Identify the paretic extraocular muscle in acquired vertical diplopia.
STEP 1
— Select Eye —
Right Eye Hypertropia (Right Hyper)
Left Eye Hypertropia (Left Hyper)
STEP 2
— Select Gaze Direction —
Worse in Right Gaze
Worse in Left Gaze
STEP 3
— Select Head Tilt —
Worse on Right Head Tilt
Worse on Left Head Tilt
Please make a selection for all three steps.
Diagnosis Result
Understanding the Parks 3-Step Test
The Parks 3-Step Test (also known as the Parks-Bielschowsky Three-Step Test) is the gold standard clinical protocol used by ophthalmologists and optometrists to isolate the paretic cyclovertical muscle in patients presenting with acquired vertical diplopia (double vision).
This calculator automates the elimination logic required to identify which of the eight cyclovertical muscles (Superior Rectus, Inferior Rectus, Superior Oblique, or Inferior Oblique in either eye) is underacting.
The Anatomy Behind the Calculation
The test relies on the specific primary actions of the extraocular muscles:
Rectus Muscles: The Superior and Inferior Recti generally act as elevators and depressors, primarily when the eye is abducted (looking out).
Oblique Muscles: The Superior and Inferior Obliques act as depressors and elevators, primarily when the eye is adducted (looking in).
Intorsion/Extorsion: Head tilting stimulates the vestibular system, causing the eyes to rotate (intort or extort) to maintain a level horizon. This reflex is used in Step 3 to isolate the muscle based on its torsional action.
How to Perform the 3 Steps
To use this calculator effectively, ensure your clinical exam follows these steps strictly:
1. Primary Gaze Alignment
Determine which eye is higher (hypertropic) while the patient looks straight ahead. Note that a Right Hypertropia is functionally equivalent to a Left Hypotropia, but the test is conventionally documented via the hypertropic eye.
2. Lateral Gaze Assessment
Have the patient look to the left and then to the right. Observe in which direction the vertical deviation (the separation between images or the physical height difference of the eyes) increases. This isolates the muscles responsible for vertical movement in that specific horizontal field.
3. Bielschowsky Head Tilt
Have the patient tilt their head toward the right shoulder, then the left shoulder. The deviation will typically worsen significantly on the tilt that stimulates the paretic muscle to perform its torsional function (intorsion or extorsion) which it can no longer do effectively.
Clinical Example
Right Superior Oblique (RSO) Palsy: This is the most common isolated cranial nerve IV palsy. The clinical signs would be:
Step 1: Right Eye Hypertropia (Right eye is higher).
Step 2: Worse in Left Gaze (RSO acts as a depressor in adduction).
Step 3: Worse on Right Head Tilt (RSO is an intorter; tilting right requires right eye intorsion).
function calculateMuscle() {
var step1 = document.getElementById('primaryGaze').value;
var step2 = document.getElementById('gazeDirection').value;
var step3 = document.getElementById('headTilt').value;
var resultBox = document.getElementById('resultBox');
var resultText = document.getElementById('muscleResult');
var detailText = document.getElementById('muscleDetail');
var errorMsg = document.getElementById('errorMsg');
// Reset display
resultBox.style.display = 'none';
errorMsg.style.display = 'none';
// Validation
if (step1 === "" || step2 === "" || step3 === "") {
errorMsg.style.display = 'block';
return;
}
var diagnosis = "";
var details = "";
// Logic for Parks 3-Step
// RH = Right Hyper, LH = Left Hyper
// WG = Worse Gaze, WT = Worse Tilt
if (step1 === "right") { // Right Hyper
// Candidates: R Depressors (RIR, RSO) or L Elevators (LSR, LIO)
if (step2 === "left") { // Worse Gaze Left
// Muscles acting vertically in left gaze: R Obliques (RSO, RIO), L Recti (LSR, LIR)
// Overlap: RSO, LSR
if (step3 === "right") { // Worse Tilt Right
diagnosis = "Right Superior Oblique (RSO)";
details = "Classic sign of Cranial Nerve IV Palsy on the right side.";
} else { // Worse Tilt Left
diagnosis = "Left Superior Rectus (LSR)";
details = "Often associated with Cranial Nerve III issues on the left side.";
}
} else { // Worse Gaze Right
// Muscles acting vertically in right gaze: R Recti (RSR, RIR), L Obliques (LSO, LIO)
// Overlap: RIR, LIO
if (step3 === "right") { // Worse Tilt Right
diagnosis = "Left Inferior Oblique (LIO)";
details = "Primary overaction of the IO or paresis.";
} else { // Worse Tilt Left
diagnosis = "Right Inferior Rectus (RIR)";
details = "Could indicate trauma, muscle entrapment, or CN III palsy.";
}
}
} else { // Left Hyper
// Candidates: L Depressors (LIR, LSO) or R Elevators (RSR, RIO)
if (step2 === "right") { // Worse Gaze Right
// Muscles acting vertically in right gaze: R Recti (RSR, RIR), L Obliques (LSO, LIO)
// Overlap: LSO, RSR
if (step3 === "left") { // Worse Tilt Left
diagnosis = "Left Superior Oblique (LSO)";
details = "Classic sign of Cranial Nerve IV Palsy on the left side.";
} else { // Worse Tilt Right
diagnosis = "Right Superior Rectus (RSR)";
details = "Often associated with Cranial Nerve III issues on the right side.";
}
} else { // Worse Gaze Left
// Muscles acting vertically in left gaze: R Obliques (RSO, RIO), L Recti (LSR, LIR)
// Overlap: LIR, RIO
if (step3 === "left") { // Worse Tilt Left
diagnosis = "Right Inferior Oblique (RIO)";
details = "Primary overaction of the IO or paresis.";
} else { // Worse Tilt Right
diagnosis = "Left Inferior Rectus (LIR)";
details = "Could indicate trauma, muscle entrapment, or CN III palsy.";
}
}
}
// Display Result
resultText.innerText = diagnosis;
detailText.innerText = details;
resultBox.style.display = 'block';
}