Hess Screen

Eye muscle imbalances can be best diagnosed by the Hess Screen the latest versions of which can be conducted by computer technology.

Typically, constant large-angle strabismus does not cause symptoms such as eye strain and headaches because there is virtually no attempt by the brain to straighten the eyes. Large-angle strabismus usually causes severe amblyopia in the turned eye if left untreated.

Whereas less noticeable cases of small-angle strabismus are more likely to cause disruptive visual symptoms, especially if the strabismus is intermittent or alternating.

In addition to headaches and eye strain, symptoms may include an inability to read comfortably, fatigue when reading and unstable or “jittery” vision. If small-angle strabismus is constant and unilateral, it can lead to significant amblyopia in the misaligned eye.

Ocular deviation may be manifest (tropia) or latent (phoria). Manifest ocular deviation can be present in all directions of gaze (comitant) or only present in specific directions of gaze (incomitant)

Concomitant deviations are relatively common. They are usually associated with hypermetropia or the anomalous placement of one or more of the extraocular muscles.

Incomitant deviations are usually caused by a functional anomaly of one or more of the extra-ocular muscles or their associated neurology. This may be a result of a defective muscle or mechanical interference in the orbit (myogenic), or a consequence of a lesion in the nerves supplying the musculature (neurogenic).

The presence of diplopia usually suggests that the incomitancy is of recent origin and indicates a disturbance to some component of the oculomotor system. This may have been caused by trauma or may indicate the presence of an intracranial tumour, aneurysm or haemorrhage.

A detailed analysis of the oculomotor fields can be obtained using the Hess screen (Thomson Hess Pro on PC).

Cover and Uncover tests: Eye movements by the subject patient are compared.

However, the Hess results of the plots for the left and right eyes, are the gold standard as they indicate which eye has a palsied muscle and provides an index for monitoring the progression of an incomitant deviation.

Motility is tested normally in nine positions of gaze for each eye but can be done in 25 positions of gaze. The grid can be specified in units of degrees or prism dioptres. A prism dioptre corresponds to a deviation of 1 cm at 1 metre, which is equivalent to 0.573 degrees.

Six eye muscles, controlling eye movement, are attached to the outside of each eye. In each eye, one muscle moves in the eye to the right, and one muscle moves the eye to the left. The other four muscles move it up or down and at an angle.

To line up and focus both eyes on a single target, all of the muscles in each eye must be balanced and working together. In order for the eyes to move together, the muscles in both eyes must be coordinated. The brain controls these muscles.

With normal vision, both eyes aim at the same spot. The brain then combines the two pictures into a single, three-dimensional image. This three-dimensional image gives us depth perception.

What are the symptoms of strabismus
How is vision affected?

Good vision develops during childhood when both eyes have normal alignment. Strabismus may cause reduced vision, or amblyopia, in the misaligned eye.

Strabismic amblyopia occurs in approximately half of the children who have strabismus.