Timothy C. Hain, MD Page last modified: June 23, 2016
THIS PAGE IS UNDER CONSTRUCTION -- IT SHOULD NOT BE RELIED UPON FOR ANY PURPOSE
This page is written to provide a background for the individual who is might have visual vertigo. It mainly discusses how the eyes work together, but also attempts to discuss interaction with the inner ear. Primarily the focus is explaining convergence insufficiency and accomodative insufficiency.
Convergence insufficiency (CI) means that an individual is unable to converge the eyes smoothly and effectively as an object of visual interest moves from distance to near and/or the ability to maintain the convergence near point.
Vergence requires the the eyes move in opposite directions, and is a type of disjunctive movements. Convergence involves moving the eyes inward, but considering disjunctive movements in general, there is also divergence as well as vertical and torsional vergence.
Convergence is part of the "near triad" which consists of turning both eyes inward (converging), increasing the focusing part of the eye (accomodation), and constriction of the pupil. The near triad is driven by circuits in the midbrain (largely 3rd nerve nucleus), and accomplished by activating the oculomotor nerves that turn the eyes inward (i.e. 3rd nerve drive to the medial recti mainly), as well as the ciliary muscle within the eye that make the lens thicker, and pupillary constrictors that make the pupil smaller.
Convergence may be voluntary (i.e. to the command "cross your eyes"), or reflex (i.e. generally associated with a demand to focus at near). According to Van Noorden, reflex convergence is subdivided into tonic convergence, accomodative convergence, fusional convergence, and proximal convergence.
- Tonic convergence is thought to be the baseline position of the eyes maintained in awake and conscious subjects. Tonic convergence is lost then in persons who are asleep. It is regulated by the cerebellum, and is modifiable with prisms.
- Accomodative convergence is convergence stimulated by a near target that requires a change in focus of the lens.
- Fusional convergence provides the extra convergence (or divergence) needed with both eyes viewing.
- Proximal convergence is convergence induced by awareness of nearness of an object.
Vergence is a slow system. While saccades may move the eyes 50 degrees in a quarter of a second, vergence in degrees may take a full second to accomplish a 6 degree change. Fusional movements may take 1-2 seconds to complete.
Accomodation is the change in focus ability of the ocular lens. The stimulus is blur, which then goes to the visual pathways, the midbrain nucleus, the third nerve, and finally the ciliary muscle. Accomodation insufficiency then is the inabilty of an individual to maintain focus at a variety of distances, but generally failing at near where the demand for accomodation is highest.
Convergence is generally measured at the bedside by providing a need for convergence such as a near target, and then alternatively covering the eyes, looking for a failure to completely converge manifesting as a refixation of the eye when the cover is switched. Of course this method does not include fusional convergence, but would include tonic and proximal convergence.
Units of convergence -- meter angle.
The theoretical unit of convergence is the meter angle -- the reciprocal of the fixation distance in meters required for each eye to fixate. Thus it is similar to a diopter. This choice of unit, rather than the more obvious degrees, was made because it is similar to the measurement of accomodation. For the same distance, the required convergence in meter-angles is the same as the required accomodation in diopters. There is a minor difference between the two because accomodation is ordinarly specified from the spectacle plane, and vergence from the center of rotation of the eyes (Van Noorden, 1980).
While the meter angle is the same for everyone, the actual amount of eye movement in an absolute unit such as degrees is different depending on the interpupillary distance.
The more practical unit of convergence is the prism diopter. A prism diopter is defined as a displacement of the visual axis by 1 cm at a distance of 1 m.
The angle needed to converge can be found by multiplying 1/d (the meter angle in meters) by the interocular distance in cm. Thus if a person converges at 1m, and has an interocular separation of 6.5 cm, the convergence needed is 6.5 diopters.
There are many methods of measuring convergence.
- Haploscopes are devices where a separate target is presented to each eye.
- Stereoscopes are simpler, and can include very simple devices such as red/green lenses combined with a method of projecting red and green targets. An excellent example fot his is the "Lancaster Red-Green" test.
- The cover-uncover test, combined with a prism, can be used to quantify convergence. Here one adds a prism until the eyes remain stable. The examinee views a fixation target. The accuracy of this test is about 3 diopters.
