Saturday, 15 February 2014

Tips for handling digital eye strain

Digital eye strain is caused by the overuse of digital devices such as computers and smart phones. Because these electronic devices are designed to be used and held within close range of the eyes, after a while, the eyes become strained as they continue to refocus to process the images on the digital screen.

According to organizations like The Vision Council, more than 70% people don’t know or don’t believe they are at risk for digital eye strain; however, anyone who is in front of a digital screen is vulnerable. Red eyes, twitching eyes, dry eyes, blurred vision, headaches, neck pain, decreased productivity and increased work errors, fatigue from staring at a digital screen, and straining to see small fonts and images are some of the signs and symptoms that occur when experiencing digital eye strain.

“In our fast-paced society, most people use a computer throughout the day while they’re at work, and they also go online to communicate with friends, read books, and even pay bills.”

“It’s just the way we operate in the 21st century. Nevertheless, people can stay digitally connected and also maintain the health of their eyes.”

Optometrists suggests following tips for avoiding digital eye strain:

1. Follow the “20-20-20 rule.” Be mindful of the amount of time that is spent looking at a computer screen without taking a break. Every 20 minutes, take a 20-second break and look at something that is 20 feet away. Looking far away relaxes the focusing muscle inside the eye and reduces eye fatigue.

2. Reduce glare. People often see reflections from objects around their computer on their computer screen. Install an anti-glare screen on the computer monitor to reduce glare on the screen. Cover windows with drapes and blinds, and use a computer hood to block some of the overhead and peripheral light. Get anti-reflective (AR) coating on eyeglass lenses.

3. Work in proper lighting. When looking at a digital screen, the surrounding light should be half as bright as what is typically found in most offices. Try to position the computer screen so windows are on the side (instead of in front or behind) the computer screen. If the interior lighting is a concern, consider reducing the number of fluorescent tubes that are installed above the computer. Also consider turning off the overhead fluorescent lights in the office and use lamps that provide halogen or incandescent lighting, or switch to lower intensity bulbs.

4. Blink often. People tend to blink less often when they look at a computer screen—approximately one third less often as they normally blink—and a lot of the blinking that takes place when looking at a digital screen are only partial lid closures. Blinking less often can cause the eyes to become dry. To reduce the chances of experiencing dry eyes when looking at a digital screen, try this exercise: Every 20 minutes, blink 10 times by closing the eyes very slowly, as if falling asleep. This will moisten the eyes, and it will also help the eyes refocus.

5. Revise the workspace. When working on a computer, people often look back and forth between the computer screen and a printed page, which can cause eye strain. To alleviate the stress and strain on the eyes, put the printed pages on a copy stand that is next to the computer monitor. Make sure the paper on the copy stand is well-lit by using a desk lamp. Poor posture can also lead to problems with clearly seeing a digital screen. Consider purchasing ergonomic furniture where the computer screen is positioned 20 to 24 inches from the eyes. The center of the digital screen should be 10 to 15 degrees below the eyes.

6. Get a regular comprehensive eye exam. Computer users should have eye exams once a year. Before the exam, be sure to measure the distance between the eyes and the digital screen. Share that measurement with the eye care provider, and remember to let the doctor know how often computers and smart phones are used. 

Friday, 14 February 2014

Bielschowsky's head tilt (3 step) test

When a healthy individual tilts their head, the superior oblique and superior rectus muscles of the eye closest to the shoulder keep the eye level. The inferior oblique and inferior rectus muscles keep the other eye level. In patients with superior oblique palsy, the superior rectus muscle’s action is not counteracted by the superior oblique muscles. This leads to vertical deviation of the affected eye when the head is tilted towards the effected eye. However, there is no deviation when the head is tilted towards the unaffected eye because the superior oblique muscle is not stimulated in the effected eye, but rather it is stimulated in the unaffected eye. When there is a discrepancy in ocular deviation based on which way the head is tilted, the patient is diagnosed with unilateral palsy of the superior oblique muscle due to damage in the Trochlear Nerve.

