Approach
Astigmatism should be treated after the first 12 months of life.
The indications for treatment are moderate to high astigmatism (≥1 dioptre) and/or a failure to correct refractive errors (emmetropise), usually as manifested by increasing or stable astigmatism during the first 3 years of life.
Patient age determines the definition of borderline astigmatism. Treatment of borderline cases is indicated when eye strain (asthenopia) is present, and typically involves spectacle or contact lens correction. All other patients can be monitored at 3-6 month intervals. Follow the recommended screening programme and manage patients by age according to local or national criteria.[30][31][34][51]
Regular astigmatism: infants aged <1 year
Spectacle correction is not usually necessary in this age group because the visual blur does not seem to increase the risk of developing amblyopia. When no (or a small amount of) astigmatism is found, a follow-up examination should be scheduled at the next age interval. High degrees of astigmatism (≥1 dioptre) necessitate follow-up examinations at 3- to 6-month intervals.[52][53] It is acceptable to give spectacle correction in this age group when isometropic astigmatism is measured at ≥3 dioptres and anisometropic astigmatism is measured at ≥2.5 dioptres between eyes (in the absence of strabismus).[31]
Regular astigmatism: children aged 1-3 years
When astigmatism is borderline (i.e., approximately 1-2 dioptres), prescription is not obligatory and repeat retinoscopy at 3- to 6-month intervals is indicated. When repeat measurements demonstrate a stable or an increasing amount of astigmatism (≥2.5 dioptre for isometropia or a ≥2 dioptre difference between eyes for anisometropia without strabismus), spectacle under-correction is recommended.[31] Under-correction of the astigmatism should be accompanied by an adjustment of the sphere power. Leaving the child with a mild degree of blur allows further correction of refractive error (emmetropisation). Prescription is usually not required for astigmatism of <1 dioptre and 6-month interval monitoring is sufficient.[54]
Regular astigmatism: children aged 3-5 years
Astigmatism ≥2 dioptre for isometropia or ≥1.5 dioptre difference between eyes for anisometropia without strabismus requires full correction with spectacles. Note that thresholds to correct anisometropia should be reduced if the child has strabismus or amblyopia.[31][55] When a child exhibits good visual acuity and binocular function, he/she can wear the astigmatic correction as needed and return annually. However, in children <5 years old, refraction should be monitored every 3 to 6 months. Prescription is usually not required for astigmatism of <1 dioptre, for which 6-month interval monitoring is sufficient.
Regular astigmatism: children aged ≥5 years
In older children and young adults with a significant amount of astigmatism (≥0.75 dioptre), refractive correction is determined by the severity of the refractive error, visual acuity, and visual symptoms.[31] Full spectacle correction is usually needed to reduce the astigmatic correction (with adjustment of the sphere power), although patients may not be fully adherent. Full correction is mandatory if amblyopia is present. Older children may benefit from soft or rigid gas-permeable contact lenses as an alternative to spectacles. Soft toric lens prescriptions are usually feasible, even for astigmatism of ≥0.75 dioptre. Prescription is usually not required for lower astigmatism, unless eye strain is present.
Regular astigmatism: ≥18 years
Full correction of the astigmatism with spectacles is required, though it is possible to reduce the astigmatic correction (with adjustment of the sphere power accordingly) in non-adherent patients. Alternatives to spectacles include soft or rigid gas-permeable contact lens prescription or, if not contraindicated, refractive surgery (laser refractive surgery or intraocular surgery [phakic intraocular lens or refractive lens exchange]).[56] Patients with low degrees of astigmatism (0.50 to 0.75 dioptre in this age group) and high degrees of myopia or hyperopia may sometimes benefit from correction of the spherical error alone.
Refractive surgery
Laser refractive surgery is quickly gaining popularity.[42] The two most common procedures are photorefractive keratectomy (PRK) and laser-assisted in situ keratomileusis (LASIK). Other options exist, such as astigmatic keratotomy and small-incision lenticule extraction (SMILE), with the latter avoiding the need for a corneal flap. Phakic intraocular lens implantation is also effective for correcting myopia and myopic astigmatism.[56]
In PRK, the anterior surface of the cornea is reshaped using laser energy, while in LASIK the laser is used to remove tissue from the corneal stroma after raising a lenticular corneal flap. LASIK appears to produce comparable visual acuity whether using a mechanical microkeratome or a femtosecond laser in patients with >0.5 dioptre of myopic astigmatism.[57] One review comparing wavefront versus conventional refractive surgery and wavefront‐optimised versus wavefront‐guided surgery also found no differences in uncorrected visual acuity, residual refractive errors, or residual higher‐order aberrations between groups at 6-12 months.[58]
The full list of indications is extensive and should be checked against local guidelines and product literature. The procedures are generally contraindicated with unstable refraction, corneal abnormalities, thin corneas, uncontrolled ophthalmic disease (e.g., glaucoma, blepharitis, dry eye, atopy, autoimmune disease), and unrealistic expectations or untreated mental illness.[42]
Good management requires informed consent (e.g., adverse effects include dry eye, eventual presbyopia, ectasia, and diffuse lamellar keratitis) and both the provision and maintenance of a clear record of the procedure (e.g., diagnosis, preoperative keratometry measurement, and both preoperative and post-operative refraction).[42][57] These records can be useful for future eye care (e.g., cataract surgery).
