Approach
Because the clinical presentation of astigmatism varies according to the patient's age and the degree of astigmatism, a high index of suspicion should be maintained with all patients.[1]
With infants and young children, the clinician cannot rely on a medical history and therefore a screening ophthalmologic exam is the proper approach to diagnosing astigmatism.
With patients who can give a proper history, those with a low degree of astigmatism may complain mainly of headache and eye fatigue, rather than poor visual acuity, and the clinician should be alert to this possibility. Large degrees of astigmatism are easier to diagnose, because visual acuity is usually reduced for both near and distance.
Retinoscopy
Retinoscopy is the most practical office procedure to determine an objective refraction.
The fundamental components of a retinoscope are a light source, a condensing lens, a mirror, and a sleeve. The clinician projects the light streak toward the patient's eye (usually in a working distance of 66 cm) and a red reflex from the retina passes back to the clinician's eye. By observing and analyzing the properties of the reflex, the examiner can determine the refractive state of the patient.
The clinician should sweep the light streak from side to side and observe the direction of the red reflex, which can be with the motion of the sweep or against it. In general terms, when a "with" motion is seen, a plus lens of gradually increasing power should be placed in front of the eye until no motion is seen (neutralization). When an "against" motion is observed, a minus lens of gradually increasing power should be placed in front of the eye until neutralization is achieved.
When neutralization is obtained in all meridians using the same dioptric power lens, the eye is said to have a spherical refractive error (myopia or hyperopia). However, when different dioptric power is needed to reach neutralization in different meridians, astigmatism is diagnosed. This difference in the dioptric power is the power of astigmatism and the principal meridians are localized to determine the axis.[29]
Wavefront analysis technology
An alternative approach to evaluate the refractive power of the lens and the cornea (anterior and posterior). Wavefront analysis technology achieves a more accurate refractive correction, with potential application in cases of irregular astigmatism. There are 4 different wavefront aberrometers, 3 of which are combined with a corneal topographer.
Considerations by age
Diagnosis in infants and young children should follow appropriate local and national guidance.[30][31] Early in an infant's first year of life, an ophthalmologic exam should be carried out to evaluate visual function. The evaluation includes slit lamp biomicroscopy, fundus exam, retinoscopy, binocular vision, and ocular motility.
If a significant amount of astigmatism (or any other refractive error) is detected, the evaluation should be repeated at 3- to 6-month intervals until stable. If stability is not achieved by 12 months of age, prescription should be considered.[32]
In older children, ophthalmologic evaluation should include visual acuity testing, slit lamp biomicroscopy, fundus exam, retinoscopy, binocular vision, accommodation, and ocular motility.
Detection of astigmatism outside the normal limits should prompt a repeat evaluation in 3-6 months or, in cases of high-powered astigmatism, a decision to prescribe, especially in patients with oblique axis astigmatism. An even more aggressive treatment approach, namely earlier prescription, should be pursued when amblyopia and/or strabismus develops.[33]
Diagnosis in young adults and adults should follow appropriate local and national guidance.[30][34] Annual ophthalmologic exam should include visual acuity testing, slit lamp biomicroscopy, fundus exam, and retinoscopy. If there are frequent changes in the degree of astigmatism, or when irregular astigmatism is suspected, further evaluation needs to be carried out, including keratometry and corneal topography.[35]
Keratoconus
Advanced keratoconus is easy to diagnose by a slit lamp exam alone. However, diagnosis of mild cases is often difficult and a high index of suspicion needs to be maintained. Familiarity with the symptoms and signs of keratoconus is therefore important, as this aids early diagnosis and helps prevent unnecessary decreases in functional vision. Standard keratometry may miss the diagnosis, especially when the area of distortion is decentered on the cornea, but corneal topography can highlight corneal changes very early in the course of the disease.[36][Figure caption and citation for the preceding image starts]: Corneal topography: keratoconusFrom the collection of the Department of Ophthalmology, Assaf Harofeh Medical Center, Israel. Used with permission [Citation ends].
Following keratoplasty
The healing process following keratoplasty is long and sutures are often not removed until at least 6 months after the implantation. High degrees of astigmatism are common both before and after suture removal, averaging 4.5 to 5.5 diopters in different studies.[37][38]
Keratometry can be used postoperatively for diagnosing and evaluating astigmatism, but it is not of sufficient accuracy for contact lens prescription. Corneal topography is preferred for determination of the shape of the transplant and the type of astigmatism.
Following corneal trauma
Corneal scarring and distortion are common following corneal trauma, and consequently astigmatism is common. As with keratoconic patients, keratometry is often used but the readings are of limited value. Corneal topography may be of greater value in assessing and following up these patients. Technologies such as anterior-segment optical coherence tomography and Scheimpflug imaging may be considered when posterior corneal curvature measurement is important (e.g., in postoperative astigmatism following cataract surgery).[39]
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