People with superior oblique palsy on one side experience double vision, which is improved or even abolished by tilting the head towards the shoulder on the unaffected side. Tilting the head towards the shoulder on the affected side will make the double vision worse by causing increased separation of the two images seen by the patient.

Fourth Nerve Palsy

A fourth nerve palsy typically causes diplopia that is worse in downgaze; hence, patients almost always report diplopia (or the tendency to close 1 eye) while reading. In some cases, examination of the affected eye reveals limited downgaze in the adducted position, but, in most cases, ocular motility appears grossly normal. Accordingly, it is essential to perform cover-uncover or Maddox rod testing to demonstrate a hypertropia that worsens on contralateral downgaze. Ipsilateral head tilting usually increases the vertical strabismus, and, therefore, patients typically (subconsciously) tilt their head to the opposite side to avoid diplopia.

The Parks-Bielschowsky 3-step test is a time-honored algorithmic approach to identifying patterns of ocular motility that conform to dysfunction of specific vertically acting extraocular muscles. The 3 steps are

1.     Find the side of the hypertropia.
2.     Determine if the hypertropia is greater on left or right gaze.
3.     Determine if the hypertropia is greater on left or right head tilt.

Beyond these 3 steps, it is also useful to determine if the vertical separation is greater in upgaze or downgaze (a fourth step) and check for relative cyclotropia.
The 3-step test is most helpful in determining whether a vertical strabismus conforms to the pattern of a fourth nerve palsy; for example, a right fourth nerve palsy shows right hyperdeviation that worsens on left gaze, right head tilt, and downgaze, with relative excyclotropia of the right eye.

Occasionally, a skew deviation mimics a fourth nerve palsy on the 3-step test but can distinguish itself by nonconformity to these rules. Practically speaking, the specific muscle(s) involved and the etiology of a vertical strabismus not due to a fourth nerve palsy is often not resolved by the 3-step plus fourth step test, because acquired vertical strabismus is often the result of the dysfunction of more than one muscle. In particular, thyroid eye disease, myasthenia gravis, or dysfunction of multiple ocular motor cranial nerves produces a wide variety of nonspecific patterns of ocular motility. The reliability of the 3-step test in identifying patterns of vertical strabismus lessens somewhat over time because of the phenomenon known as “spread of comitance”.
Bilateral fourth nerve palsy should always be considered whenever a unilateral palsy is diagnosed, especially after head trauma. Bilateral fourth nerve palsy presents with:-
  • ·      crossed hypertropia (ie, the right eye is higher on left gaze, and the left eye is    higher on right gaze)
  • ·        Excyclotorsion of 10° or greater (each eye rotates outwardly; best measured with  double Maddox rod testing)
  • ·         a large (≥25 D) V pattern of strabismus
Brazis PW. Palsies of the trochlear nerve: diagnosis and localization—recent concepts. Mayo Clin Proc. 1993;68(5):501–509.

Fourth nerve palsies are often congenital. An anomalous superior oblique tendon, an anomalous site of its insertion, or a defect in the trochlea are now recognized as causes of some congenital fourth nerve palsies; similarly, some cases of presumed congenital fourth nerve palsy are secondary to a benign tumor (eg, schwannoma) of the fourth nerve. Patients are often asymptomatic until the fourth to sixth decades of life, when their vertical fusional amplitudes diminish and diplopia develops. Most patients maintain a chronic head tilt. The long-standing nature of the head tilt can often be confirmed by reviewing old photographs . Patients with a long-standing fourth nerve palsy have a relatively large vertical fusional range (>3 prism diopters).

In patients older than 50 years, isolated fourth nerve palsy is typically caused by micro-vascular ischemic disease, and function always improves and typically resolves within 3 months. The fourth nerve is particularly vulnerable to closed-head cranial trauma due to the unique dorsal midbrain crossing anatomy. In addition, the fourth nerve can be damaged by disease within the subarachnoid space or cavernous sinus.

