Subjective Refraction Technique: Stenopeic Slit

Anatomy and Physiology

Astigmatism is a type of refractive error that occurs when parallel rays of light entering the eye are not well-focused on the retina.[11] Light rays propagate in two different planes in an astigmatic eye; the incident light rays fail to converge at a single focal point and instead have two separate foci.[12] The retinal images of distant and near objects appear blurred due to differences in the degree of refraction in different meridians; the distance between the foci is the conoid of Sturm.[13] Astigmatism may be simple, compound, or mixed and regular or irregular. The stenopeic slit method can be used to determine regular and irregular astigmatism.[8] Astigmatism may be described by the location of the foci, the underlying etiology, or the location with the steepest meridian.

When describing astigmatism by the location of the foci, simple myopic astigmatism occurs when one focus is in front of the retina and the other is on the retina. In compound myopic astigmatism, both foci are in front of the retina but at different locations.[14] Simple hyperopic astigmatism is characterized by one focus behind the retina at a virtual location and the other on the retina.[15] Compound hyperopic astigmatism occurs when both foci are at different locations behind the retina.[14] Mixed astigmatism results when the refractive media creates two foci. One focus is positioned in front of the retina, and the other is behind the retina.[16] Irregular astigmatism results from the refractive media creating principle meridia not perpendicular to each other.[17][18]

When describing astigmatism by its underlying etiology, corneal astigmatism is caused by irregularities in the anterior or posterior corneal curvature, as seen with keratoconus, corneal scarring, or a limbal dermoid.[19] Lenticular astigmatism is due to variations in the crystalline lens, such as curvature, position, or refractive index, as with lenticonus, subluxation, or nuclear sclerosis, respectively.[20] Retinal astigmatism is due to an oblique macular placement.[21]

Astigmatism may also be with-the-rule (WTR) or against-the-rule (ATR) based on the location of the steepest meridian. In WTR astigmatism, the vertical meridian is the steepest and has the most convergent focusing power. This vertical meridian will create a horizontal focal line closer to the source of light and the front of the eye than the focal line created by the horizontal meridian. Contrarily, in ATR astigmatism, the horizontal meridian is the steepest and has the most convergent focusing power. This horizontal meridian will create a vertical focal line closer to the source of light and the front of the eye than the focal line created by the vertical meridian.[22]

Indications

The stenopeic slit is an important accessory to detect and refine the amount and orientation of astigmatism. The stenopeic slit is often used when other assessment methods requiring extensive equipment are unavailable. The stenopeic slit technique detects higher amounts of astigmatism and irregular astigmatism that can occur in cases of keratoconus, corneal scarring, postsurgical irregularities, or severe dry eye. When irregular astigmatism is present, the principal meridians are not at a 90-degree angle. The stenopeic slit can isolate the two principal meridians when obliquely oriented.

A stenopeic slit is also used in cases of corneal opacities to find the optimal site for optical iridectomy.[23] After mydriatic-induced pupillary dilation, the stenopeic slit is rotated in front of the eye; the axis which gives maximum clarity is chosen for optical iridectomy.

Vertex distance can also be assessed with the help of a stenopeic slit.

Contraindications

Since the refinement of the axis and power with a stenopeic slit is a subjective assessment method, the patient must be able to understand and follow the instructions. A patient unable to effectively engage with the clinician due to cognitive deficit or other barriers is not a good candidate for this assessment technique. The patient must also tolerate the positioning and wearing of a trial frame.

The stenopeic slit technique is complicated by high amounts of astigmatism that have resulted in meridional amblyopia, habitual squinting, and patients who have developed a habit of "blur-interpretation."

Finally, the best-corrected visual acuity has to be adequate to recognize changes in image clarity provided during the test.

Equipment

A stenopeic slit is a slit-shaped, elongated, pinhole-like aperture positioned in the center of a disc. The disc is the same size as a trial lens and can be placed in a trial frame. The slit is less than 1 mm wide and approximately 15 mm long. The stenopeic slit device is usually included in a trial lens case as one of the accessories used to determine astigmatism, along with hand-held Jackson cross cylinder lenses.[24]

The stenopeic slit reduces the effective pupil size in the meridian perpendicular to it, producing an axial shift of the circle of least confusion (CLC) and, consequently, a change of the retinal blur size that depends on the orientation of the slit. The stenopeic slit removes the peripheral rays in the meridian perpendicular to the slit, isolating the meridian at which refraction is to be performed.[10]

Personnel

The refinement of the astigmatism axis and power using a stenopeic slit can be performed by anyone who has received appropriate training; however, interpretation of the results is best conducted under the supervision of an optometrist or an ophthalmologist.

