The Crowding Phenomenon is a term used to describe difficulty or inability to discriminate visual acuity chart letters when they are presented next to each other in a row. Although this phenomenon may be experienced by anyone, it is most often characteristic of patients with amblyopia. Amblyopia is a relatively common vision disorder that occurs when the nerve pathway from one eye to the cortical area of the brain does not develop during childhood. Strabismus (eye misalignment) is the most common cause of amblyopia; however a significant difference in refractive error in the eye, uncorrected high refractive error, or vision deprivation as seen with ptosis and congenital cataracts can also interfere with normal cortical visual development.
What does this mean to the ophthalmic technician? Studies have proven that a child with amblyopia will test higher on a visual acuity test when shown single letters versus a full line of letters. Let’s look at a real-life clinic scenario:
Andrew, a 5 year old child, is referred to the eye clinic by the school nurse after performing poorly on a vision exam. Jill, an ophthalmic technician, tests Andrew’s vision by showing him single Snellen letters and records his vision as 20/20 OD and 20/40. The doctor prescribes a patching regimen and schedules a 2 month return visit. Brad is the ophthalmic technician who performs the pre-testing when Andrew returns. He tests the vision by showing Andrew full lines of Snellen letters and records his vision as 20/20 OD and 20/60 OS. Has the vision actually decreased? We have no way of knowing since the original test was performed with single letters and the follow-up test was performed with a full line of letters.
Using single letter testing on a patient with amblyopia can misled the doctor into believing that the vision is better than it really is. For this reason, visual acuity should always be measured with an entire line of letters. If you wish to note what the vision is with single letter testing also, record the vision as: OD: 20/60, single letter 20/40.