Retinitis Pigmentosa is a progressive condition resulting in a loss of vision. It may be associated with a number of systemic conditions or syndromes, or may be primary in nature. The rod-cone presentation is the more classic with a loss of peripheral vision to a resultant ring scotoma or tunnel vision, and a decrease in night vision, nyctalopia. The central vision is frequently spared and many of these patient present with excellent central acuity, but with a compromised field of vision. This case describes a classical RP patient who is having increased difficulty with mobility as a result of his field loss. It will address a potential avenue of management and treatment to expand the useful field of view and assist this patient in his travels.
Key Words: Retinitis Pigmentosa, amorphic lens, field expansion, ring scotoma, reverse telescope
Retinitis Pigmentosa (RP) should be regarded as the description for many different dystrophies and degenerations of the photoreceptors and the retinal pigment epithelium(RPE). Most of these conditions are genetic in nature(1) and almost all are progressive. Progressive deterioration of the RPE and/or the photoreceptors define a myriad of conditions which include several named dystrophies.
The photoreceptors that are affected may be rods or cones, and they may be affected centrally or peripherally, although the classical presentation is peripheral rods. It hasn’t been established if the loss of the photoreceptors is primary or secondary to the loss of the RPE, but it is known that changes occur in the RPE and that the photoreceptor loss is responsible for the loss of vision.
The genetic history as well as accompanying ocular, systemic, and physical findings need to be considered in the diagnosis. Additionally, it is necessary to consider toxicity by various medications that may also alter the RPE in some fashion and result in loss of the photoreceptors.
Electrodiagnostic testing also proves very useful in the confirmation of the diagnosis of RP, and in the determination of the classification of the RP(2). Some of the accompanying ocular signs of the disease that may need to be addressed include, posterior subcapsular cataracts(3), vitreous haze, bull’s eye maculopathy, peripheral and central field loss, reduced visual acuity (4), and reduced night vision.
Patient #3 is a 52-year old gentleman with a history of Retinitis Pigmentosa. He was referred to our office by his health insurance agent. He was first seen on May 9, 2005. His family history is positive for RP having a father and uncle with the condition. Additionally, two of his daughters have been diagnosed with the disease as well through electro-diagnostic studies performed at the Bascom Palmer Eye Institute. His ocular history is significant for bilateral posterior subcapsular cataracts removed approximately 20 years ago, without implants leaving him aphakic.
He also had bilateral vitrectomies at that time to “remove some cloudy tissue in his vitreous”. He has a history of angle closure glaucoma in his right eye and has had laser iridectomies OU as well. He has never smoked, drank alcohol, or used any drugs which were not prescribed for him. Currently he is taking no medications and has had no other surgical history.
He is negative for any history of STD’s as well. His unaided visual acuities are OD, OS, OU are counts fingers at 2 feet. With his habitual refraction of OD +13.50-.75×060 his acuity at distance is 20/20 and OS +14.00 sphere acuity is 20/20 ( he also wears aphakic soft contact lenses, +16.00 OU and they additionally give him 20/20 distant acuities OD, OS, and OU).
Slit lamp biomicroscopy is negative for anything other than the iridectomies described above, aphakia, and the surgical pupillary borders OD. His irises are medium to light blue. His pupils are normally reactive to light, although the right one is not round nor equal to the left as a result of the damage to it during cataract surgery.
His intraocular pressures by Goldman applanation tonometry were 18 OD and 16 OS. Motor fields are full in all directions of gaze, he has a negative afferent pupillary defect. His head and face appear to be normal with no unsual tilts or turns. His mood is normal, no depression, anger, or agitation noted. He has a Von Herrick ratio of 1:2 in all quadrants OU. He was dilated using 1% proparicane, 1% mydriacyl, and 2.5% neofrin. Internal with a Binocular Indirect Ophthamoscope yields Elschenig classifications of the cups as type I, funnel shaped, OU.
His A/V ratio is ¾ OU and his cup to disc ratio is .2 OU. His cups have a depth of 2.00 diopters OU and his macular reflexes are crisp and clear. His peripheral retina demonstrates bone spicules scattered throughout the retinas, and some attenuation of the retinal arterioles. There appears to be no abnormal pallor to the discs. OU.
His color perception on the Farnsworth D-15 is normal administered binocularly. Amsler grids centrally are unremarkable. Refraction is the same as his habitual, no changes at this point. His visual fields show central fields of 8 degrees in the 180th meridian OD and 5 degrees OS. The central islands are somewhat irregular and distorted in shape with an additional 3 degrees of field in the 90th meridian OD and 5 degrees OS.
