Incomitant Strabismus/Module 1: Clinical Investigations of Incomitant Strabismus/Part 3: Parks 3 Steps & Bielschowsky Head Tilt Test

Background
When the head is tilted to either side of the vertical meridian, the eyes undergo physiological cycloduction to compensate. Otherwise, images would be seen as being tilted. The compensatory movements are such that one eye excycloducts (I have a preference for this than the term extorts) while the other incycloducts, eg. when the head is tilted to the left, the RE must excycloduct and the LE must incycloduct.

For the RE to excycloduct, the RIO and RIR must contract (remember that only inferior people extort!)

For the LE to incycloduct, the LSO and LSR must contract.

NB: The opposing vertical (and horizontal) actions of these muscles to maintain orthophoria while the person tilts their head. So, the RIO and RIR both excycloduct, however, one is an elevator the other a depressor…

'Step 3' of the Parks 3 Step Method
Remember that by step 3, you have eliminated 6 cyclovertically-acting muscles, leaving you with one oblique and its contralateral antagonist vertical rectus muscle, eg. LIO and RIR, or RSO and LSR.

Head tilt testing is the only way to determine which muscle is palsied.

An Example
Step 1: R/L (RIR-, RSO-, LIO- or LSR-)

This is self explanatory

Step 2: R/L increases on R gaze (RIR- or LIO-)

The problem obviously involves a muscle which belongs to the 'R gaze family of cyclovertically-acting muscles', so RIR or LIO (the other two being RSR and LSO, but they were excluded after step 1. NB. that these muscles belong to the same 'sequelae')

Step 3: R/L increases on L tilt

How do you interpret this finding?

Both the RIR and LIO excycloduct, one excycloducts the RE the other the LE, so you must produce excycloduction of each eye in turn: R tilt will force the LE to excycloduct (evaluating LIO function), L tilt will force the RE to excycloduct (evaluating RIR function).

In a clinical situation (or in a clinical exam!) just ask yourself:

Where is the problem? …On L tilt.

Which eye excycloducts on L tilt? …The RE.

Which of the suspected muscles belong to the RE? …The RIR.

Therefore, the RIR is the palsied muscle.

(NB. that the LIO has no influence on L tilt, yet this is the problem side. So, the LIO is not involved.)

It's that simple!

The rationale is as follows. On L tilt, the RE must excycloduct (the LE must incycloduct, but neither of the suspected muscles are incycloductors, so the palsy is not in the LE - LIO is automatically excluded). R excycloduction will be produced by RIO and RIR. You know that RIO is intact and is an elevator. The R/L (or R hyper) increases because the RIO is acting unopposed since the RIR is palsied.

NB. that the difference in the vertical deviation on tilting would be less in a vertical rectus palsy than in an oblique palsy because the vertical action of an unopposed oblique would be considerably less than that of an unopposed vertical rectus muscle.