Essential Instruments To Detect Muscle Balance

Dr Vineet Shrivastava
B.E.M.S., D.O., (FOOREC)
FCLC (Gold Medalist)
Consultant Optometrist & Educator Banswara, Rajasthan

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Muscle balance gives a good and comfortable vision. Any disturbance in muscle balance can cause phoria (Latent squint) or tropia (Manifest squint) which causes cosmetic disfigurement of the face as well as significant disturbances in the visual pathway. This can make the patient experience a remarkably lesser vision along with some other disturbances like cosmetic disfigurement, doubling of vision (Diplopia), asthenopia, headache, nausea, vomiting, giddiness etc. To measure the amount of phoria or tropia, some special instruments are required in the respective hospital or clinic to help provide an exact and clear investigation data. These instruments may be clinical or therapeutic – clinical orthoptic instruments can be used to detect the amount of squint whereas therapeutic instruments are used to give exercises or therapy for enhancing the vision and cosmetic look. Read on to know about some of the specific instruments that are used to improve the condition of the patient.

Maddox Rod

This is a handheld clinical orthoptic instrument which consists of a group of red cylindrical rods that converts a white point source of light into a red line that is perpendicular to the axes of the Maddox Rods. The instrument is used to detect a degree of phoria for a distance. When the Maddox Rod is placed vertically over the right eye, the patient sees a horizontal red line and a white light. When the Maddox Rod is placed horizontally over the right eye, the patient sees a vertical red line and a white light. If the eyes are aligned, the red line will go through the white light. If the eyes are not aligned, the red light will deviate in the opposite direction of the eye which will be relative to the white light.

The test relies on colour dissociation using red/green complementary filters. This maximises the ocular deviation. A red target is illuminated or projected at the juncture where each tangent line crosses. A green light is projected by the patient and each plot is recorded. The test is repeated for the opposite eye resulting in a chart showing an inner and outer range of ocular rotation for each eye.

Lees Screen

This apparatus is also based on the principle of providing dissociation but does not utilise coloured filters for the purpose. This instrument is essentially a twin-screen where the two screens are placed rigidly and permanently at right angles to each other. The screen is used to obtain the same information about incomitant strabismus but by using a different method of dissociation. The mirror prevents both the eyes from viewing the same screen simultaneously, although the patient perceives both images seen by the fovea of each eye as if they are projected straight ahead.

Any disturbance in muscle balance can cause phoria (Latent squint) or tropia (Manifest squint) which causes cosmetic disfigurement of the face as well as significant disturbances in the visual pathway

Maddox Wing

This is a handheld clinical orthoptic instrument which makes use of mechanical dissociation of the fields of the two eyes. It is used to detect the amount of phoria for the nearer distance. Maddox Wing has a face piece, a diaphragm, a handle, and a chart. The fields of the two eyes are separated by the diaphragm. This is an effective method of detecting the presence of, and measuring the degree of, heterophoria. In this test, one eye sees a double tangent scale that is calibrated to read in prism dioptres while the other eye sees a white arrow pointing upward, and a red arrow pointing horizontally to the left. As the two retinal images are quite different, there is no attempt of a fusion point and the eyes stay in the passive position. The arrows that are seen by the left eye point to the numbers seen by the right eye. The numbers represent the vertical and horizontal components of the phoria, which can be read directly by the observer.


This is an instrument which is utilised for the measurement of subjective and objective angles of a squint. It stands on a heavy cast iron base with one movable foot adjustable by a screw. Gun metal columns are attached to this base which carries the illuminating system, slide holders and reflecting mirrors and lenses. The arrangement of the Synoptophore results in the rapid flashing of the light to stimulate the under-functioning macula. Synoptophore is an excellent instrument as it is adjustable for several measurements. Horizontal, vertical and rotator movements are controlled and measured in scales.

Worth’s Four Dot Test

This test is also known as Worth’s Four Light Test. This instrument consists of an illuminated box with four apertures. Coloured glasses are put before each aperture – one red, two green and one white, yellow or ground glass. This test assesses the patient’s flat fusion ability at a distance and a nearer point. This hand-held test is also used to detect a small unilateral central scotoma. The instrument is also used in the differential diagnosis of unilateral decreased visual acuity.

Hess Screen

The Hess Screen test was designed by Walter Rudolf in 190. This test uses a black screen on which a square-meter tangent scale is marked. The tangent nature of the coordinate lines converts equidistant points, seen in a virtual sphere like a perimeter, into a two-dimensional chart.


This instrument is used to detect amblyopia with eccentric fixation. The Visuoscope is a type modified ophthalmoscope that is provided with a disc which can be brought in front of the lighting so that a star can be focused on the retina. The patient is asked to fix the projected star.

Maddox Prism Verger

This is an excellent instrument which estimates the vergence power by rotating the prisms. Extraocular muscle function testing examines the function of the eye muscles. A health care provider observes the movement of the eyes in six specific directions. Some specific instruments are specially designed for eye muscle balance. These instruments are capable of providing the type, amount and angle of squint which certainly helps in diagnosing and managing the case.