History, corneal reflexes and cover tests

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Clinical History & Abnormal Head Posture (Ocular Torticollis)

A meticulous history establishes the age of onset, constancy versus intermittency, diurnal variation (characteristic of ocular myasthenia gravis), and diplopia orientation (horizontal, vertical, or torsional; distance versus near).

The Three Spatial Components of Ocular Torticollis

Patients with incomitant strabismus, nystagmus, or cranial nerve palsies adopt an abnormal head posture (AHP) to align the visual axes, restore binocular single vision, avoid diplopia, or position the eyes at a nystagmus null point:

1. Face Turn (Horizontal Rotation): The head turns around the vertical Z-axis toward the field of action of the paretic muscle, effectively keeping the eyes in the opposite gaze field (e.g., a patient with a right lateral rectus palsy turns their face to the right, placing their eyes in levoversion where the paretic right lateral rectus is not required).
2. Head Tilt (Lateral Torsional Tilt): The head tilts around the sagittal Y-axis toward the right or left shoulder. In cyclovertical palsies, the head tilts to minimize demand on the paralyzed muscle. For example, in a right superior oblique (RSO) palsy, the RSO is a primary intorter. Tilting the head to the right shoulder stimulates the otolith-ocular counter-rolling reflex demanding right ocular intorsion; tilting the head to the left shoulder demands right extorsion (relaxing the RSO). Thus, patients with RSO palsy adopt a compensatory left head tilt.
3. Chin Elevation or Depression (Vertical Pitch): The head rotates around the transverse X-axis:
   • Chin Elevation (Up): Eyes are placed in relative depression. Used in elevator weakness, bilateral Brown syndrome, or 'V'-pattern esotropia.
   • Chin Depression (Down): Eyes are placed in relative elevation. Used in depressor weakness, depressor palsy, or 'A'-pattern esotropia.

Ocular versus Non-Ocular Torticollis

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Corneal Light Reflection Tests: Hirschberg, Krimsky & Bruckner

Corneal light reflection tests evaluate ocular alignment when patient cooperation, age, or visual acuity precludes formal cover testing:

1. The Hirschberg Test

The examiner shines a penlight at the patient's eyes from a distance of $33\text{ cm}$ while the patient fixates the light. The position of the corneal light reflection relative to the pupil center is noted:

• In normal alignment, the light reflection is centered or slightly nasal (physiological positive angle kappa).
• Hirschberg calibration is approximate and varies with ocular geometry; a commonly taught estimate is about 15–20 prism dioptres per millimetre. Do not equate 7° with 15–22 prism dioptres: the exact definition gives $100\tan 7^\circ\approx12.3\Delta$. Coarse landmarks are useful only as estimates:
  • Pupillary Margin (~2 mm decentration): $\approx 15^\circ \approx$ 30 to 35 prism dioptres.
  • Mid-Iris (~4 mm decentration): $\approx 30^\circ \approx$ 60 to 70 prism dioptres.
  • Corneal Limbus (~5 to 6 mm decentration): $45^\circ$ corresponds by definition to 100 prism dioptres; a limbal reflex position is only a rough estimate.
• Direction of Displacement:
  • Temporal displacement of reflection -> Esotropia.
  • Nasal displacement of reflection -> Exotropia.
  • Inferior displacement of reflection -> Hypertropia.
  • Superior displacement of reflection -> Hypotropia.

2. Prism neutralisation of corneal reflexes

Krimsky methods introduce prisms until the corneal reflex relationship is neutralised. Descriptions of “standard” and “modified” placement vary, so state explicitly whether the prism is before the fixating or deviating eye. Use base-out for esodeviation and base-in for exodeviation, check fixation and compare with the fellow reflex. These estimates are helpful when a reliable prism cover test cannot be obtained, but an abnormal angle kappa can bias them.

3. The Bruckner Test

The examiner views both pupils simultaneously through a direct ophthalmoscope set to plano ($0\text{ D}$) from a distance of $1\text{ meter}$ in a dim room. The color, brightness, and symmetry of the fundus red reflex in both pupils are compared:

• Normal Response: Symmetrical, equal, dull orange-red reflexes in both pupils.
• Abnormal Response: An asymmetrical, brighter, or whiter reflex identifies pathology. The deviating eye in strabismus reflects light off the less-pigmented extrafoveal retina, producing a brighter reflex than the foveal reflex of the fixating eye. An asymmetrical Bruckner reflex also flags unilateral high refractive error (anisometropia) or media opacities (leukocoria).

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Cover Testing Methodology: Tropia versus Phoria

Cover testing is the gold standard for detecting and quantifying ocular deviations. It requires an accommodative fixation target (e.g., a Snellen optotype $6/9$ at $6\text{ m}$ or a detailed picture at $33\text{ cm}$) to control accommodation, tested with and without refractive correction:

1. Unilateral Cover Test (Cover-Uncover Test)

• Detecting Manifest Strabismus (Tropia): The examiner covers the fixating eye while carefully watching the uncovered fellow eye:
  • If the uncovered eye makes a corrective saccade to take up fixation, a manifest heterotropia is present (inward nasal movement = exotropia; outward temporal movement = esotropia; downward movement = hypertropia).
  • If the uncovered eye remains stationary, it was already fixating. The cover is removed, binocularity is re-established, and the procedure is repeated covering the fellow eye.
• Detecting Latent Strabismus (Phoria): If neither eye moves when the fellow eye is occluded, the examiner observes the covered eye as the occluder is removed:
  • If the eye drifted under the cover and makes a refixation saccade upon uncovering to regain binocularity, a heterophoria is present.

2. Alternating Cover Test (Cross-Cover Test)

The occluder is moved back and forth between the two eyes, pausing for 2 to 3 seconds over each eye, without allowing the patient even a fraction of a second of binocular viewing. This completely breaks binocular fusion, revealing the total deviation (latent phoria plus manifest tropia).

3. Prism Cover Neutralization

• Simultaneous Prism Cover Test (SPCT): A prism is placed in front of the deviating eye at the exact millisecond the fixating eye is occluded. Quantifies the true manifest tropia before fusion is disrupted; vital in microtropia and intermittent exotropia.
• Alternating Prism Cover Test (APCT): Performed during cross-covering. Prisms are placed over one eye with the base oriented opposite to the deviation:
  • Base-Out (BO): Neutralizes esodeviations.
  • Base-In (BI): Neutralizes exodeviations.
  • Base-Down (BD): Neutralizes hypertropias.
  • Base-Up (BU): Neutralizes hypotropias.
  • Prism power is increased until all refixation movement ceases (the neutral endpoint), and confirmed by stepping 2 prism dioptres higher to observe a slight movement reversal. Measured at distance ($6\text{ m}$) and near ($33\text{ cm}$). Measurements in upgaze and downgaze identify 'A' and 'V' patterns (defined as a difference $\ge 10$ prism dioptres for 'A' patterns and $\ge 15$ prism dioptres for 'V' patterns between $25^\circ$ upgaze and $25^\circ$ downgaze).

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