Examination Under Anaesthesia

For Medical Students

What It Tests

An examination under anaesthesia tests the same things as an awake examination with the muscles fully relaxed.

That removes the single biggest limitation of every test in this chapter, which is the patient's ability to resist the manoeuvre.

It is a different instrument rather than a more careful version of the same one, and findings obtained under anaesthesia are not comparable with findings obtained awake. 4.1.8 covers recording that distinction.

How It Is Performed

Perform it after induction and before any incision, as a deliberate sequence rather than an afterthought.

Examine both knees, because the comparison is as important here as anywhere and the unaffected knee is now also fully relaxed.

Perform the Lachman, the anterior and posterior drawer, varus and valgus stress at two flexion angles, the dial test at two angles, and the pivot shift. 4.13.5 covers the dial test.

Record each finding with its grade, its endpoint and its flexion angle, and record explicitly that the examination was under anaesthesia.

Record it before the arthroscopy, because once the joint has been entered the examination has changed.

What a Positive Means

A pivot shift that was absent awake and is present under anaesthesia is the commonest and most useful finding, and it is the main reason to perform the examination. 4.10.6 covers the test.

Increased laxity in a plane that was not suspected changes the operation, which is the whole point of doing it before the incision.

Varus or external rotation laxity found under anaesthesia raises a lateral or posterolateral injury that the awake examination missed. 4.13.4 covers the corner.

A finding that matches the awake examination confirms it and is still worth recording, because it establishes the pre-operative state.

Everything found here is more reliable than the awake equivalent and less comparable with the follow-up, which is the trade-off.

MRCS Level

How It Is Performed

In a viva the expected points are the timing, the sequence, both knees, the recording, and the statement that it precedes any incision.

The candidate should say which test benefits most, which is the pivot shift, and why, which is that guarding abolishes it awake.

Say that an examination under anaesthesia at the time of any knee procedure is a free opportunity to record a pre-operative state, and that it is frequently not taken.

Say that findings must be labelled as under anaesthesia, because an unlabelled finding will be compared with awake findings later and will mislead.

What a Positive Means

The reasoning expected is that every laxity test depends on the patient permitting the motion, and anaesthesia removes that variable entirely.

So the laxity measured under anaesthesia is closer to the structural laxity of the joint and further from the functional laxity the patient experiences.

That makes it better for surgical planning and worse for predicting symptoms, which is an important distinction and the reverse of the history's properties. 4.10.1 covers the history.

A knee that is grossly lax under anaesthesia and functionally stable awake is not a contradiction: it is the difference between structure and function, and 3.17.1 covers why that distinction runs through the whole field.

How Good It Is

This is the most informative laxity assessment available and it is the most reliable examination in this chapter by a clear margin.

The pivot shift's sensitivity rises substantially under anaesthesia, which is the single clearest demonstration anywhere in Section 4 that a test's performance depends on the conditions rather than the technique.

Its cost is an anaesthetic, so it is never a screening test and is always a planned part of a procedure.

Its findings are not comparable with awake findings, which limits its use for follow-up.

No figures appear, under decision 023.

FRCS Level

How Good It Is

At decision level the question is when the answer is worth an anaesthetic, and the answer is when it would change the operation.

A patient whose rotational state would determine whether a lateral procedure is added is such a case. 3.17.5 covers that balance.

A patient in whom a multiligament injury is suspected is another, because the pattern of laxity determines the staging and the sequence. 4.23.6 covers the assessment.

A revision assessment is a third, because the pre-operative state was frequently never documented and the current state determines the plan. 4.10.12 covers it.

A patient whose management will not change on the result does not need one, and performing it anyway is an anaesthetic without an indication.

Where a knee is being operated on for any reason, the examination is free and should be recorded regardless, which is a different proposition from arranging an anaesthetic for it.

Where It Misleads

The commonest error is not performing it when a knee is already being anaesthetised, which discards a free and more reliable examination.

The second is recording the findings without labelling them as under anaesthesia, which makes them misleading at follow-up.

The third is performing it after the arthroscopy, when the joint has been distended and entered.

And treating a gross laxity found under anaesthesia as an indication in a patient who is functionally stable, which confuses structure with function.

Fellowship Level

Where It Misleads

At this level the examination under anaesthesia is frequently the assessment that resolves a referred patient, and the reason is that it is the only reliable rotational assessment available. 4.10.9 covers the gap it fills.

In a revision assessment it establishes what the current laxity actually is, which the awake examination in a guarding, previously operated knee often cannot.

In a multiligament knee it establishes the pattern, which determines both the staging and which structures are reconstructed in which order.

In a hypermobile patient it is the examination most likely to overstate the problem, because their baseline is lax and anaesthesia removes the muscular control they rely on. 4.23.3 covers the assessment.

The discipline that matters most is recording it fully, because a fellowship clinic's examination under anaesthesia is frequently the best record a knee will ever have and it is wasted if it is summarised as grade two.

No figure here supports these claims, under decision 023.