The diagnosis of oculomotor palsy is sometimes difficult. Deciding which muscle or muscles are affected can be very complex, even for an experienced ophthalmologist or neurologist. It is usually based on tests, either objective or subjective, which either cannot be accurately parameterised or are so complex to measure correctly that they are only available to an ocular motility specialist (prisms).

HESS AND HES-LANCASTER TEST

Already since the end of the 19th century there have been different methods to explore and document ocular motor pathology. Hirsberg in 1874 devised a method in which the diplopic images perceived by the patient on a tangent screen were fused by means of prisms. The method was very complex and did not completely dissociate both eyes, which invalidated the procedure. Other authors such as Ohm in 1907, Krusius in 1908... designed methods that did not become popular. It was Hess in 1908 who laid the foundations for the modern examination of oculomotor palsies. His method was based on the following principles:

1. Complete dissociation is used when wearing red/green glasses and one red and one green token. So one eye can only perceive the red token and the other eye can only perceive the green token.
2. It is a macular-macular test, so the deviation is direct and its representation is as if each fovea is looking directly at the screen at the point that the patient deviates. This contrasts with other tests that use a maculo-paramacular principle and in which what actually exists is confusion.
3. The lines representing the scanning points follow a hyperbolic tangent pattern, which is closer to reality.
4. It is a large screen that allows the scanning distance to be greater than 35 cm, thus avoiding accommodative and convergence influences.
5. It allows the scan to be recorded and therefore stored on paper for later consultation or comparison. A partir de ese momento y siguiendo sus principios han aparecido diferentes modificaciones. Las más importantes han sido la de Lancaster y la de Lee.

With the advent of personal computers, different systems have been implemented that follow Hess' original exploration principle. These systems have been programmed in different languages and their execution environment is the Windows operating system. They consist of a computer that controls the scanning and a monitor, more or less large, on which the test is presented.

All the methods used suffer from rigidity in the customisation of the scan, so that the essential parameters of the test cannot be altered.  The size, colour or shape of the cores cannot be changed, nor can the size, colour or shape of the screen. Similarly, the torsional element cannot be scanned in most cases.

The systems used so far lack flexibility and adaptability to the particular needs of the scanner. The latter is anchored to the unmodifiable characteristics provided by the designer. No variations can be introduced, except those that do not change the scanning parameters.

FIELD OF DIPLOPIA TEST

In addition to the Hess method and its variations, the study of the diplopia field can be used for the study and monitoring of oculomotor palsies. The usual method has been to use a Goldmann perimeter. It is a completely different test from the Hess method in both its rationale and instrumentation:

1. Using the principle of diplopia, the patient indicates when the witness is seen single or double.
2. A red glass is used over one of the eyes, thus improving the discrimination of diplopia.
3. The screen used is a hemisphere with an extension field of 180° at a scanning distance of 30 cm.
4. It is a kinetic test, the witness moves from the periphery of the screen towards its centre following predetermined axes.
5. The patient must follow the baton.
6. The points at which the diplopia begins are noted.
7. A graph can be drawn showing the single binocular field of vision or the field of diplopia.

It is currently impossible to obtain a Goldmann perimeter, so only with a computer model could such an examination be carried out.

The study of oculomotor paralysis involves two medical subspecialties: ophthalmology and neurology. Usually, the patient goes to the ophthalmologist for diplopia and the latter, on finding oculo-motor paralysis, refers the patient to the neurologist. It is the neurologist who finally carries out the relevant studies to reach an aetiological diagnosis. Sometimes it is the neurologist who, when faced with diplopia or a suspicion of paralysis, sends the patient to the ophthalmologist for confirmation.

Thus, the technical study of oculomotor paralysis has fallen to the ophthalmologist. It is in the ophthalmology department, and specifically in the motility department, that the necessary tests for instrumental diagnosis can be found. It is unlikely that a Hess screen will be available in a neurology surgery... and this is becoming increasingly common in ophthalmology surgeries as well. This is due to the impossibility of obtaining either a classic test or one based on a computer system.

The system designed is intended to make up for these shortcomings.