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16th Jan 2010.

Eye care for Parkinson’s

Doctors need to look harder into the eyes of Parkinson’s patients.

That’s what I have learned from an email that I received from Mr. Hero Teo (www.heroteo.com), a good friend and highly inquisitive Parkinson’s patient:

Dear Dr. Chew,

I know a Parkinson’s patient who has double vision (a single object is seen as “double”). As a result, he has difficulty in driving, reading newspaper and watching television. He could not read even with the use of special glasses, hence, he prefers to use the magnifying lens to read or see objects at close range.

I myself have uncomfortable sensation due to dry eyes. My eyes feel tired easily despite the frequent use of artificial tears, which is supposed to moisten the surface of my eyes. I stop driving at night or rainy as I have poor vision.

I think the dryness of my eyes is caused by reduced blinking rate, which is one of the common symptoms of Parkinson’s.

Are all these visual problems related to Parkinson’s?

People are usually concerned about how Parkinson’s can affect the muscles of the face, throat, voice, limbs and bowel. Somehow, despite being one of the five basic senses, the eyesight of Parkinson’s patients seldom receives attention. The direct effect of Parkinson’s on the eyes is often overshadowed by more obvious features such as hand tremor and stiffness of the limbs. Fortunately, Mr. Hero Teo opened my eyes to the visual disorders in Parkinson’s.

In general, when Parkinson’s patients complain of visual symptoms, the common underlying causes are medical disorders which are not related to Parkinson’s, such as cataract, glaucoma (increased pressure in the eyes), diabetes, etc. As such, Parkinson’s patients would usually consult the eye specialists first whenever they develop any visual symptom. Among these patients who have visited the eye specialists, some are told to have “normal” eyes (there is no problem with your lens), while the others have persistent visual symptoms despite treatment of obvious eye disorders (e.g. cataract). In both groups of patients, the visual disturbances are actually due to Parkinson’s itself.

As the visual disorders due to Parkinson’s originate in the brain, and not the eyes, it is not surprising that some of these patients have been told to have “normal” eyes. In simplest terms, the manifestations of Parkinson’s are the result of interruption in the flow of signal from the brain to the eye muscles.

The effect of Parkinson’s on the eyes should be given emphasis because an important study has shown that Parkinson’s can frequently cause visual symptoms even during the early stage of illness, before the patients receive any Parkinson’s medications (Biousse V et al, 2004).

It should also be remembered that the Parkinson’s medications can affect vision. Anti-cholinergic medications (e.g. Benzhexol or Artane) are known to worsen glaucoma.

It is necessary to address the visual symptoms of Parkinson’s as they may further impair the daily activities of patients, especially those who are still driving.

There are several abnormalities of the visual system which are directly caused by Parkinson’s, as described below.

a) Impaired visual function

Decreased colour discrimination (e.g. blue / green) and decreased contrast sensitivity have been demonstrated in Parkinson’s patients (Price MJ et al, 1992). Contrast sensitivity is the visual ability to see objects that may not be outlined clearly or that do not stand out from their background. When there is decreased contrast sensitivity, patients have difficulty making out the shape of light-coloured objects on light background, e.g. reading gray letters on a white background. Due to decreased contrast sensitivity, some Parkinson’s patients have difficulty driving at night or in the late evening (as illustrated in Mr. Hero Teo’s email). Adequate lighting of the environment can help overcome the colour vision and contrast sensitivity.

The improvement of colour vision and contrast sensitivity with Parkinson’s medications (Buttner T et al, 1992; Buttner T et al 2000) suggest that dopamine plays a role in visual function. In certain cells in the retina (the innermost layer of the eye), dopamine has been shown to play a role in their function. In fact, post-mortem studies on Parkinson’s patients showed reduced level of dopamine in the retina (Harnois C, 1990).

Visuo-spatial deficit refers to a condition whereby patients have difficulty in estimating the amount of space and distance between objects around them. For example, they may not be able to walk along a narrow passage, e.g. a corridor with chairs at the side. Driving will be more difficult for these patients, especially when they need to park their vehicle.

b) Impaired eyelid function

Compared with normal people, Parkinson’s patients have reduced rate of eye blinking, which is related to the deficiency of dopamine in the brain. The reduced eye blinking is responsible for the “stary” eyes which are characteristic of Parkinson’s. In addition, reduced eye blinking further aggravates the dryness of eyes caused by decreased production of tears, as eye blinking helps to maintain the wetness of the eyes. Dryness of eyes leads to symptoms such as eye irritation, burning sensation and blurring of vision (as illustrated in Mr. Hero Teo’s email). Artificial tears can be helpful in this situation.

