Saturday, January 31, 2009

Visual Complaints by Patients with Parkinson's Disease

I've gotten many questions over the years about changes in vision that bother many PD patients. So I prepared a brief article discussing the visual changes common in this illness. We recently published the article in the newsletter "PD UPDATE".

Many patients with PD at some time or other notice that their vision is declinng. Of course this happens normally with aging, but there are some specific complaints that patients have that are more likely to occur during the course of Parkinson’s disease. Some patients may experience episodic blurring of vision and others may have transient double vision where objects are seen in duplicate, most often side by side. Some patients notice difficulty with reading either because they cannot focus on near objects or because they cannot scan the page easily. They may tire from reading much more easily than in the past. Patients may also develop problems driving, especially during twilight or at night. Each of these symptoms or complaints has an explanation in the context of the process of the illness.

First of all, blurring of near vision when objects are brought close to the eyes is a normal process that occurs with aging due primarily to a decline in the ability of the lens to change shape automatically (“accommodate”) in the language of the ophthalmologist). However this can be terribly aggravated by certain medicines used to treat PD such as bentzropine (Cogentin) or trihexyphenidyl (Artane). Episodes of double vision usually are due to the asymmetric fatigue of the extraocular muscles of the eye. In normal vision, the eyes are ‘yoked’ to maintain conjugate gaze, like a pair of horses that are harnessed together and controlled by subtle adjustment of the reins. There are sets of muscles for each eye that are responsible for moving the eyes together as one unit, in perfect coordination. Since PD develops asymmetrically, the muscles on one side of the body may be more stiff and slow than the muscles on the other side. The eye muscles likewise are susceptible to the slowness and stiffness that affects other muscles in PD. So when muscles of one eye are slower or stiffer than the other eye, the result is double vision. When it is slight and long standing, the brain adjusts for it. But when the asymmetry in muscle stiffness is great enough there is definite double vision. This may be corrected by the anti-PD medication (levodopa), but as the medicines wear off, some patients may experience a transient double vision because the eyes are no longer yoked together, much as the chariot might veer to one side when one horse becomes more fatigued than the other or when the reins of one horse are pulled more tightly than the reins of the other horse. Difficulty in scanning a page while reading is similarly related to easy fatiguability of the eye muscles. Many times this is experienced as a transient fatigue and may be related to wearing off of levodopa effects. Problems with driving at night or during twilight hours is related to the difficulty with peripheral vision and discrimination of shades of grey. Although a PD patient may have normal acuity tested in the usual way and corrected with the appropriate lenses, he may still have difficulty discriminating shades of grey. This defect, measured by specific tests that assess “contrast-sensitivity”, is known to occur in PD patients. Loss of contrast sensitivity in PD patients has been related to the loss of dopamine neurons in the retina (the “visual receptor screen” at the back of the eye) which transduces light information into nervous signals to be interpreted by the brain as an image. Hence, contrast sensitivity can transiently improve with levodopa treatment. Finally there is a problem of higher visual processing that is more subtle. When specific neuropsychological tests of cognitive function were administered to many PD patients, it became evident that a significant proportion of patients scored poorly on problems that required perception of objects in three dimensional space and their relatonship to each other. It is not clear what the functional consequences of this deficit is in everyday life, but it would not be surprising to associate it with difficulties in driving a car.

To solve the problem of blurry vision it is important to make sure it is not related to the use of drugs like Artane or Cogentin. If double vision is a persistent problem, then a good exam by a neuro-ophthalmologist (a specialist of the visual nervous system) should be performed. The specialist may recommend special lenses with prisms to correct the double vision. The most important step a patient can take after seeing a good ophthalmologist and/or neurophthalmologist is to make sure the anti-PD medicines are properly adjusted since many of the visual complaints might
improve with a better schedule of medication. And if a patient is prone to automobile accidents, the most prudent advice is to avoid driving.

written by J. Sanchez-Ramos
copyright PDUPDATE.