Preparation

The stenopeic slit technique should be performed in a dimly lit or dark room. The patient should be educated about the procedure. The patient is asked to view an acuity chart at a standard 6-meter or 20-foot distance. The trial frame should be cleaned thoroughly with an alcohol swab, and all lenses should be cleaned with the cloth provided in the trial set before placing them in the trial frame.

Technique or Treatment

After viewing a visual acuity chart, a trial frame is placed. The stenopeic slit technique is performed monocularly, and vision in the contralateral eye should be blocked with an occluder from the trial set placed in the trial frame.

Steps in the Determination of the Astigmatic Axis and Power

1. Obtain the best spherical correction by placing spherical lenses over the eye. Begin with plus power lenses and introduce increasingly more minus (or less plus) power lenses.
2. Place a +1.50 DS lens in the trial frame lens holder overtop of the best spherical correction lens. This step is called "fogging."
3. Place the stenopeic slit in front of the lenses in the trial frame.
4. Manually rotate the slit 360 degrees until the patient reports the clearest vision through the slit. This is the first principal meridian.
5. Slowly reduce the power in 0.25 DS increments until the patient reports the best visual acuity.
6. Rotate the slit by 90° and reduce the power in 0.25 DS increments until the patient reports the best visual acuity. This is the second principal meridian.
7. Draw the two meridians in an optical cross format.
8. The difference in power between the two meridians is the amount of astigmatism.
9. For the minus cylinder format notation of the refractive error, the first principal meridian orientation is also the axis of the minus cylinder.
10. For the plus cylinder format notation of the refractive error, the second principal meridian orientation is also the axis of the plus cylinder.

If reducing the power in steps 2 through 5 results in increasingly blurred vision, there are more than 3 diopters of astigmatism; the first principal meridian may actually have more plus power. In this case, the clearest image is obtained by increasing the plus power of the lenses in 0.25 DS increments as part of step 5. The test can then be continued as described in step 6.

The Thorington Method

The Thorington method is an alternate refraction technique using the stenopeic slit. When employing the Thorington method, fogging is performed with a +0.50 DS lens placed in the trial frame to move the posterior line of focus closer to the retina. A logMAR chart with multiple lines of letters at a 6-meter distance is used as the viewing target. The patient should be able to read at least 2 lines above the limit of visual acuity with the fogging lens in place. The stenopeic slit is initially positioned at 90°, and the patient is instructed to use the knob on the trial frame to keep rotating the slit until its position gives the clearest view.

The Thorington method is rarely employed in subjective refraction to find the axes of refractive error of an astigmatic eye because of the general perception that the Jackson cross cylinder (JCC) method provides greater accuracy. However, the JCC method can be tedious for patients with reduced vision, as patients can have difficulty detecting any changes between the views produced by two lenses of different axis or power. The Thorington method is very useful in cases of high astigmatism and poor visual acuity or where the retinoscopic reflex is difficult to visualize.

Determination of Vertex Distance

Place a trial frame to determine the vertex distance using the stenopeic slit and ask the patients to close their eyes. The slit device should be kept in the trial frame. Pass a thin ruler through the slit until the ruler touches the eyelid; note the measurement and add 1 mm to compensate for eyelid thickness. A stenopeic slit is also useful in cases of high astigmatism or low visual acuity.[25]

Complications

The stenopeic slit test is a noninvasive procedure with rare complications. Patients may develop conjunctivitis if the trial frame is not cleaned properly before use. Patients may provide incorrect answers during this subjective test resulting in an incorrect refractive correction.

Clinical Significance

Astigmatism causes deterioration of the retinal image and affects the quality of vision. Therefore, the diagnosis of astigmatism and its proper treatment are critical. This is especially true for children who can develop meridional amblyopia if the power of their astigmatism is significant. The optical defocus caused by astigmatism can cause asthenopia, monocular diplopia, and blur. 