Ectasia and keratitis are important complications. The risk of ectasia is increased with excimer laser ablations that leave very thin residual stroma (e.g. <250 micrometres for LASIK [though there is no guarantee at any absolute value], abnormal topography, and a percentage of tissue altered ≥40%). Diffuse lamellar keratitis is a non-infectious aggregation of inflammatory cells that is usually treatable with corticosteroids. However, if the condition does not respond to treatment, consider microbial keratitis or increased interlamellar fluid. Other complications include flap striae and displacement, epithelial ingrowth, central toxic keratopathy, and post-operative infection.[42]
Irregular astigmatism: keratoconus-related
In very mild cases of keratoconus, a functional visual acuity may be achieved by spectacle correction. However, most patients require rigid gas-permeable (RGP) contact lenses in order to improve visual acuity. In some circumstances, more specialised types of contact lenses may be required.[1]
Specialised contact lenses
If keratoconic patients become intolerant of RGP lenses, it is possible to use a rigid lens fitted over a hydrogel lens (piggy-back lenses) to improve comfort and to provide an adequate wearing time with good visual acuity. The downsides of this technique are the handling and maintenance of two types of lenses, the difficulty in providing sufficient oxygenation to the cornea, and difficult centration of the rigid lens over the hydrogel one.
Hybrid lenses overcome the handling problems of piggy-back lenses, combining the optics of a rigid lens with the comfort of a hydrogel lens in one piece. This is done by forming a soft rim onto a hard central portion. Hybrid lenses have numerous potential problems, but they are useful for some patients with keratoconus and other corneal distortions.
Scleral lenses are very large rigid lenses, the haptic portion of which rests on the sclera. They provide excellent optics by neutralising the distorted cornea and are surprisingly comfortable and easy to use. Their major disadvantages are the time and skill required in fitting the lens and their cost.
Other treatment options
Intrastromal corneal ring segment (INTACS) inserts are micro-thin rings that are inserted into the corneal stroma. The inserted implants reshape the corneal curvature from within, thus modifying its refractive power. INTACS implantation can improve visual acuity and refraction in most patients with keratoconus. Infectious keratitis, a possible sight-threatening complication, has been reported.[59]
Collagen cross-linking by the photosensitiser riboflavin and ultraviolet-A light (epithelium-off cross-linking) seems to be effective in stabilising the cornea and might delay the progression of keratoconus.[60][61] Transepithelial cross-linking, in which the epithelium is retained, may potentially reduce patient discomfort and time to healing. There is, however, a lack of evidence to confirm these putative benefits.[61]
Mild to moderate keratoconus can be managed by photorefractive keratectomy (PRK), in which an excimer laser is used for customised ablation of the superficial corneal layers, thereby modifying their topography. PRK has been found to improve videokeratography indices and visual acuity in these patients over a 2-year follow-up period.[62]
In photo-astigmatic refractive keratectomy (PARK), the excimer laser is used to form a corneal slit, the width of which is determined by the degree of astigmatism. It has been shown to be safe and effective in mild keratoconus and in carefully selected patients with refractive and corneal stability.[63][64]
Clear lens extraction with toric intraocular lens implantation is also an option, though only in cases with stable corneal topography for a period of at least 1 year, and when maximum keratometry reading is not above 55 dioptre.[65]
Keratoplasty, which can be penetrating (full corneal graft) or deep lamellar (partial thickness corneal graft), should be considered in severe and refractory cases. Partial thickness corneal grafting is indicated only when the host cornea is partially intact (either the epithelial or the endothelial part). Though the surgical technique is more difficult compared with penetrating keratoplasty, the healing process is shorter and therefore it is preferred when indicated.[36]
Irregular astigmatism: post-keratoplasty
The first months following keratoplasty are usually characterised by an oedematous corneal graft and significant amount of astigmatism. Changes in the refractive state are frequent, especially when suture removal is commenced. Therefore, it is wise to postpone the optical correction until relative stability is reached. Mild cases of astigmatism may be treated with spectacles.
