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

Leber Hereditary Optic Neuropathy (LHON)

Definition (1 mark)

Genetics (3 marks)

• Caused by maternally inherited mitochondrial DNA (mtDNA) point mutations affecting respiratory chain complex I genes
• Three primary pathogenic mutations:

  Mutation	Gene	Frequency	Prognosis
  G11778A (most common)	MT-ND4	50–90%	Worst prognosis
  T14484C	MT-ND6	~15%	Best chance of spontaneous recovery
  G3460A	MT-ND1	~15%	Intermediate

• Mutations are usually homoplasmic (100% mutant mtDNA); heteroplasmic in ~15%
• All children of an affected mother inherit the mutation, but only ~10% of cases occur in females — incomplete penetrance with suspected nuclear modifier genes and X-linked susceptibility factors
• Environmental triggers (smoking, alcohol, nutritional deficiency) may precipitate visual loss in carriers

Epidemiology (1 mark)

• Typically affects males aged 15–35 years
• Atypical presentations in females or outside this age range (10–60 years) occur
• LHON should be considered in any bilateral optic neuropathy of unknown cause, regardless of age or sex

Clinical Features (4 marks)

Symptoms

• Acute or subacute, painless, severe central visual loss
• Initially unilateral in ~50%, with the fellow eye affected within weeks to months
• Bilateral simultaneous involvement may occur

Signs — Acute Stage

• Disc hyperaemia with blurring/obscuration of disc margins
• Telangiectatic microangiopathy — dilated, tortuous capillaries on disc surface extending onto peripapillary retina
• Pseudo-oedema — swelling of peripapillary nerve fibre layer (no true leakage)
• Dilatation and tortuosity of posterior pole vasculature
• Colour vision impairment
• Relative afferent pupillary defect (RAPD) — though surprisingly, pupillary light reactions may remain fairly brisk

Signs — Late/Chronic Stage

• Vessels and pseudo-oedema regress
• Severe optic atrophy supervenes
• Nerve fibre layer dropout most pronounced in the papillomacular bundle

Key clinical clue: Telangiectatic microangiopathy may be present in asymptomatic female carriers

Investigations (2 marks)

• Visual fields: Central or centrocaecal scotomas with preserved peripheral vision
• OCT: Peripapillary retinal nerve fibre layer (RNFL) thinning in affected eyes; unaffected carriers may show temporal RNFL thickening (compensatory mitochondrial accumulation)
• Fluorescein angiography (FA): Crucially, no leakage from disc or microangiopathic vessels (distinguishes pseudo-oedema from true disc oedema)
• Genetic testing: Testing for the three common pathogenic mutations — mandatory in any unexplained bilateral optic neuropathy

Prognosis (1 mark)

• Generally poor — most patients reach a final VA of 6/60 or worse
• Better prognosis if:
  • Age of onset < 20 years (especially < 10 years)
  • 14484 mutation (highest rate of spontaneous partial recovery)
• Some spontaneous visual recovery possible even years after onset
• 11778 mutation carries the worst prognosis

Treatment (3 marks)

Key Distinguishing Features (Summary) (1 mark)

Accommodative Esotropia

Definition and Pathophysiology (2 marks)

Classification (2 marks)

1. Refractive Accommodative Esotropia (3 marks)

• AC/A ratio is normal
• Caused by excessive hypermetropia (typically +2.00 to +7.00 D)
• The large degree of accommodation needed to focus on even a distant target triggers convergence beyond the fusional divergence amplitude
• Deviation is similar for distance and near (usually <10Δ difference)
• Typical age of onset: 18 months – 3 years (range: 6 months – 7 years)

Subtypes:

• Esotropia present when glasses are removed
• Deviation completely eliminated and BSV restored at all distances with full optical correction of hypermetropia
• The goal of management

• Deviation is reduced but not eliminated by full optical correction
• A non-accommodative (structural) component co-exists
• Amblyopia is frequent; bilateral congenital superior oblique weakness may be associated
• Suppression is common; ARC may occur (but lower grade than in microtropia)

2. Non-Refractive Accommodative Esotropia (2 marks)

Subtypes:

• High AC/A due to increased accommodative convergence (accommodation is normal; convergence is excessive)
• Normal near point of accommodation
• Esotropia greater at near than at distance (key distinguishing feature)
• Distance deviation may be minimal or absent

• High AC/A ratio due to reduced accommodation (convergence is normal; accommodation is deficient)
• Near point of accommodation is recessed
• Child must over-accommodate to see clearly at near → excessive convergence

Sensory Adaptations (1 mark)

• Suppression of the squinting eye is most common
• Amblyopia is frequent — must be actively screened for and treated
• Anomalous retinal correspondence (ARC) may develop but is of lower grade than in microtropia

Management (4 marks)

1. Amblyopia Treatment

• Must be addressed before any surgery
• Patching or penalisation of the dominant eye

