lay in not serving a writ within the limitation period for the medical negligence action.

If that limitation period had expired before 18 August 1984 the cause of action against the solicitor would have accrued more than six years before the issue of the writ.

So the vital question, tried at first instance as a preliminary issue, was: did the plaintiff have the relevant knowledge in respect of her medical negligence claim before 19 August 1981?At the trial the judge found that the patient had had constructive knowledge before August 1981 in that she must have known before then that she had suffered a significant injury as a result of the operation and should and could have reasonably discovered the cause by seeking appropriate expert advice.In the Court of Appeal, Balcombe LJ distinguished (as suggested by counsel) four categories of knowledge, namel y 'broad', 'specific', 'qualitative' and 'detailed', of which the last two were not necessary to constitute 'knowledge' for the purposes of limitation.

He indicated that 'broad' knowledge was not enough in itself, ie knowledge that the operation had been carried out in such a way that something went wrong and resulted in foot-drop.

The plaintiff needed also to have 'specific' knowledge, namely that the operation had been carried out in such a way as to damage a nerve and so cause the injury of foot-drop.

'Qualitative' knowledge was not necessary, ie knowledge that the operation had been carried out 'unreasonably', nor, of course, was 'detailed' knowledge, ie sufficiently detailed to enable the plaintiff's advisers to draft the claim.

In conclusion, Balcombe LJ agreed with the trial judge that the plaintiff had, constructively, the necessary specific knowledge by 19 August 1981.Hoffman LJ said that the plaintiff's counsel was in effect arguing that she needed to know that the relevant act or omission involved fault.

It is not surprising that, viewed in that light, the submission failed.

The judge said that s.14 required one to look at the way the case was put, distil what the plaintiff was complaining about and ask whether she had in broad terms knowledge of the facts on which that complaint was based.

He said that all the facts pleaded in the allegation of medical negligence were known, actually or constructively, to Mrs Broadley shortly after the operation.

The only factor that was not known was that what had been done was negligent, and that of course was not material to the issue of limitation.Leggatt LJ also endorsed the conclusion of the trial judge, saying that the plaintiff knew soon after the operation that something was wrong with her foot which was not an inevitable consequence of the operation and one would expect her to take advice and therefore to discover that the nerve must have been damaged in the operation in a way that was not inevitable.

He said that it was not necessary that the plaintiff have knowledge of the mechanics of damage.

Therefore, the plaintiff had the relevant knowledge by a date more than six plus three years before the issue of her writ against the solicitor and so her claim against the solicitor was brought out of time.In my view the judgment of the Court of Appeal is authority for the following propositions.

First, when a patient knows (actually or constructively) that he or she has been injured by an operation in a way that was not inevitable and knows in broad terms (ie in essence) how it arose, even though he or she does not know the precise mechanics, the limitation period will start.Secondly, the patient should seek advice once he or she has reason to suppose that medical treatment has resulted in an injury which was not a normal hazard, for otherwise he or she runs the risk of being held to have acquired constructive knowledge (ie of how the accident happened in broad terms) far earlier than the time he or she acquired actual knowledge.Thirdly, these propositions will apply, mutatis mutandis, to begin the ordinary personal injury claim.

The limitation period will begin when the plaintiff knows, or should by reasonable inquiry have discovered, that he or she has suffered a significant injury and knows also, or should reasonably have discovered, what act or omission in broad terms (ie in essence) has caused it.Four further cases on limitation have recently come to light, all of which will be reported in due course in the Medical Law Reports.Buckley J gave judgment in Colegrove v Smyth on 12 March 1992.

The claim was for damages for negligent delay in diagnosing congenital displacement of the hip.

It was alleged that the condition should have been diagnosed two years earlier than it was, ie at the age of 12 months instead of three years, and that the delay was responsible for the painful and debilitated condition that the plaintiff had endured ever since.

The unusual feature of the case was that the plaintiff was born in 1959.In essence, the view of the judge was that there was no reason for the plaintiff to have suspected that she had suffered injury, ie impairment of her condition, through any delay in diagnosis until, applying for a job in 1984, the examining doctor suggested as much to her.

As far as she knew until then, she had been born with a disability that had been treated as well as could be expected.

This, as far as it goes, seems clearly to be correct in principle.

The judge also stated that what was said by the examining doctor would not have given the plaintiff the necessary knowledge but would have put her on enquiry.

Her knowledge would not be complete until confirmed by an expert report (as pointed out above, the Court of Appeal's judgment in Nash v Eli Lilly envisaged the need for expert confirmation in appropriate cases).What is perhaps more difficult to understand is the treatment by the judge of the evidence from the plaintiff that she remembered that when she was eight a doctor had said much the same thing to her and, it appears, she had understood what he was saying.