Diagnostic evaluation for isolated, non traumatic fourth nerve palsy usually yields little information because most cases are congenital, ischemic, or idiopathic. In patients in the vasculopathic age group, a full medical evaluation looking for vascular risk factors, including diabetes, hyperlipidemia, and hypertension is appropriate. Older patients should be followed to ensure recovery. Lack of recovery after 3 months should prompt neuroimaging directed toward the base of the skull to search for a mass lesion. Other possible causes of an acquired vertical strabismus include orbital restrictive syndromes (eg, thyroid eye disease or previous trauma). Skew deviation, partial oculomotor nerve palsy, or myasthenia gravis should be considered in atypical cases.

Tuesday, 11 February 2014

Magnification and magnifiers

Many people with low vision find magnifiers useful to help them do short everyday tasks such as reading their post or instructions on a packet. Magnification increases the retinal image size. For people with a scotoma this may make an object easier to see, because although the retinal image size increases the area of visual loss remains the same size (Figure 1).

Figure 1: A schematic and simplified representation of how magnification can help a person to read short text.

1. Relative size magnification

This is a linear relationship: doubling the size of the object makes the image on the retina twice as large, creating x2 magnification. This form of magnification is usually limited to about 2.5x because of the physical limitations of enlarging an object. Examples of this type of magnification are large print books, watches or timers (Fig. 2).

2. Relative distance magnification

This is also a linear relationship: halve the distance of the object and the retinal image becomes twice as large, creating x2 magnification. For example, viewing the television from 2m rather than 4m gives x2 magnification (Figure 3).

This type of magnification can also be used for near tasks, e.g. bringing print closer to the eye from 40cm to 10cm gives x4 magnification.

Children and young adults can use accommodation to provide this form of magnification, mainly for short duration near tasks. Myopes who take off their glasses can achieve some magnification without the need for accommodation.

Plus lens magnification

A plus lens creates magnification by allowing the person to adopt a closer viewing distance. When the plus lens is placed so that the object viewed is at the anterior focal point of the lens, the object is focused clearly on the retina and accommodation can be relaxed. Most hand and stand magnifiers work on this very simple principle. The plus lens can be close to the eye, in a spectacle lens, or remote from it, in a hand or stand magnifier.

Limitations of plus lens magnifiers

Field of view: Patients often ask for larger magnifiers, hoping that this will

Figure 2: Making things bigger creates relative size magnification.

Figure 3: Moving things closer creates relative distance magnification.

Figure 4: A wide range of hand magnifiers is available, including folding and illuminated versions

increase their field of view. However, as the power of a magnifier increases, the diameter of the lens decreases, due to the weight of the lens and physical constraints in manufacturing. Instead, they should be encouraged to hold the magnifier as close as possible to the eye, thereby increasing the field of view.

Short working distance: Although the distance from the eye to the magnifier can be varied, the distance from the magnifier to the object is often very short, especially with stronger magnification. This makes it difficult to place implements such as a pen or screwdriver under stronger magnifiers, and directing adequate light on to the object can be problematic.

Hand magnifiers

Hand magnifiers are useful for short ‘survival’ tasks such as looking at packets or the dials on a cooker. Most people find them socially acceptable and they are easy to carry in a pocket or handbag. There are countless designs available at low cost in a wide range of powers, and many are internally illuminated (Figure 4). People with hand tremors or grip problems may, however, find them impossible to use.

Stand magnifiers

Stand magnifiers allow the maintenance of a precise magnifier-to-object distance, which is advantageous because of the small depth of focus of plus lens magnifiers. This means they are particularly useful for sustained tasks or where there are physical difficulties, such as tremor. The most commonly prescribed stand magnifiers are internally illuminated because the stand can obstruct light from getting to the object (Figure 5). Some lower-powered stand magnifiers allow tools, such as a pen, to be used (Figure 6). The disadvantage is that they are very bulky.