Blepharospasm is uncontrolled and involuntary contraction of the muscles of both eyes, which leads to closure of the eyes. It is an eye disorder which is associated with Parkinson’s. In severe cases, the eyes are completely closed. Blepharospasm is treated with injection of botulinum toxin (Botox) into the affected eye muscles. The toxin causes paralysis of the affected muscles, and thus, reduces the muscle spasm.

Apraxia of eyelid opening (AEO) is a condition whereby there is difficulty in elevating the upper eyelid. Following spontaneous eye blinking, the patient is unable to open the eyes, to the extent that some of them have to pry open their eyes using their hands. For the ignorant observers, these patients seem to look “sleepy”. AEO can be disabling as it interferes with the daily activities such as watching television and working with computer. One of my Parkinson’s patients, who use to surf the Internet everyday, eventually gave up on the computer due to this eye disorder. AEO is treated with injection of botulinum toxin (Botox) into the upper eyelid muscles.

c) Impaired eye movements

Difficulty in convergence refers to inability of both eyes to turn inward and focus on a near object. This explains the common complaint of difficulty reading among Parkinson’s patients (as illustrated in Mr. Hero Teo’s email). When they try hard to read, it leads to eyestrain, double vision and blurring of vision. The anti-cholinergic medications (e.g. Benzhexol or Artane) can also interfere with the convergence mechanism of the eyes.

Treatment options for treating difficulty in convergence are lenses and prisms. For such treatment, it is advisable to consult the eye specialists.

Some Parkinson’s patients also have problem with fast movement (saccadic) of the eyes such as difficulty in locating a fast-moving object, e.g. looking out for vehicles passing by or tennis balls. This is attributed to the difficulty in initiating the contraction of eye muscles, which is also seen at the limb muscles. Others have problem with the slow movement (smooth pursuit) of the eyes such as difficulty in tracking or following moving objects, e.g. moving across the page while reading.

The impairment in fast and slow eye movements, which tends to be more pronounced at the advanced stage of Parkinson’s, results in double vision. In general, these eye movement disorders respond to Parkinson’s medications (Gibson SN, 1985; Rascol O et al, 1989).

It has to be emphasized that the impairment in eye movements in Parkinson’s is relatively milder than in parkinsonism-plus syndrome (i.e. Progressive Supranuclear Palsy). In the latter condition, the restriction in eye movement is already quite pronounced at the onset of illness, and by the advanced stage both eyes are unable to move at all.

d) Visual hallucination

Please refer to the Mental complications in the “Medical Facts” section in this website.

Conclusions

The range of visual symptoms experienced by Parkinson’s patients is diverse. Parkinson’s itself, and also co-existing medical disorders such as cataract, glaucoma and diabetes are responsible for these visual manifestations. Combined effort of neurologists, eye specialists and psychiatrists is needed to provide the best possible eye care for Parkinson’s patients, in order to improve their quality of life.

References

Biousse V, Skibell BC, Watts RL, et al. Ophthalmologic features of Parkinson’s disease. Neurology 2004; 62: 177–80.
Buttner T, Kuhn W, Patzold T, et al. L Dopa improves colour vision in Parkinson’s disease. J Neural Transm 1994; 7: 13–19.
Buttner T Muller T, Kuhn W. Effects of apomorphine on visual functions in Parkinson’s disease. J Neural Transm 2000; 107: 87–94.
Gibson SN, Pimlott RM, Kennard C. Ocular motor and hand tracking in Parkinson’s disease and their response to therapy. Abstract. J Neurol Neurosurg Psychiatry 1985; 48: 605.
Harnois C, Di Paolo T. Decreased dopamine in the retinas of patients with Parkinson’s disease. Invest Ophthalmol Vis Sci. 1990; 31(11): 2473-5.
Price MJ, Feldman RG, Adelberg D, et al. Abnormalities in color vision and contrast sensitivity in Parkinson’s disease. Neurology 1992; 42: 887–90.
Rascol O, Clanet M, Montastruc JL, et al. Abnormal ocular movements in Parkinson’s disease: evidence for involvement of dopaminergic systems. Brain 1989; 112: 1193–214.