Double Vision Is Common in Parkinson’s, Large Study Finds

Double vision (diplopia) is common in people with Parkinson’s — affecting up to an estimated 30% of patients — and is linked to both motor and non‐motor disease symptoms, a new large-scale, longitudinal study has found.

The study, “Prevalence and risk factors for double vision in Parkinson disease,” was published in the journal Movement Disorders Clinical Practice.

Visual impairment is reported by some Parkinson’s patients, with one of the most common complaints being double vision. However, the investigators noted that clinical study groups are “prone to under-ascertainment, as neurologists may not be comfortable addressing visual symptoms, and patients frequently do not disclose non-motor symptoms such as double vision unless specifically asked.”

Thus, the condition’s prevalence and risk factors remain unknown.

To fill this knowledge gap, researchers at the University of Pennsylvania examined the prevalence and risk factors for double vision in a large-scale electronic survey conducted between March 2015 and June 2020.

According to researchers, because diplopia is easily treatable, “characterizing the burden and risk factors in PD [Parkinson’s disease] is essential for developing screening and treatment guidelines.”

The survey included 26,790 patients with Parkinson’s and 9,257 people without the disease who served as controls. The patients had a mean age of 67, and 44.6% were female, while the healthy controls had a mean age of 89.9 and included 76.6% women.

The participants all had taken part in the ongoing Fox Insight Study, an online clinical study launched by the Michael J. Fox Foundation that aims to collect large amounts of self-reported data about the health experiences of adults with and without Parkinson’s.

To assess the prevalence of double vision, patients received questionnaires and patient-reported outcome assessments every 90 days, or about every three months. Each survey contained different validated patient-reported outcome measures and Parkinson’s specific questionnaires, such as the Movement Disorders Society Unified Parkinson’s Disease Rating Scale (MDS-UPDRS) part II, which assesses motor aspects of experiences of daily living. Other surveys were the Non-Motor Symptom Questionnaire (NMS-Quest), which measures the presence of non-motor symptoms, and the Penn Parkinson’s Daily Activity Questionnaire (PDAQ-15), used to assess cognitive instrumental activities of daily living.

Patients with Parkinson’s — who had a mean disease duration of 3.6 years — were followed longitudinally, or over time, and completed a median of five questionnaires. The number of completed questionnaires ranged from two to seven.

At the study’s start, the prevalence of double vision in Parkinson’s patients was significantly higher than it was among the healthy controls (18.1% vs. 6.3%). Additionally, more than a quarter (28.2%) of all Parkinson’s patients reported double vision at least once during the study, compared with 9.1% of controls.

The results of the survey also showed that patients who reported double vision were 1.32 times more likely to be ages 65 or older, female (1.08 times), non-white (1.38 times), have greater disease duration (1.61 times), and report greater motor, non‐motor, and daily activity limitations.

When adjusting the data for age — over 65 versus under 65 — race, sex, disease duration, and motor and non-motor symptoms, the association between double vision and Parkinson’s indicators of disease severity remained significant, the researchers said. They noted the strongest link was for non-motor symptoms, at 1.54 times more likely.

“[Parkinson’s disease] patients should be screened for visual symptoms in addition to other non-motor symptoms,” the researchers concluded.

The study had several limitations, the scientists noted, including limited generalizability, the underestimation of double vision in the controls, the fact that participants are not assessed in person, the lack of information regarding double vision frequency or severity, and the inability to distinguish between the condition occurring in one eye (monocular) or both eyes (binocular).

Nonetheless, these findings demonstrated “diplopia is common and associates with motor and non-motor severity in PD [Parkinson’s disease],” according to the researchers.