Motility patterns, three-step testing and Hess charts

Motility Assessment & 9 Diagnostic Positions of Gaze

Ocular motility is systematically evaluated across the nine diagnostic positions of gaze:

• Primary Position: Straight ahead at optical infinity.
• Four Secondary Positions: Direct dextroversion (right), levoversion (left), sursumversion (elevation), and deorsumversion (depression).
• Four Tertiary Positions: Dextroelevation (up-right), dextrodepression (down-right), levoelevation (up-left), and levodepression (down-left).

The six cardinal positions of gaze isolate the action of a single yoke muscle pair in each direction. Underactions are graded clinically from $-1$ ($25\%$ limitation) to $-4$ (complete absence of movement beyond the midline). Overactions are graded from $+1$ to $+4$.

The Parks-Bielschowsky Three-Step Test

The Parks-Bielschowsky three-step test is an indispensable diagnostic algorithm designed to isolate a single paretic cyclovertical muscle (the vertical recti: RSR, RIR, LSR, LIR; or the obliques: RSO, RIO, LSO, LIO):

Step-by-Step Diagnostic Execution

1. Step 1: Which eye is hypertropic in primary position?
   • Identifies whether the paretic muscle is an abnormally weak depressor of the hypertropic eye or an abnormally weak elevator of the hypotropic eye (narrows possibilities to 4 muscles):
   • If Right Hypertropia (RHT): Paretic depressor of OD (RIR, RSO) or paretic elevator of OS (LSR, LIO).
   • If Left Hypertropia (LHT): Paretic depressor of OS (LIR, LSO) or paretic elevator of OD (RSR, RIO).
2. Step 2: Is the hypertropia greater in right gaze or left gaze?
   • In right gaze, the vertical recti of the right eye and the obliques of the left eye have maximum mechanical efficiency.
   • In left gaze, the obliques of the right eye and the vertical recti of the left eye have maximum mechanical efficiency.
   • Intersection with Step 1 narrows the differential to 2 muscles.
3. Step 3: Is the hypertropia greater on right head tilt or left head tilt (Bielschowsky Head Tilt Test)?
   • Evaluates the vestibulo-ocular counter-rolling reflex driven by the otolith organs:
   • Right Head Tilt: Demands intorsion of the right eye (agonists: RSO + RSR) and extorsion of the left eye (agonists: LIO + LIR).
   • Left Head Tilt: Demands intorsion of the left eye (agonists: LSO + LSR) and extorsion of the right eye (agonists: RIO + RIR).
   • Pinpoints the single paretic muscle.

Worked Example: Right Superior Oblique (CN IV) Palsy

• Step 1: Right hypertropia in primary gaze -> Possibilities: RSO, RIR, LSR, LIO.
• Step 2: Hypertropia worsens in left gaze (where RSO is a pure depressor) -> Narrows to RSO or LSR.
• Step 3: Hypertropia worsens markedly on right head tilt:
  • Tilting the head right demands right ocular intorsion. The intorters of the right eye are the RSO and RSR.
  • Because the RSO is paralyzed, the brain sends a massive intorsion command to the RSR.
  • The RSR is a powerful elevator as well as an intorter. In normal eyes, the depressive vector of the RSO neutralizes the elevating vector of the RSR.
  • In RSO palsy, the unopposed elevating action of the over-firing RSR drives the right eye into dramatic elevation, exacerbating the right hypertropia.
  • Pattern supporting the diagnosis: Right superior oblique weakness. Restriction, skew deviation and myasthenia can mimic this pattern; the three-step test is not definitive by itself.

Hess and Lees Screens: Record the Pattern, Then Explain It

The Hess screen uses colour dissociation, while the Lees screen uses mirror dissociation. Each records relative alignment in different directions of gaze. Adequate vision, fixation and comprehension are needed; a chart cannot replace examination of the patient.

In an uncomplicated recent unilateral paresis, the affected eye commonly has a smaller field, with greatest underaction in the weak muscle's field. The fellow eye may have a larger field because increased drive to the weak muscle also drives its yoke muscle. Longstanding contracture and adaptation can spread incomitance, making the chart less specific. Bilateral disease or poor fixation can defeat the simple smaller-field rule.

Mechanical restriction can also produce secondary yoke overaction. Abrupt field truncation may suggest a tether, but its shape and the size of the fellow field do not definitively distinguish a fracture from a nerve palsy. Use the history, ductions, saccades, forced ductions where appropriate and orbital imaging.

Forced duction requires appropriate anaesthesia, technique and consent. Avoid pressure or manipulation when an open globe is suspected. A child with a white-eyed trapdoor fracture may have little bruising despite serious tissue entrapment; an apparently mild external injury does not establish safety.

Use charts for documenting the baseline and change over time, explaining diplopia and planning prism or surgical treatment. A conclusion should connect the measured pattern to other evidence rather than label a chart appearance characteristic.