- Convergence break points can be measured by placing increasingly large amounts of prism until the patient sees double. This is the limit of the patients fusional amplitude in the direction being tested.
- Convergence break point can also be assessed subjectively by simply moving a fixation target until one of the eyes turns out. The distance that this occurs is the Near point of convergence (NPC). The usual NPC is about 8. In individuals with convergence insufficiency it may be large as 25 to 30 cm.
Normal individuals, at 6m distance, have about 14 diopters of convergence and about 6 diopters of divergence. At 25 cm, normal vergence is about 38.02 while there is about 16.5 diopters of divergence (table 13, Van. Noorden). In normal persons, the amplitude of vergence varies considerably, even if the eyes are working normally.
The near point of convergence is easily trained. It is said that the ability to converge on a near object is an easily learned trick and for this reason testing of the NPC is of little diagnostic value (Van Noorden).
Measuring accomodation -- units are diopters
The closely related measure of accomodation is also made in diopters - -which are the reciprocal of the fixation distance. Thus if the fixation distance is 1m, the accomodation is said to be 1D. The limiting distance on accomodation is called the "near point of accomodation". The range of accomodation is between the far point (usually infinity) to the near point.
Interactions between accomodation and convergence - -the AC/A ratio.
The "AC/A" ratio expresses the amount of convergence elicited by a unit of stimulus of accomodation. The AC/A ratio is not tightly regulated but rather tends to be different for individuals. Generally speaking the AC/A ratio is too low, leaving a need for convergence at near, which is fufilled by non-accomodative types of convergence. The AC/A ratio can be measured by several methods including:
- Heterophoria method -- measure deviation difference between distance and near (usually 33 cm). The AC/A ratio is then equal to PD + (Dn-Do)/D, where PD=interpupillary deviation in Cm, D is the fixation at near in diopters, and Dn, Do are deviations at near and distance. (Van Noorden, 1980)
- Clinical method -- a simple comparison of deviation in distance and near -- if the two measurements are equal, then the AC/A ratio is said to be normal.
- Gradient method -- various lenses are used to change the accomodative demand, and the phoria is measured. Here the effect of proximal convergence is lost, and thus the AC/A ratio is generally smaller than when computed through other methods.
- Fixation disparity method. Here a mixture or prisms and lenses are used. This test is complex and little used
- Haploscopic method. This method is well suited to measure AC/A, but requires special equipment.
Normally the AC/A ratio is about 2, but there is a large variability around this mean, and a substantial number of normal subjects are as low as 1.5 or as high as 4.0.
Ideally, all three of the near triad actions should occur in synchrony and end up matching each other. In other words, a given requirement to turn the eyes inward, should also be matched by a change in the focus ("f-stop") -- accomodation of the eye.
Patients with inadaquete ocular alignment may succeed in maintaining alignment using vergence, which can result in eye strain -- called asthenopia. Eye strain symptoms range from heaviness or soreness of the eyes, eye pain, frontal and occipital headaches, and sometimes an aversion to using their eyes to read or study. If they fail to maintain alignment, then they may develop double vision (diplopia). These symptoms are usually relieved by rest.
If there is uncorrected refractive error, patients may also develop eye strain due to inability to focus. This is called accomodative asthenopia. Patients do not develop double vision in this situation, but may exhibit similar eye strain. Interestingly, some persons are given grossly unequal refractive power in each eye, which is called "mono-vision". This creates a situation where the eyes must be used separately for focus.
Man has a built in mechanism to avoid asthenopia called suppression. By suppressing the image from one eye, asthenopic symptoms are generally diminished or abolished. Individuals with a well functioning sensory system and strong need to fuse do not readily suppress. On the other hand, people who had strabismus early in life are often very tolerant of ocular misalignment.
In theory, individuals might also have a mismatch between accomodation and vergence at a particular distance, causing a situation where the needs for focus might conflict with the needs for binocularity.
Curiously, authors during the early part of the 20th century attributed migraine headaches to small errors of refraction. Gould for example (1904), proposed that "Eyestrain produces almost an infinity of morbid results, and migraine, typical or not, is, if not absolutely, almost always, one of the products of the malfunction of astigmatic eyes". Snell supported Gould, suggesting that eyestrain caused the great majority of headaches, "often of a very aggravated character" (p. 55), and that a cylindrical correction (for astigmatism) of even 0.25 dioptres would give great benefit. (1904). The consensus as of 2016 is that minor refractive error is rarely a source of headache, nausea, vomiting or aura.