The differential diagnoses considered include:
Rod-cone retinitis pigmentosa
Cone-rod retinitis pigmentosa
Drug or chemical toxicity
Vogt-Spielmeyer- Batten disease
The ocular signs of RP may be as follows:
In Renal disease RP may be present but mostly manifests itself in an atypical presentation of whitish-gray dots in the superficial layer of the retina, these aren’t present in this patient, nor is there a history of renal disease. There is no history of drug or chemical induced toxicity that would affect the RPE or the photoreceptors. In Ushers Syndrome there is a history and evidence of hearing loss which is also not the case here as this patient has perfect hearing(5). Likewise, Kearns-Sayre syndrome involves not only hearing loss, but also diabetes so this is not a likely possibility(6).
The Vogt-Spielmeyer-Batten disease is characterized by a bull’s eye maculopathy, which is absent in this patient as well(7). This patient has a positive family health history for RP. He has a crisp macular reflex without even a hint of bull’s eye maculopathy. His color perception is 15/15 on the Farnsworth D-15 test. He has a ring scotoma and bone shaped spicules. He has a history of posterior subcapsular cataracts, and a positive history of ERG for rod-cone changes. Given the foregoing the diagnosis for his condition is rod-cone retinitis pigmentosa.
Low Vision Evaluation
Rod-cone retinitis pigmentosa often leaves the central vision untouched. It typically progresses from the periphery and contracts toward the macula, decreasing peripheral vision and creating a ring scotoma. This is exactly what happened in this patient. He wanted to expand his field of view to enable him to move about with greater confidence and facility.
His acuities were 20/20 OU, and no magnification was required to assist him in reading signs, seeing the television set, or the screen on his computer. He was able to read .6M type at nearpoint with +2.50 bifocal adds, or reading glasses over his contact lenses. He wore his contacts almost all of the time and his spectacle correction was reserved for the first few minutes in the morning or the last few minutes in the evening when his contacts weren’t on. He occasionally wore them if he had a dry or irritated feeling in his eyes, but said this was very rare, perhaps one or two days a year.
To expand his visual field and increase his awareness it is necessary to minimize the size of the objects he is looking at while gaining the desired effect. One way to approach this is to reverse a telescope. By doing so we essentially minimize the objects we are viewing by the reverse of the relative magnification of the scope while increasing the field of view by a similar percentage. With a 2.2X full diameter telescope mounted in reverse binocularly we effectively increased his field by almost double. The problem with this is all of the objects in the field of view are decreased in size in all dimensions by a proportional amount.
This results in a great deal of confusion as it relates to spatial orientation. I have used this approach with limited success in the past reversing a 3.0x telescope fitted in the bioptic position. I used an Edwards ½” scope and mounted it with most of the scope outside of the lens. This is one of the scopes that will permit this type of mounting. The new Politzer scopes from Designs for Vision lend themselves to this type of application quite well. However, there is a telescopic lens that is specifically designed for this purpose. It is called an “amorphic telescope”.
This lens functions by adding minification in the horizontal meridian only while leaving the vertical meridian untouched. This has the advantage of leaving things their normal height while minimizing or “thinning” them horizontally. By doing this it is possible to expand the horizontal field of view and decreasing the problems that generally occur in spatial orientation that are usually accompanied by minification. An amorphic diagnostic was borrowed from Designs for Vision and the lenses were tried in a trial frame over the patients contact lenses.
It was found that the patient still retained 20/25+ acuity while almost doubling his field of view with the 1.8x amorphic telescope. This telescope was prescribed and ordered for this patient and he was instructed that it would probably take 4 to 6 weeks of therapy sessions and many hours of practice to acclimate to the device. When it was dispensed a month later, on June the 2nd, 2005, he spent 2 hours with the occupational therapist to orient him to the device and give him a program of exercises to work on the first week. He never returned for any additional therapy as he was comfortable with the device from the moment he left the office. He called the next day to cancel the next weeks session.
When he returned on September 10, 2006 all findings, both objective and subjective were stable. His functional visual field with the scope in the horizontal meridian (180) remains approximately double the original field and is still consistent with the field as measured at the time of his original examination. He continues to wear the scope every day and says “he wouldn’t leave home without it”. This scope has definitely increased his level of mobility and especially his confidence. He wasn’t depressed on initial evaluation, but he is almost elated now.
Retinitis pigmentosa is a diagnosis that may take several forms and stem from many varied causes. It may be associated with Syndromes like Ushers and Kearns-Sayre Syndrome. The process itself involves the loss of photoreceptor cells in the retina. This loss is either primary or secondary to the loss of the RPE cells. It isn’t certain at this time which is the case. However, it seems that the loss is of two basic types, rod-cone and cone-rod.
