head when the clinician performs the NPC.’²⁶ Mann’⁹ feels that these responses are indicative of neurotic tendencies. The author feel that they are, rather, indications of the amount of effort being. exerted to force convergence of the eyes. For example two individuals show the same break on an NPC. One has forced the system to its maximum, while the other has relaxed convergence as soon as there is associated discomfort. Although both findings are normal and identical, the clinical picture is different. The individual who utilizes excessive effort will probably demonstrate symptomatology.

D. Near Point Analysis

Many authors have attempted to relate phoria (demand) to posi

“Most convergence insufficiencies present themselves without a known systemic or psychological cause.”

tive relative convergence. Landolt²⁴ felt that only one-third of the total convergence could be used at 33 cm and, therefore, 54 p.d. of convergence should be on reserve for maximum expenditure. Tait’²⁷ said the reserves should equal twice the demand. Similar ratios have been reported by Neumueller’²⁸ and Hofstetter.²⁹

Despite the differences in numbers, all the authors agree that the reserves must be larger than the demand in order to avoid ocular fatigue. This method of analysis does not account for the amount of conscious effort used to overcome the introduction of BO prism which may radically affect reserves. Even though the analysis method incorporates both the phoria (demand) and BO fusional ranges (reserves), it does not provide an accurate means of predicting asthenopia since variables such as amount of time spent on near work, pain

threshold, and type of work are not considered.

E. Accommodation

Recently, accommodation has been implicated as a possible cause of CI. The Optometric Extension Program’s³⁰ method of analysis has theorized that accommodation is the cause of all non-classical CIs. (Classical CI includes low BO, receded NPC, exo at near, and low NRA.)

OEP has proposed that accommodation is a slower entity; that it mediates convergence; and that it is the last to develop phylogenetically. Therefore, it is more apt to break down under near point stress. Furthermore, they have noted that many CIs have been remediated solely with the application of plus lenses.

Accommodation, in a recent study by Prem Prakash et al³¹ showed reduction in 23% of the CIs. Von Noorden et al,³² Bugola’²² and Raskind³³ have also suggested that accommodation is the cause of CIs which do not respond to conventional convergence training. They advocate the prescription of plus lenses and B! prism.

The authors have noted that may CIs demonstrate reduced accommodative facility on the ± 2.00 D flipper test, low blues of accommodation. In addition, the treatment of CI with accommodative rock often results in improved BO to blur, BO to break, NPC and reduced exophoria at near. The latter is probably a result of a patient with a normal ACA ratio using the least amount of accommodation to maintain clear and single binocular vision. The patient places the proximal point of the depth of field at the plane of regard and, therefore, uses less accommodation. With training the patient is able to move his accommodation so that the center of the depth of field and the plane of regard coincide. Since there is a strong association between accommodation and convergence, convergence in- creases of the exophoria decreases.

There are also some CIs which

result from accommodative fatigue. These may be remediated with the application of plus lenses for near. The accommodative effort, before lenses are applied, is too great for the system to handle. Here again the patient has used the least amount of accommodation and convergence possible for single, clear binocular vision. However, since the individual is at his/her physiological limit, discomfort ensues. The addition of plus, therefore, reduces the accommodative load and makes it easier for the person to utilize the correct amount of accommodation to assist convergence. The person will usually report clearer and more comfortable binocular vision with the plus lenses.

Worth’³⁴ defined three grades of fusion: superimposition, flat fusion, and stereopsis. He felt that superimposition was the most rudimentary form, the easiest to demonstrate, and the last to disappear with the loss of binocularity. Stereopsis, on the other hand, was the most sophisticated form of binocular vision. Therefore, Worth felt stereopsis should be the first to disappear in the presence of binocular interference. His observations may have been correct for esotropia, but individuals with intermittent exotropia and normal binocular vision will show losses in grade 1 fusion (superimposition) before losses in stereopsis. for example people with ~ normal binocularity, have suppression represents a normal physiological process, eg. facultative suppression with a monocular microscope or a person’s insensitivity to physiological diplopia. Also, suppression tests require suppression of the less dominant image, i.e., if a black target is presented to the right eye and a white background to the left eye in a ste reoscope, the left eye must suppress the white background so that the black object is not seen in rivalry or differently than when monocularly viewed. In addition, retinal rivalry is a normal physiological phenomenon which requires alternate suppression because the brain cannot inte

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