Different techniques to assess the axis of astigmatism include keratometry, corneal topography, aberrometry, autorefractor measurements, retinoscopy, the Jackson cross cylinder method, and astigmatic dial methods that include the fan-and-block method, the astigmatic dial technique, the cross dial technique, and others.[26][27][28]

Astigmatism can be managed with spectacles, contact lenses, or surgical correction through refractive surgery, toric intraocular lens implantation, and limbal relaxing incisions.[29]

As the orientation of the stenopeic slit changes, the retinal blur size in an astigmatic eye changes due to an axial shift of the circle of least confusion. When the stenopeic slit is aligned with one of the principal meridians of astigmatism, the spherocylindrical refractive error becomes purely spherical. This orientation produces the sharpest image because the refractive error is the lowest. Patients have different tolerance for visual blur; therefore, the decision on the astigmatic power prescription is dictated by their tolerance level. However, factors such as age and the need to prevent long-term complications may dictate the need to correct astigmatism.

A higher degree of astigmatism is noted in infants and neonates and is reported to be even higher in preterm newborns. An inverse correlation is noted between postconceptional age and birth weight. The occurrence of astigmatism in childhood naturally declines with the process of emmetropization. In almost all cases, significant astigmatism resolves spontaneously in the first year of life. This resolution occurs because of a physiological decrease in the toricity of the cornea and anterior lens surface, combined with reduced variability of corneal-lenticular surfaces.

However, higher degrees of infantile astigmatism are associated with increased astigmatism and myopia in school-aged children.[30] WTR astigmatism occurs most often, followed by ATR astigmatism; oblique astigmatism is much rarer.[31] These changes are usually attributed to alterations in the position and tension or tightness of the eyelid and extraocular muscles, biomechanical properties of the cornea, and genetic factors.[32] Numerous guidelines exist to help practitioners decide on the degree of astigmatism that needs to be corrected.

The visual system may not be very sensitive to the blur induced by astigmatism during the initial stages of life, but from then on, uncorrected astigmatic error, especially when oblique, can induce retinal blur during the early developmental phase leading to a condition called meridional amblyopia.

Factors such as the palpebral fissure slant can affect corneal toricity, which in turn induces astigmatism. Patients with Trisomy 21 and Treacher-Collins syndrome show oblique astigmatism due to an upward or downward slant of the palpebral fissure.[33] Reduced corneal rigidity can also increase astigmatism caused by eyelid pressure.[34] The pupil size also affects astigmatism. Larger pupil size is correlated with high cylindrical power and WTR astigmatism. A higher degree of astigmatism induces chromatic aberrations like coma, which becomes an important consideration before doing refractive surgeries.[35] Tear film changes also affect the cornea and can result in astigmatism.[36]

Management of Astigmatism

Nonsurgical management options for astigmatism are spectacles, toric soft contact lenses, and rigid gas permeable contact lenses (RGPCLs). RGPCL options for astigmatism include front-surface, back-surface, and bitoric lenses. In some cases, spherical RGPCLs are combined with spectacle correction. Surgical options for astigmatism include refractive surgery procedures, toric intraocular lens implants, and limbal relaxing incisions.[37][38][39]

Astigmatic Lens Types

Plano-cylindrical lenses contain one meridian that has no focusing power, the plano meridian, and the other meridian that focuses light, the cylindrical meridian. The plano meridian creates a focal line at infinity, while the cylindrical meridian creates a focal line at a finite distance. Therefore, in the case of the plano-cylindrical lens, one of the focal lines is at infinity, and because the circle of least confusion is halfway in between the two focal lines, its location is at infinity as well.[12] 

Sphero-cylindrical lenses contain both meridians with focusing power, and both will create a focal line at a finite distance. The circle of least confusion is halfway between the two focal lines.

For both lenses, the focal line is perpendicular to the orientation of the meridian.

Enhancing Healthcare Team Outcomes

Astigmatism negatively affects vision quality. Correction of power and proper axis is imperative, especially in symptoms like eyestrain, squinting, dry eye, and headaches. Patients with these or similar complaints require a thorough ocular evaluation best performed by an eyecare professional such as an ophthalmologist or optometrist. Patients that can not be corrected to a visual acuity of 6/6 or 20/20 and have progressive or high astigmatism should be evaluated for corneal ectasia; corneal topography becomes essential in such cases. Similarly, when astigmatism is identified as the underlying cause of meridional amblyopia, referral to a practitioner that can manage amblyopia is indicated. 

Headache, diplopia, and blurring of vision can also be associated with underlying neurological or medical disorders, so other healthcare professionals must consider an existing refractive error or squint before considering the patients for systemic evaluation and radiological tests.