Spectacle-mounted plus lens magnifiers

The best optical solution to the difficulties of plus lens magnifiers is to mount them in spectacles: this gives the best magnification and greatest field of view. However, the majority of patients do not like any magnifier that focuses less than 25cm from the spectacle plane. For people who are able to accept shorter working distances, spectacle-mounted plus lenses are sometimes tolerated because they give the best magnification and field of view, and allow their hands to be free (Figure 7). Spectacle-mounted low vision aids can be prescribed monocularly or binocularly

Figure 5: Illuminated stand magnifiers are the most commonly prescribed stand magnifiers. 

Figure 6: A pen may be used under some low powered stand magnifiers.

Figure 7: Spectacle-mounted low vision aids allow the person to do tasks that need both
hands free, but only at a short working distance.

if prisms are incorporated to help convergence. Over +10.00DS, the person is unlikely to maintain binocularity. As well as providing magnification, some allow for the correction of refractive errors; high powered bifocal near additions are also available.

3. Real image magnification

Optical magnifying systems are limited to a magnification of about x20. Real image magnification produced electronically is available in much larger magnifications of x50 and over.

Closed circuit televisions

Closed circuit televisions (CCTVs) produce real image magnification electronically using a camera to create a magnified image on a monitor screen. They are usually used for near or intermediate tasks.

In theory, CCTVs should be the solution to all the frustrations of low vision aid users. They can produce high degrees of magnification, contrast reversal and enhancement, zoom facilities and binocularity of the image with none of the postural difficulties of many other magnifiers. In practice, however, they are expensive, quite difficult to use and often bulky. Only a small proportion of the low vision population use CCTVs, and most do so for longer, sustained reading tasks while they use optical low vision aids for short, survival tasks.

The most common type of CCTV is a TV screen mounted above an ‘X-Y’ table where the object is placed or held (Figure 8). Standard CCTVs cost about £1,500 but many models are much more expensive. TV readers are more affordable (£100 to £500). They consist of a hand-held camera which is plugged into the patient’s own television (Figure 9). The magnification is limited, often fixed at one value and dependent on the size of the television screen. Although they are cheap and quite portable, they are difficult to manipulate.

In recent years a number of head mounted CCTVs have been developed, such as the Jordy. The camera and TV screens are mounted in a virtual reality-type headset, and the control box is attached to the belt. These remain very expensive, heavy, difficult to use and cosmetically poor and, as yet, they cannot be used when walking around.

Unlike optical low vision aids, CCTVs are not provided on the NHS. Employment and education services will usually provide them if deemed necessary for the person’s

Figure 8: Various models of CCTV are available. The material to be viewed is placed on an X-Y table.

Figure 9: A TV reader.

Figure 10: A flat field magnifier.

work or schooling. Older people usually have to purchase their own. Many public libraries, some voluntary organizations for blind people and some social services departments have them available for trial use. Most manufacturing companies will let people try the CCTV in their own home for a short period before purchase. Due to the great expense and difficulty involved in using CCTVs, this approach should be strongly recommended to patients. 

Flat field magnifiers

These are single lenses of hemi-cylindrical or hemispherical form, designed to be put flat onto the object (usually text). The thicker the magnifier is in relation to its radius of curvature, the higher its magnification. This is unlikely to exceed x3 because of size and weight. Flat field magnifiers are very useful for children with a visual impairment as they look like a paperweight or ‘crystal ball’ (Figure 10).

4. Angular (or telescopic) magnification

Telescopes and binoculars are very effective in producing magnification for distance, while allowing the person to stay at their chosen distance from a task, such as viewing a street sign or blackboard. They can also be used for near tasks. Their main  disadvantage is restricted field of view. Also, distortion of space and movement perception prohibits walking around while using the telescope. Their use requires considerable manual dexterity, skill and practice, particularly to follow moving objects. Only a very small proportion of people with low vision use them (Figure 11).

Low vision therapy

Although as yet there is no conclusive evidence1, it is thought that people may benefit from training which maximizes the usefulness of low vision aids and vision in daily life. Some rehabilitation workers are trained to provide low vision therapy, which may take place outdoors with distance aids or in the person’s home environment.

Figure 11. Devices that produce angular magnification. A distance Galilean telescope used for TV viewing, terrestrial telescope and a pair of binoculars