Myopic astigmatism is a specific type of astigmatism wherein the light focuses before it reaches the retina. This condition is classified as a combination of astigmatism and myopia, or nearsightedness. If both principal meridians are nearsighted, but one is more pronounced than the other, it is considered to be a compound myopic astigmatism. Convergence Insufficiency on the other hand is a condition wherein the eyes are unable to turn inward or converge properly during near work which causes a difficulty to focus normally due to the deficiency in eye teaming which creates a strong tendency for the eyes to drift outward. Convergence insufficiency is likewise associated as exophoria at near.
Myopia, Astigmatism, Exophoria, Convergence, Convergence Insufficiency
Myopia or nearsightedness, is a refractive error, which means that the eye does not bend or refract light properly. In this sense, the light that enters the eye focuses the image in front of the retina. In cases of myopia, a person is to see things at near with much more clarity than fixating at distant targets. Myopia occurs when the eye is has a longer axial length than normal would have or an eye that has a cornea which is relatively steeper than an emmetrope’s. The degree of myopia or nearsightedness affects the eye’s ability to focus on distant objects. People with high amounts of myopia can see clearly only objects just a few inches away, while those with lower amounts of myopia may still see objects at a distance with enough clarity.
Astigmatism on the other hand is a condition wherein the rays of light
entering the eye focuses on different areas, in the case of Compound Myopic Astigmatism, both the power meridian and the axis meridian forms its focus in front of the retina. Astigmatism usually is caused by an irregularity in the shape of the cornea, which instead of the cornea having a symmetrically spherical shape, it is shaped more like a football, with one meridian being curved either steeper or flatter than the other meridian. In some cases, astigmatism is caused by the irregularity of the shape of the crystalline lens inside the eye. This type of astigmatism is called lenticular astigmatism, which has a different nature from the more common corneal astigmatism. As a result of either type of astigmatism, a person’s vision for both near and distant objects appears to be either blurred or distorted to a certain extent. Objects seen through the eyes of a person with astigmatism may either seem to be too tall, too thin, too wide, too short or simply having any alterations in terms of apparent size. Convergence insufficiency is a sensory and neuromuscular anomaly of the binocular vision system, characterized by a reduced ability of the eyes to turn inwards or towards each other which causes the eyes to sustain convergence. Symptoms of convergence insufficiency include diplopia or a doubling of vision and headaches when participating in near work. Individuals experiencing convergence insufficiency may complain that they have a noticeable difficulty concentrating on near work such as reading, using of the computer, etc. and can observe that objects such as words in a book seem to blur out after prolonged periods of reading or if reading when tired. People with convergence insufficiency may possibly have normal or 20/20 vision. To diagnose convergence insufficiency, certain tests and procedures may be done, such as; taking of a medical history, this may include questions about problems a patient would have with focusing, blurred or double vision, headaches, and other signs and symptoms. Likewise the near point of convergence (NPC) of a patient should be measured. An assessment of positive fusional vergence should also be done to determine the patient’s reserve in terms of convergence. Convergence insufficiency may be managed with convergence exercises determined by optometrists. Some cases of convergence insufficiency are successfully managed by prescription of eyeglasses, sometimes with therapeutic prisms incorporated in the prescription.
A 17 year old female student was subjected to an eye examination last September 6, 2013 at the Philippine Sports Commission – Rizal Memorial Sports Complex. She complains about blurring of vision with objects at far for both of her eyes and likewise complains about headaches in the frontal area when doing nearwork which she complains as bothersome due to the headaches disabling her from reading for a long time.
The patient’s last eye examination was done about 2 years ago she was prescribed with eyeglasses with a refractive power of -4.00 sph = -1.00cylx180 for the right eye which gives her a visual acuity of 20/70 or 6/21 and -3.50 sph = -1.00 cylx180 which also gave a visual acuity of 20/70 or 6/21. Her naked visual acuity for both eyes is 20/200 for far and 20/30 for near while pinhole visual acuity is measured at 20/25 for both of her eyes.
The patient’s refractive error as measured through an auto refractometer was OD -6.00sph = -1.50 cylx180 and OS -5.50sph = -1.50 cylx180 which was further refined through subjective refraction to a refractive correction of OD -5.50sph = -1.50cyl x180 and OS – 5.00sph = -1.50cylx180 which gave her a visual acuity of 20/20 for both eyes.
Alternate cover testing was also performed and a small amount of exophoria was revealed when the patient was fixating at a distance and likewise exophoria was also observed when the patient was fixating at a near object.
Due to the unavailability of other materials necessary for a comprehensive eye examination in the field, further evaluation and investigation was not performed. Differential Diagnosis
Astigmatism, Accommodative Insufficiency, Convergence Insufficiency Discussion
In terms of refractive error, Compound Myopic Astigmatism can be observed with the patient’s final prescription. But analyzing and comparing the patient’s chief complaints from the results obtained from the eye examination there is a clear inconsistency between the frontal headache and
the compound myopic astigmatism. It is known that frontal headaches are commonly associated with hyperopia rather than myopia but frontal headaches can also be associated with accommodative insufficiency and convergence insufficiency. Accommodative insufficiency could easily be ruled out considering that the patient is myopic and her refractive correction has increased more towards minus therefore nuancing out a further need to accommodate. This leaves out convergence insufficiency as a suspect, the symptoms have been aligned in terms of the headache, the difficulty in reading for a long period of time and the observable exophoria for near and far. Management
Subject the patient to a comprehensive eye examination to assess her visual system and to verify the prognosis of convergence insufficiency.