The judge concluded that this in itself was sufficient neither to constitute knowledge nor to put the plaintiff on enquiry so that when she reached maturity she would be obliged to investigate the situation.

He avoided the force of this evidence by finding that the words of the doctor in 1967 amounted to 'no more or less than that there had been a delay in treatment.

There is no suggestion there that that was attributable to any act or omission'.

I do not understand the distinction.The judge gave a second reason for 'discounting' this evidence, namely that information heard at the age of eight recalled only 17 years later under pertinent questioning from her solicitor did not require him to conclude that in the intervening period the plaintiff had been in possession of that information.Knowledge, once acquired, cannot be lost, as was said in the Nash case, but that was not said in relation to minors.

It may well be that the judge's conclusions can be supported as a matter of policy or even as a matter of common sense, in that one does not like to attribute 'knowledge' to a child at such a tender age, ie to hold that what was heard at the age of eight constitutes knowledge still possessed at the age of 18.Finally, it is worth noting that the judge said he would in any event have exercised discretion under s.33 of the Act in favour of the plaintiff.In Baig v NE Thames Regional Health Authority (unreported, 10 June 1994) Rougier J said that in order to constitute actual knowledge of 'the act or omission' a patient who had undergone an operation had to know of something that was done or was not done to which the injury was attributable; it was not enough merely to know that he had emerged injured from an operation (though such an event would probably require that he start enquiries at that stage).The case of Fletcher v Sheffield Health Authority, CA, 21 April 1994 raised the point as to whether the limitation issue should be tried as a preliminary issue or whether it was so closely bound up with the evidence that would be led on the issues of liability and causation that it was better for both matters to proceed to trial together.

The Court of Appeal said that the judge had clearly been right in seeing too great an overlap between the limitation issues and the substantive issues to sensibly allow a preliminary hearing.Space does not permit an analysis of the facts of the case.

Suffice it to say that negligence in and around the birth of the plaintiff in 1959 and negligence in and around a series of orthopaedic operations in 1975/76 formed the basis of the claim.

The lesson to be learned from the case is that it is not always the better course to go for a preliminary hearing on limitation, but only when the issues are relatively discrete.The most important of these three Court of Appeal cases is Dobbie v Medway Health Authority, CA, 11 May 1994.

The facts can be briefly summarised.

In 1973 the patient consented to a left lumpectomy.

On awaking from the anaesthetic, she found to her horror that her whole breast had been removed, as the surgeon had reckoned, without carrying out any tests, that the lump was cancerous.

To add insult to injury, she was told a little later that histology had showed he was wrong.

Small wonder that she suffered psychiatric distress as well as the loss of her breast.

However, she did nothing about the matter until 1988.Otton J decided in February 1992 that she had the relevant knowledge when she learned the tissue was benign and he declined to exercise his discretion in her favour.

The Court of Appeal unanimously agreed with him on both issues.The decision is understandable.

The patient knew in 1973 that her breast had been removed unnecessarily and without consent.A claim for assault would, consequent upon the decision of the House of Lords in Stubbings v Webb [1993] 2 WLR 120, be subject to a limitation period of six years, so that was out of time.

The claim for medical negligence in removing the breast without testing the lump for malignancy was held to be out of time on the ground that the plaintiff had the relevant knowledge when she knew the tissue had been found to be benign.In all three Court of Appeal cases (Nash, Broadley, and this one) the court accepted that the plaintiff had to know the essence of the act or omission alleged to be negligent to which her injury was reasonably capable of being attributed.

However, it is not entirely clear what each member of the court in Dobbie was identifying as the relevant act or omission to which the patient knew her injury was attributable.Beldam J is the clearest on this.

He said it was 'the act of the surgeon in removing the breast and the omission to carry out a test before doing so which would have indicated that the removal was unnecessary'.

This seems clearly to be right.

What, however, is not clear is whether the patient knew soon after the operation that pre-operative investigations could be part of the procedure and that such a test would probably have shown there was no need for a mastectomy.

Without evidence that she had that knowledge it is hard to see how the court could be satisfied that the patient had knowledge that she had suffered a significant injury that was capable of being attributed to the act or omission alleged to constitute negligence.Whether or not one agrees that the plaintiff had actual knowledge soon after the operation, it seems reasonable to require of her that she should have started enquiries at that time, seeing that she knew she had suffered an injury as a result of the surgeon's actions (she would not have known this, of course, if she had not been told that the tissue was benign because she would have gone on believing that the mastectomy had been a benefit to her and not an injury).

So, on any view, she would and should have gained the requisite knowledge within a fairly short time thereafter.