2. Optical Correction (Spectacles)

• Full cycloplegic refraction under cycloplegia is mandatory — even in infants (irrespective of age, all children with esotropia should undergo cycloplegic refraction)
• For refractive type: Full correction of hypermetropia eliminates or reduces the deviation
• After age 8: manifest (non-cycloplegic) refraction; prescribe maximum tolerable "plus"

3. Bifocals

• Indicated in convergence excess esotropia — relieves near accommodation and thereby accommodative convergence, allowing bifoveal fixation at near
• Executive bifocal type preferred: the segment line crosses the lower border of the pupil
• The minimum "add" required is prescribed; gradually reduced and eliminated in early teens
• Also used in hypoaccommodative esotropia where AC/A is not grossly elevated

4. Surgery

• Considered only if spectacles do not fully correct the deviation and amblyopia has been maximally treated
• Bilateral medial rectus recessions — first choice when near deviation > distance deviation
• For equal distance/near deviation: bilateral MR recessions or unilateral recession/resection
• For residual amblyopia: surgery on the amblyopic eye
• Partially accommodative esotropia: surgery aims to correct only the residual squint present with glasses; best delayed until the child requests it (avoids early consecutive exotropia)
• Faden (posterior fixation suture) operation or pulley fixation of MR — used as primary procedure or after under-correction from bimedial recessions in convergence excess

Prognosis:

• Prognosis for spectacle withdrawal depends on magnitude of AC/A ratio and degree of hypermetropia/astigmatism
• Fully accommodative esotropia controlled by glasses → some patients achieve microtropia or true BSV over time

Summary Table (1 mark)

Accommodative Esotropia — 20 Marks

Definition and Pathophysiology (2 marks)

AC/A Ratio (1 mark)

• Normal AC/A ratio: ~3–5 Δ per dioptre of accommodation
• High AC/A ratio: convergence response is disproportionately large → near deviation > distance deviation
• Abnormalities of AC/A ratio are the central mechanism in accommodative esotropia

Classification (1 mark)

Type 1 — Refractive Accommodative Esotropia (4 marks)

• AC/A ratio is normal
• Caused by uncorrected hypermetropia, typically +2.00 to +7.00 D
• Large accommodation required even for distance fixation → proportionate convergence exceeds fusional reserve → manifest esotropia
• Deviation is similar for distance and near (difference usually < 10Δ)
• Age of onset: 18 months – 3 years (range: 6 months – 7 years)
• All children with esotropia must undergo cycloplegic refraction, irrespective of age

Subtypes:

• Esotropia present without spectacles
• Deviation completely eliminated with full optical correction of hypermetropia
• BSV restored at all distances with glasses
• Goal of optical management

Right esotropia without glasses (top); straight eyes with full correction (bottom)

• Deviation reduced but not eliminated by full optical correction
• A non-accommodative (structural) component co-exists
• Amblyopia is frequent; bilateral congenital superior oblique weakness may be associated
• Suppression common; ARC may occur (lower grade than in microtropia)

Type 2 — Non-Refractive Accommodative Esotropia (3 marks)

Subtype A — Convergence Excess (most common):

• High AC/A due to increased accommodative convergence (accommodation is normal; convergence is excessive)
• Normal near point of accommodation
• Esotropia significantly greater at near than at distance — hallmark feature
• Distance deviation may be minimal or absent

Subtype B — Hypoaccommodative Esotropia:

• High AC/A due to reduced accommodation (convergence is normal; accommodation is insufficient)
• Recessed near point of accommodation
• Child over-accommodates to see clearly at near → excessive convergence response

Sensory Adaptations (1 mark)

• Suppression of squinting eye — most common
• Amblyopia — frequent; must be screened and treated before surgery
• Anomalous retinal correspondence (ARC) — may develop; lower grade than in microtropia

Management (6 marks)

Step 1 — Treat Amblyopia

• Patching or penalisation of the fixing eye
• Must be maximally treated before any surgical intervention

Step 2 — Optical Correction

• Full cycloplegic refraction — prescribe full hypermetropic correction
• Under age 8: cycloplegic refraction; full "plus" prescribed
• Over age 8: manifest refraction; maximum tolerable "plus" prescribed
• Fully accommodative esotropia → glasses alone restore BSV
• Partially accommodative esotropia → glasses reduce but do not eliminate the deviation

• Bifocal spectacles — relieve near accommodation → reduce accommodative convergence → allow bifoveal fixation at near
• Executive bifocal preferred: segment line crosses the lower border of the pupil
• Prescribe the minimum "add" to achieve alignment at near
• Segment strength gradually reduced and eliminated by early teenage years
• Also used in hypoaccommodative esotropia if AC/A is not grossly elevated

Step 3 — Surgery

Prognosis (1 mark)

• Fully accommodative esotropia: excellent if glasses worn consistently; may eventually achieve microtropia or true BSV
• Partially accommodative: residual structural component usually requires surgery for cosmesis
• Prognosis for spectacle withdrawal depends on:
  • Magnitude of AC/A ratio
  • Degree of hypermetropia and astigmatism
• Some patients require glasses only for close work long-term

Summary Comparison Table (1 mark)