There is a important interaction between the vergence system and vestibular (inner ear balance) system. In order to keep the eyes aligned on a point of regard while the head is moving, the vestibular system senses head rotation and linear acceleration, and activates the eyes to counterrotate so as to keep gaze constant even though the head is moving. This is what enables us to see the tennis ball while moving our head. (Takimoto et al, 2016)
The problem becomes more difficult at near, because the eyes are not located at the center of rotation of the head, but rather are about 10 cm anterior to the axis of rotation. This means that when one is regarding a near target (such as 10 cm away), the amount of eye movement needed to keep the target fixated is much greater than the amount needed to view a similar object 100 cm away. This additional eye movement is supplied by the otoliths (linear acceleration sensors) that produce eye movement that are roughly inversely proportional to the distance of the target from the center of the eye.
Thus individuals with disorders of their otoliths, might reasonably have a selective problem with stabilizing their vision while there head is moving, at near -- think of reading your cell-phone in the back seat of your car.
As the method of computing the distance of the target likely may be related to the accomodative drive to the eyes, which are also using the distance to the point of regard as their input, there might reasonably be interactions between the fusional system having to do with blur and diplopia, and the vestibular system having to do with keeping the point of regard stable, while in motion.
CI is a common disorder at all ages.
AI is also extremely common, but largely related to advancing age.
Causes can be generally divided into those related to the eyes themselves, the brain circuitry, and the output circuits. There are also some causes that don't fit too well into any category.
When one eye is blind, there is no longer a need for convergence. Similarly, when one eye has substantially lower visual acuity, or when some arrangement has been made to alter the requirements for accomodation between eyes (e.g. one eye near, one eye far), then again convergence becomes a cosmetic activity rather than one that is relevant to visual acuity as fusion becomes difficult.
In internuclear ophthalmoplegia, the speed that the eyes move inward is slowed by damage to the MLF. This disturbs convergence, especially during saccades. Similarly, lesions of the midbrain which contains the 3rd nerve nucleus can be devastating for convergence, accomodation or both.
The cerebellum regulates convergence, and cerebellar damage is one of the few sources of grossly disjunctive eye movements. For example, one eye might be moving much more than another.
The cerebellum also regulates convergence tone. Divergence can be affected by the Chiari malformation.
Concussion of course is a diagnosis based on a train of events (basically head trauma and thinking disturbance), rather than a specific location of injury. Thus the location of injury in concussion is often unclear, so we have made it a category of its own.
CI is reported frequently after concussion . Master and associates reported "A total of 100 adolescents were examined, with a mean age of 14.5 years. Overall, 69% had one or more of the following vision diagnoses: accommodative disorders (51%), convergence insufficiency (49%), and saccadic dysfunction (29%). In all, 46% of patients had more than one vision diagnosis. "(Master et al, 2016) We think that this frequency is very high, especially saccadic dysfunction.
Pearce et al (2015) found that" CI was common (~42%) in athletes evaluated within 1 month after an SRC. Athletes with CI had worse neurocognitive impairment and higher symptom scores than did those with normal NPC."
Overall, it appears that the literature supports the idea that CI is common after concussion.
When an eye is unable to move due to restriction or paralysis, convergence may be weakened or impossible. Mainly this occurs after eye doctors do surgery in an attempt to straighten the eyes for cosmetic purposes. This also might occur from damage to one of the oculomotor nerves (i.e. 3rd nerve), damage to the eye muscles, paralyzing conditions of the eyes such as PEO, or entrapment of eye muscles from a "blow-out" fracture.
Methods of treatment largely include prisms (for CI), lenses (for AI), and training.
There are differences of opinion as to whether the AC/A ratio is learned or innate. Van Noorden (1980) appears to be undecided. He does note that it is generally thought that exercises do not change the AC/A ratio, and as well, lenses aimed at changing the refractive error do not change the AC/A ratio. On the other hand, cycloplegic drops do change the AC/A ratio,and cause it to become much larger while the drops are acting.