Later on post-operatively, preferably after suture removal, it is possible to use RGP contact lenses. Piggy-back, hybrid, or scleral contact lenses can be used in these patients, but these lenses have a tendency to encourage corneal blood vessel growth and so are used only if RGP lenses are unsuitable.
Selective suture removal is also a method for reduction of astigmatism.[37][38] Other approaches include photo-astigmatic keratectomy and topography-guided excimer laser surgery.[66][67]
Irregular astigmatism: post-corneal trauma- or scarring-related
The preferred treatment modality is RGP contact lenses. Correction of astigmatism in this group by spectacles or soft contact lenses is not likely to achieve satisfactory visual acuity, but in mild cases it is acceptable to try. The dimensions and power of the contact lenses are determined as for the keratoconic patients using the keratometric reading and corneal topography only as a rough starting point, making the final adjustment by evaluating the fluorescein pattern with a diagnostic contact lens on the cornea.
Alternatives include piggy-back, hybrid, or scleral contact lenses, although these are far less convenient for the patient. In severe and refractory cases, penetrating keratoplasty is indicated.
Irregular astigmatism: pterygium-related
Pterygium is a benign growth of the conjunctiva, commonly encountered on the nasal side of the sclera, though the temporal side may also be involved. The pterygium can induce a refractive change that often leads to visual impairment in a mechanism that is not totally clear. It has been shown to have a considerable effect on topographical indices of the cornea, presenting as flattening in the horizontal meridian, which is associated with astigmatism. The pterygium size appears to be related to the magnitude of the induced astigmatism. When a small pterygium is associated with a low-degree astigmatism, it is acceptable to attempt spectacle correction.[68]
Higher-powered astigmatism is an indication for pterygium excision.[68] Larger pterygia adversely affect astigmatism, asymmetry, and irregularity of the cornea. Pterygium >45% of the corneal radius may induce significant degrees of astigmatism. Indeed, one of the indications for pterygium removal is visual impairment. The surgery usually leads to reduced astigmatism and improved corneal topography, regularity, and symmetry.
Irregular astigmatism: eyelid pathology-related
Pressure from the eyelids has been shown to induce short- and long-term corneal topographical changes. Such changes in corneal topography and corneal astigmatism accompany certain common eyelid abnormalities (e.g., chalazia, capillary haemangioma). Disorders of eyelid position (e.g., ptosis) and surgery on the eyelids have also been found to cause changes in corneal topography and astigmatism. The magnitude of astigmatism is reduced by treating the primary eyelid pathology (e.g., excision of the chalazion, ptosis repair).[69][70]
Irregular astigmatism: following eye surgery
Some eye surgeries (e.g., cataract, buckle procedure) can give rise to astigmatism. The corneal incisions in cataract surgery induce variable amounts of astigmatism, the degree of which is influenced by the size, architecture, and location of the incision. Therefore, small incisions, associated with a more rapid wound healing and less corneal distortion, are preferred. Buckle procedure for retinal detachment and other eye surgeries are also associated with altered corneal surface indices and astigmatism. When astigmatism is mild, it is acceptable to use spectacle or contact lens correction.
When astigmatism is of greater magnitude, spectacle or contact lens correction may be insufficient. In these cases, several surgical techniques can be used. In astigmatic keratotomy (AK), arcuate incisions are placed along the full arc of the steep area and the level of astigmatic correction is controlled by varying the incision depth. This technique allows accurate control of the level of astigmatic correction with minimal risk. Alternatively, photorefractive keratectomy (PRK) or laser in situ keratomileusis (LASIK) is effective and safe for correcting residual astigmatism after cataract surgery.[71][72][73]
Astigmatism management during cataract surgery
Cataract surgery can serve as a good opportunity to reduce or even cancel pre-existing astigmatism. Limbal relaxing incisions are best reserved for patients with low levels of astigmatism. However, toric intraocular lens implantation is the most predictable and cost-effective method to correct high amounts of astigmatism.[42] Other intra-surgical options for treating residual astigmatism are to place the clear corneal incision on the steepest meridian (less effective with modern techniques) or to perform a two-stage procedure with excimer laser ablation (more predictable but expensive).[74]
Lenticular refractive surgery may be preferable to keratorefractive surgery for early cataract formation in the presence of significant lenticular astigmatism.[42]
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