Blood going undetected in urine and a lack of screening programs could explain the association, according to one expert.
Sticks and stones may break my bones, but an inability to differentiate colours can also hurt me.
Bladder and colorectal cancers are often identified after patients notice blood in their urine or stool. But what happens in people with visual impairments that prevent them from seeing these red spots or flashes of blood?
Do these individuals have a greater risk of delayed detection and diagnosis, which subsequently puts them at risk of more severe disease and death? These are the questions posed by a recent exploratory study from America, published in Nature Health.
“There is [enough] evidence to suggest that patients with colour vision deficiency (CVD) and cancers that initially present with blood in urine or stool may delay care-seeking due to lack of recognition of the blood… [allowing] the cancer to invade more tissue, leading to more advanced cancer histology at the time of presentation,” the study authors wrote.
“However, it remains an open question whether this translates to differential mortality among patients with CVD as compared to patients without CVD.”
To test this hypothesis, researchers identified 135 CVD patients with bladder cancer and 187 CVD patients with colorectal cancer from the TriNetX electronic health record network.
These patients were matched with cancer patients without CVD in a 1:1 fashion to control for the impact of age, age at cancer diagnosis, sex, race, ethnicity and systemic comorbidities (e.g., hypertension, diabetes). The average follow-up time across all patients was 4.37 years.
After finalising the two matched cohorts, the researchers performed a survival analysis to examine whether there were differences in survival probability between patients with and without CVD.
Patients with both bladder cancer and CVD had a lower survival probability compared to people without CVD. Specifically, patients with bladder cancer and CVD had a 52% increase in the risk of 20-year mortality compared to people with normal colour vision. However, there was no association between CVD and survival in the colorectal cancer patients.
Professor Masahito Jimbo, head of the University of Illinois’ Department of Family and Community Medicine, offered a potential explanation for the findings in an accompanying commentary.
“Although patients with CVD may experience similar difficulty in detecting blood in stool (which can be a manifestation of colorectal cancer), this is less relevant owing to the established colorectal cancer screening strategy, which typically involves colonoscopy or a stool-based blood test starting at age 45.
“Thus, the inability to detect blood in stool may not be as important, as patients are typically screened for colorectal cancer before they become symptomatic.”
The current study was limited by its use of de-identified, aggregated medical record data and administrative coding system of medical diagnoses – meaning that there may have been coding errors or patients who were missed if their CVD was not officially diagnosed and noted within their medical records.
“This may have led to misclassification bias, in which the effect size would be diluted because patients with undiagnosed CVD may have been included in the cohort without CVD. So, the effect size may be larger in reality than the analysis calculated,” the researchers noted.
Related
Professor Jimbo also noted a potential limitation regarding the definition of CVD (“it is a heterogenous disorder and it is not clear whether every patient with CVD has a similar inability to recognise blood in their urine”) but felt the authors should be commended for their efforts.
“They also suggest a study to evaluate whether screening populations with CVD for bladder cancer leads to improved survival. The fact that bladder cancer is more prevalent in men, who also have an increased prevalence of CVD, makes such a study potentially important. If it is shown that screening patients with CVD for bladder cancer indeed saves lives, guidelines should be modified to tailor such screening to this select population,” he concluded.
Figures from the Australian Institute of Health and Welfare’s Cancer data in Australia report predicted there would be 14,784 and 3345 persons diagnosed with colorectal and bladder cancer, respectively, in 2025. These predictions suggested that the number of men and women diagnosed with colorectal cancer would be reasonably similar, but that there would be more than three times as many men than women diagnosed with bladder cancer.
Self-reported data from the Australian Bureau of Statistics’ 2017-18 National Health Survey estimated over 500,000 are colour blind, with the majority assumed to be males.