In both of these conditions the external appearance of the eyes is generally normal, posterior subcapsular cataracts may be observed in the later stages of the process. The pupillar response may be normal or abnormal, and the vitreous may show some levels of haze as the result of fine cells present there. The typical rod-cone form involvesatrophic areas of the peripheral retina that result in hyper-pigmentation and the formation of “bone spicules”.
This is accompanied by night blindness, nyctalopia, and a loss of peripheral visual field function resulting in a ring scotoma, tunnel vision. Generally in this form the macula is that last to go and retains a good deal of its resolution right up to the end along with it’s color perception. In the cone-rod form of the disease the macula may be affected early resulting in a bull’s eye maculopathy. The peripheral field may initially remain intact to some degree and the central vision is compromised or lost along with a shift to an absence of color vision as well.
The visual field is an important aspect of visual function. It is strongly associated with the ability of visually impaired patients to have confidence and mobility functions(9). Restricted peripheral fields as found in progressive retinitis pigmentosa make moving about very difficult. Patients with severely restricted fields, ring scotomas known as tunnel vision frequently experience problems such as collisions, stumbling, and failure to find objects.
Various field expanders based on the principle of minification have been proposed, such as handheld divergent lenses(10), reversed telescopes(11), and an amorphic lenses(8).
The use of minification seems to be logical, but failure of these devices frequently occurs as spatial orientation with reverse telescopes is difficult to adapt to. There is a loss of resolution that also occurs, but is generally not the reason for failure here. Resolution loss is usually a tradeoff for wide field in conventional minification devices. To deal with the loss of resolution, a field expander worn in a bioptic position (a small device mounted on a spectacle lens, above or below the center of the lens) has been suggested (12).
However, patients using a bioptic minifier need to glance frequently into the expander to notice objects that they would not otherwise be aware of. These field expanders are usually used for mobility and not for reading or near point functions where the patient is stationary and can generally move their eyes in efficient and trained scanning patterns to read or work with their hands in a fixed field area.
The purpose of the field expansion is primarily for mobility in these patients and hence the ultimate level of acuity and resolution may be somewhat compromised in an acceptable amount to permit the patient satisfactory acuity to move about in the safer environment of expanded peripheral awareness created by the reversed scope. When the amorphic telescope is employed the spatial dis-orientation is reduced and resolution is not compromised as significantly as would otherwise be with the simple reverse of a standard telescope.
Therefore, it is not necessary to position this scope in the bioptic position as these devices aren’t meant for full time wear, but rather to be employed during mobility, and fitting them on center greatly aids in their function. The general purpose of a standard bioptic telescope is the gathering of information. When we drive a car, we look in the rear-view mirror for a second or two to gather information about what is behind us, we don’t drive constantly use this mirror, but use it only for a small amount of time to get the information we need to proceed safely.
In a like manner we use a bioptic telescope to glance ahead at a light or a sign in the distance to gather information about what is out there, we aren’t using it to move in the straight-ahead direction. However, the function of a reverse telescope is actually to improve movement, it is the device we want to look through not briefly to gather information, but constantly as we move forward, it is the best source of complete information we have.
In fact, to look away from the scope would increase the magnification and decrease the field and put us in the same position as a person going from the base lens in their glasses up to their bioptic. It is this thinking that must be adopted to understand the importance of this device to be used in the primary position of gaze, still permitting the patient to look around it to gather magnified information that they may need for a brief period of time before returning to the field expansion scope to move ahead.
When an amorphic telescope is employed for this purpose the minification in the vertical meridian is not significantly affected and as such the resultant acuity and resolution are much better. Therefore, it is easier for the patient to adapt to this scope then the standard telescopes which minify in all meridians equally. It is for these reasons that the patient was able to adapt so quickly, easily, and happily to this form of field awareness expansion.
This case demonstrates the functional loss of visual field that occurs in the classic variety of rod-cone retinitis pigmentosa. It clearly gives us a means to expand the peripheral awareness and functional field of view of patients who are challenged in their mobility functions by the restrictions imposed by their field losses.
While this is a viable solution for these rod-cone patients it may be of little to no benefit to the cone-rod patient suffering with bull’s eye maculopathy and a decreased central acuity as a result of this. These patients if they still possess healthy islands of vision in the para and peri central areas surrounding the macula may benefit from the approaches we would take with patients who have the more classic vision loss found in age-related macular degeneration.