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This patient presents with classic signs and symptoms of convergence insufficiency (CI). CI is a common symptomatic binocular vision anomaly seen in optometric practice. The definition of CI is an 'inability to obtain or maintain sufficient convergence for comfortable binocular vision'.1 A normal near point of convergence is considered in the range of 6-8cm2 (Figure 1).
The prevalence can vary depending on the population examined. In a study by Rouse et al3 they found 3.8 per cent (24 out of 620) of randomly selected children aged 8-12 within an optometry school clinic were identified with clinically significant CI. A Spanish study found a prevalence of 0.8 per cent (21 out of 265) in a group of symptomatic patients.4
The most likely cause of this patient's CI is the change in her visual demand since starting her new first job, which involves significant close focusing and convergence. In this case, there is insignificant refractive error to correct and one would suspect her lowered amplitudes of accommodation are secondary to the CI. This should be carefully monitored as her treatment for CI takes place.
Management
Convergence insufficiency can usually be managed successfully in optometric practice. The condition should be explained and agreement and co-operation from the patient or parents must also be obtained. The first objective to consider is to find out the cause of decompensation and remove it if possible. In this case, a discussion of visual hygiene such as taking regular breaks should take place. One example being the '20/20' rule: change focus from near distance (for example, the VDU) to far distance 20m away (such as out of a window) every 20 minutes for a few seconds.
Any refractive error should then be corrected. In some cases, suppression may need to be treated first. For example, bar reading, wearing a red filter over non-suppressing eye and tracing patterns with a red pencil. A course of orthoptic exercises (see below) should then be prescribed. Follow up should take place in 1-2 weeks and is essential to encourage motivation and monitor progress.
In this case, several orthoptic exercises such as 'pen to nose' and a 'dot card' could be used. The pen to nose exercise requires the use of an accommodative target held at arm's length, gradually bought in towards the patient's nose. The end point is appreciating horizontal diplopia at which the target should be moved away until single vision is restored and the process repeated, with the aim of improving the near point each time. The patient should be encouraged to 'try' to make the pen single again before receding it. The goal target distance should be demonstrated to the patient and it should be explained for adults (whose convergence may be weaker than a child's) that it is normal not to reach their nose.
Variations of this can be employed if diplopia is not appreciated, dependent on the age of the patient. A parent or friend can be shown how to look for divergence at the end point and prompt the patient. Alternatively, a pen torch light target with a red filter over one eye can be used, which will appear as a red and white light when the eyes diverge.
The 'dot card' is another simple to administer exercise which can be used in conjunction with the pen to nose test. This can be easily demonstrated in the consulting room using a sheet of A4 paper cut in half and drawing a black line length ways. Black dots are then positioned at 5cm intervals along the line. The patient holds the line in front of her nose and looking down should aim to see the dot the furthest away. Appreciating diplopia should be explained, and she should see a cross at the dot (Figure 2). She should then be encouraged to converge to the next dot nearer, each time appreciating the cross. This exercise cannot be readily performed if the original NPC is less than 20cm, although larger sized card can be used.
Pen to nose exercises can be used initially to bring the NPC into the range of <20cm before commencing the dot card. Alternatively, the use of a brock string with a bead allows lengths of up to 1 metre and can allow better cooperation in younger children. This is tied to a door handle or chair and held to the patients nose. Then with a helper, the bead is moved towards the patient, while seeing the string as a cross (in physiological diplopia) at the bead.
Using a variety of exercises described above can be more interesting for the patient. The session should be carried out for 3-5 minutes, 2-3 times a day. The eyes should then be relaxed for a few minutes afterwards avoiding reading or close work, or by closing them.
It may be beneficial to explain that there may be asthenopic symptoms initially immediately after, but this shows the patient she has done the exercise correctly.
Follow up and discharge
This patient should notice an improvement and elimination in her visual symptoms after a few weeks. One would expect 1-2 follow up appointments fortnightly before discharge. She should be able to reach her target NPC of 6-8cm, smoother jump convergence, good recovery of near exophoria (Figure 3) and exhibit normal amplitudes of accommodation. The patient should be advised that, should her symptoms return, to check for a reduced NPC and restart exercises as necessary should this recur.
