Convergence insufficiency is the inability to obtain WW1 for any length of time without undue effort. It is the most common cause of ocular asthenopic symptoms.
Etiology of Convergence Insufficiency
1. Primary or idiopathic:
In many cases, exact aetiology of convergence insufficiency is not known. It may be associated with a wide IPD and delayed or inadequate functional development. General debility, psychological instability, overwork and worry may be the precipitating factors.
2. Refractive errors:
Convergence insufficiency may be associated with uncorrected high hyper metropia and myopia. Diseases of accommodative convergence mechanism result in convergence insufficiency in such patients as follows:
- High hypermetropes (more than 5 D) usually make no effort to accommodate and thus there is deficient accommodative convergence as well.
- Myopes may not need accommodation and thus lack accommodative convergence.
- Patients who have worn too full a plus spherical correction may also exert less accommodation and thus less accommodative convergence.
With advent of presbyopia, the near point of eye recedes and so there is less use of con vergence. Neglect of presbyopia may lead to fixation of this anomaly.
On the other hand, patients may also develop convergence insufficiency with the first-time use of presbyopic correction. This has been explained by the fact that the relief of sustained accommodatie effort afforded by the use of presbyopic correctio causes a decrease of accommodative convergence.
4. Muscular imbalances:
Extraocular muscular imbalances in the form of exophoria, intermittent exotropia and vertical muscle imbalances, if neglected for a long time, may be associated with convergence insufficiency.
5. Consecutive convergence insufficiency
It may occur following either recession of medial rectu or resection of lateral rectu muscles.
Clinical features of Convergence Insufficiency
Convergence insufficiency becomes a clinical problem in children with increased school work, prolonged periods of reading, desk workers and precision workers. It is usually not a problem in farm- and manual-labour workers.
Symptoms of convergence insufficiency are similar to that of heterophoria, and in general the term asthenopia is used to denote the symptom complex. Unsuitability of the glasses is the most frequent complaint of patients using glasses and having asthenopic symptoms. Such patients change their refractionist and glasses frequently without any satisfaction. Asthenopic symptoms may be grouped as under:
1. Symptoms of muscular fatique
These result due to continuous use of the neuromuscular power and are usually marked with near work. These include:
- Eyestrain and a sensation of tension in and around the globes is a common complaint of such patients.
- Headache and eyeache after prolonged use of eyes especially for near work, which are relieved when the eyes are closed for a while. Some patients may show even migrainous tendencies.
- Difficulty in changing the focus from distant to near objects.
- Itching, burning and soreness of eyes and even hyperaemia of the nasal half of the conjunctiva may occur after prolonged close work.
2. Symptoms due to failure to maintain binocular vision.
- Blurred near vision and crowding of words while reading.
- Intermittent crossed diplopia for near vision under conditions of fatigue is not uncommon.
- Characteristically, one eye will be closed or cov ered while reading to obtain relief from visual fatigue.
Diagnosis of Convergence Insufficiency
Diagnosis of convergence insufficiency is confirmed by the following:
1. Remote NPC: Convergence insufficiency is said to exist, if NPC is more than 10 cm from the base line.
2. Decreased fusional convergence for near: When measured on synoptophore, the convergence insuf ficiency is said to exist, if there is difficulty in attain ing 30° of convergence.
3. Prism convergence: is low but prism divergence is normal.
4. Exophoria: at near with orthophoria at distance may occur. However, convergence insufficiency may be associated with orthophoria and even exophoria.
5. NPA is normal and corresponds to the age of the patient. However, measurement of NPA is essential in each case to diagnose and manage patients suffer ing from a combined insufficiency of convergence and accommodation. Further, rarely accommoda tive spasm may occur, if voluntary accommoda tion and convergence are stimulated in an effort to overcome the convergence insufficiency.
Convergence insufficiency needs to be differentiated from the following conditions presenting with almost similar symptoms
1. Convergence insufficiency versus convergence paralysis
Convergence insufficiency needs to be differentiated from the following conditions presenting with almost similar symptoms.
- In convergence paralysis, there is total lack of ability to overcome any amount of base-out prism, whereas in convergence insufficiency several dioptres of convergence amplitude can be demonstrated.
- On receiving a convergence impulse, a patient with convergence paralysis will show pupil lary constriction but inability to converge; in a patient with convergence insufficiency, pupil lary constriction will occur while converging on an approaching target, followed by dilation of the pupil when convergence can no longer be maintained.
2. Convergence insufficiency versus accommodative effort syndrome
- Usually an exophoria at near is associated with convergence insufficiency, while patients with accommodative effort syndrome have esophoria.
- Convergence insufficiency is helped by the -3 D test, which compensates for the lack of good fusional convergence, whereas a patient with accommodative effort syndrome breaks into a tropia during this test.
- Plus lenses will worsen the convergence insuf ficiency due to relaxation of accommodative convergence, while they will improve the symp toms in the accommodative effort syndrome for the same reason.
Treatment of Convergence Insufficiency
Convergence insufficiency has an excellent prog nosis in the majority of cases. Children are treated when fusional vergences are poor and the patient is showing signs of becoming exotropic. Adults with this condition receive treatment only in the presence of symptoms. Treatment of convergence insufficiency includes optical treatment, orthoptic treatment, prismotherapy and surgery.
1. Optical treatment
Proper refraction should be carried out and the cor rect glasses should be prescribed for any associated refractive error. Myopes are given full correction and hypermetropes undercorrection to stimulate their accommodation which will simultaneously stimulate convergence. In adults older than 40 years, proper presbyopic correction should also be done.
2. Orthoptic treatment
Aim of orthoptic exercises is to improve the binocular convergence and to increase the amplitude of fusional convergence.
When all the exhaustive orthoptic exercises fail, then prismotherapy may be tried to relieve symptoms.
- Base-in prism reading glasses or bifocals with prism in the lower segment are useful as reliev ing prisms.
- Relieving prisms and bifocals in young age shouldbe avoided.
4. Surgical treatment
As a last resort, when all other measures fail, especially when convergence insufficiency is associated with a large exophoria at near vision, medial rectus muscle resection can be performed in one or both eyes. In some cases, exophoria at near fixation tends to recur.