Hess Chart (and Lees Screen)

Purpose

• Extraocular muscle palsy (3rd, 4th, or 6th nerve paresis)
• Mechanical or myopathic limitation (thyroid ophthalmopathy, blow-out fracture, myasthenia gravis)

The Hess Screen (Technique)

• The screen displays a tangent pattern on a dark-grey background with red lights at cardinal positions of gaze — a central field (15° from primary position) and a peripheral field (30°); each square = 5° of ocular rotation.
• The eyes are dissociated using red/green goggles: red lens over the fixating eye, green lens over the non-fixating eye.
• Red lights are illuminated one at a time; the patient superimpose a green pointer over each red light.
• The goggles are then reversed and the test repeated for the other eye.
• Software exists to plot Hess charts on a standard computer screen.

The Lees Screen (Alternative)

• Two opalescent glass screens at right angles, bisected by a two-sided plane mirror — this dissociates the eyes without coloured lenses.
• The examiner indicates a target on the illuminated screen; the patient places a pointer on the non-illuminated screen where they perceive the target to be.
• The patient is rotated 90° to repeat for the other eye.

Interpretation Rules

• The greatest restriction on the smaller chart falls in the direction of action of the paretic muscle.
• The greatest expansion on the larger chart falls in the direction of action of the yoke muscle.
• The angle of deviation is estimated from the disparity between the plotted point and the template (each square = 5°).

Changes with Time

1. Early (acute palsy): Underaction of the paretic muscle + overaction of its yoke muscle → charts are very incomitant and diagnosis is straightforward.
2. With secondary contracture: The ipsilateral antagonist develops secondary overaction → this causes inhibitional palsy of the antagonist's yoke, which appears as underaction — and may mimic a palsy of the contralateral muscle.
3. Late (longstanding): The two charts become progressively concomitant; it may be impossible to identify the originally paretic muscle.

Clinical Examples

• Left chart (paretic eye): much smaller; underaction of all muscles except LR
• Right chart (yoke eye): larger; overaction of all muscles except MR and IR
• Left exotropia visible: fixation spots deviated laterally; secondary deviation > primary deviation (hallmark of paretic squint)

• Right chart: underaction of right SO (depression in adduction)
• Left chart: overaction of left IO (yoke of right SO)

Hess Chart

Definition and Purpose (2 marks)

• Cranial nerve palsies (3rd, 4th, 6th)
• Thyroid eye disease
• Blow-out fracture of the orbit
• Myasthenia gravis

Principle (2 marks)

Equipment and Setup (2 marks)

• A tangent screen with a grid pattern; each square = 5° of ocular rotation
• Central field = 15° from primary position; peripheral field = 30°
• Eyes dissociated using red/green goggles: red lens over the fixating eye, green lens over the non-fixating eye
• Red lights are illuminated at cardinal positions; the patient uses a green pointer to superimpose over each red light
• Procedure is repeated with goggles reversed (other eye fixates)

Lees Screen (Alternative) (2 marks)

• Uses two opalescent glass screens at right angles, bisected by a two-sided plane mirror — no coloured lenses needed
• The examiner indicates a target on the illuminated screen; the patient places a pointer on the non-illuminated screen at the perceived matching position
• Patient is rotated 90° and the procedure repeated for the other eye
• Considered more accurate as it eliminates colour rivalry

Reading and Interpretation (4 marks)

• Right (smaller) chart: restricted laterally → underaction of right LR
• Left (larger) chart: expanded medially → overaction of left MR (yoke of right LR)
• Secondary deviation > primary deviation confirms a neurogenic/paretic cause

Changes with Time (4 marks)

• Underaction of paretic muscle; overaction of its yoke muscle
• Charts are markedly incomitant → diagnosis straightforward

• Ipsilateral antagonist (e.g. right IR in right SR palsy) develops secondary contracture and overaction
• This causes inhibitional (secondary) palsy of its yoke muscle (left SO)
• May be mistaken for a primary palsy of the contralateral muscle

• Both charts become progressively concomitant
• Original paretic muscle may no longer be identifiable
• Sequential charting over time is crucial to avoid misdiagnosis

Clinical Examples (2 marks)

• Right chart (smaller): restriction to the right
• Left chart (larger): expansion to the left

• Left chart (smaller): underaction of all muscles except LR
• Right chart (larger): overaction of all muscles except MR and IR
• Left exotropia: fixation spots deviated laterally in both charts

• Right chart: restriction in depression-adduction (field of right SO)
• Left chart: expansion in elevation-adduction (overaction of left IO, yoke of right SO)

Clinical Uses and Limitations (2 marks)

• Diagnosis of which muscle/nerve is affected
• Monitoring recovery or deterioration
• Timing of surgical intervention (charts should be stable for ≥6 months before surgery)
• Differentiating neurogenic, myopathic, and mechanical causes

• Requires the patient to have some degree of binocular vision — not valid in dense amblyopia or suppression
• Results must be interpreted alongside prism cover test and field of binocular single vision
• Long-standing palsies with spread of comitance can be difficult to interpret