Causes of decompensation
- Accommodative anomalies due to the close link between accommodation and accommodative convergence, the lack of use in the former can lead to a reduction in function of the latter.5
This can occur in:
- Uncorrected hypermetropes (for example +5.00 and above) who make little effort to accommodate
- Uncorrected myopes who are used to seeing close work at their near point without needing to accommodate
- First time corrected presbyopes, who now do not need to exert any accommodation and hence accommodative convergence.
- Change in visual environment or demand. This is especially common in students before exams or a change in job.2
- Secondary to other binocular vision anomalies2 such as
- intermittent exophoria/exotropia of divergence excess type
- exphoria or exotropia of convergence weakness type (with or without low AC/A ratio)
- vertical muscle defects
- accommodative difficulties (see above).
- Poor general health.
This can be lead to a breakdown of the control of many binocular conditions. Other associations found in the literature include CI presenting as part of Moebius' sign that can indicate thyrotoxicosis and pineal gland tumours1
- Medications eg tranquilizers.
- Anatomical and developmental factors.
- Disuse of one eye or uncorrected refractive error.
When to refer
If symptoms persist or worsen at follow up, the practitioner has to decide whether to continue with another treatment option. It should be established if the patient was compliant or understood the convergence exercises. Other exercises to improve positive fusional reserves may also be required, for example training fusional reserves using base out prisms or using an amblyoscope. In some cases, it may be appropriate to prescribe base in prism temporarily to relieve symptoms during a stressful time eg exams. Referral must be considered if pathology or psychological factors are suspected.
It is reported that some rare cases of CI associated with a large exophoria at near do not respond to conventional orthoptic exercises. These may be candidates for surgical intervention on the medial recti but there is the danger of consecutive esotropia at distance.5
Differential diagnoses
CI presenting with associated accommodative insufficiency can show a poor response to orthoptics. The AC/A ratio can also be low or absent. Von Noorden reports that most of these cases had a history of trauma or childhood illness.5 These cases tended to benefit from low convex lenses and base in prisms.
The differential diagnosis of CI is convergence paralysis. It presents with the inability to converge, causing diplopia at near. Accommodation may or may not be present. Pupil reflexes may be absent on convergence but a light reflex is maintained. Note that adduction is normal but may present with internal ophthalmoplegia or a vertical gaze palsy. Causes of this can be functional (eg hysteria) or organic. These include trauma (especially whiplash injuries) to the tabes dorsalis, midbrain lesions (causing loss of contraction of the medial recti), brainstem lesions (eg sub-dural haematoma) or in disseminated sclerosis.5
Summary
Many patients present to the optometrist with asthenopic symptoms associated with close work. Convergence insufficiency is a fairly common cause and is easily treatable. Exercises such as the 'pen to nose' and the dot card are simple and cheap to administer. The practitioner should be aware of any other untoward presenting history eg head trauma and associated signs and refer any cases which do not improve. ?
References
- Evans BWJ. Pickwell's Binocular Vision Anomalies (4th Edition), Oxford: Butterworth Heinemann, 2002.
- Bishop A. Convergence and convergent fusional reserves - investigation and treatment. In 'Binocular Vision & Orthoptics.' Edition: Evans B and Doshi S, 2001.
- Rouse MW, Hyman L, Hussein M and Solan H. Frequency of convergence insufficiency in optometry clinic settings. Convergence Insufficiency and Reading Study (CIRS) Group. Optometry and Vision Science, 1998 Feb 75(2): 88-96.
- Lara F, Cacho P, Garcia A and Megias R. General binocular disorders: prevalence in a clinic population. Ophthalmic & Physiological Optics, 2001 Jan 21(1): 70-4.
- Von Noorden G and Campos EC. Binocular vision and ocular motility, theory and management of strabismus. (6th edition), Missouri: Mosby Inc, 2002.
? Sosena Tang is an optometrist working in hospital and multiple practice in Hertfordshire. She has a special interest and completed research in binocular vision as part of her MSc in clinical optometry