12.5 Exophthalmometry, Head & Posture Assessment and Wavefront Diagnostics

Exophthalmometry

Exophthalmometry measures the anteroposterior position of the corneal apex relative to the lateral orbital rim. It is the objective measurement behind every proptosis assessment, and thyroid eye disease is its commonest indication.

Hertel exophthalmometer technique:

1. Seat the patient at your eye level, facing you.
2. Place the instrument's footplates firmly on the lateral orbital rims, in the deepest part of the rim notch, with even pressure.
3. Read and record the base setting — the distance between the footplates. This number must be recorded every time, because a reading taken at a different base is not comparable.
4. Align so that in each mirror the zero line of the scale is superimposed on its own reflected image, which removes parallax error.
5. Ask the patient to look straight at your opposite eye.
6. Read the position of the corneal apex against the millimetre scale in each mirror.
7. Record as: "Hertel base 102: OD 19 mm, OS 24 mm."

Interpretation:

Causes of proptosis: thyroid eye disease (by far the commonest in adults, and the commonest cause of both unilateral and bilateral proptosis), orbital cellulitis, orbital tumours, lymphoma, vascular malformations, carotid-cavernous fistula, and in children rhabdomyosarcoma and orbital cellulitis.

Technique errors to avoid: varying the base setting between visits, pressing unevenly on the rims, allowing the patient to tilt the head, and parallax from failing to align the scale with its reflection. A Naugle exophthalmometer, which rests on the superior and inferior orbital rims instead, is used when the lateral rims are absent or surgically altered.

Head and posture assessment

The blueprint names "assess and record head and posture abnormalities" as a COMT-level task. An abnormal head posture is a clue, not a habit — patients adopt it because it improves vision, and forcing them out of it reveals the underlying problem.

Describe three components separately:

1. Face turn — to the right or left (rotation about the vertical axis)
2. Head tilt — toward the right or left shoulder (rotation about the anteroposterior axis)
3. Chin elevation or depression — up or down (rotation about the horizontal axis)

The head tilt rule for superior oblique palsy. The superior oblique intorts the eye. When it is weak, the eye extorts, producing torsional diplopia. Tilting the head away from the affected side removes the need for intorsion on that eye and relieves the diplopia — which is also why the Bielschowsky head tilt test shows the hyperdeviation increasing on tilt toward the affected side. A patient with a right superior oblique palsy therefore tilts left.

Documentation: record the posture in all three components, whether it is constant or intermittent, whether it disappears with occlusion, and photograph it where the clinic protocol allows. A childhood photograph showing the same posture is strong evidence of a long-standing, likely congenital, cause.

Wavefront diagnostics

Wavefront aberrometry measures how a wavefront of light emerging from the eye deviates from a perfect plane. Where a phoropter describes the eye with two numbers plus an axis, aberrometry describes it with a whole series of terms.

Lower-order aberrations — defocus (sphere) and astigmatism (cylinder) — account for roughly 85 to 90% of the total aberration in a typical eye and are correctable with spectacles or contact lenses.

Higher-order aberrations are everything else, and they cannot be corrected with a sphero-cylinder:

Measurement principles:

• Hartmann-Shack (the most common): a lenslet array samples the outgoing wavefront and the displacement of each spot from its ideal position gives the local slope.
• Tscherning and ray tracing systems project a grid into the eye and analyse the retinal image.

Results are reported as Zernike coefficients and summarised as root-mean-square (RMS) error in micrometres. The detailed Zernike framework is covered in the clinical optics section of this guide.

Pupil size is everything. Higher-order aberrations increase steeply with pupil diameter, so a measurement at a 6 mm pupil is not comparable with one at 4 mm. Always record the pupil diameter at which the measurement was taken. This is also why patients complain of glare and haloes at night, when the pupil dilates, while seeing perfectly in daylight.

Clinical uses:

1. Refractive surgery planning — wavefront-guided and wavefront-optimised ablation profiles.
2. Explaining unhappy 20/20 patients — the post-surgical patient with excellent chart acuity and disabling night symptoms usually has elevated higher-order aberrations.
3. Keratoconus — elevated vertical coma is characteristic, and aberrometry supports topography in diagnosis.
4. Intraocular lens selection — aspheric lenses are designed to offset the cornea's positive spherical aberration.
5. Specialty contact lens design — wavefront-guided scleral lenses for highly aberrated corneas.

Limitations: results depend on accommodation being relaxed, on a stable tear film (a dry surface produces spuriously large aberrations that improve after a blink), on adequate pupil size, and on fixation. Repeat measurements after lubrication and a full blink before accepting an outlier.