Long COVID and contrast sensitivity: early reports and open questions

Long COVID — formally called post-acute sequelae of SARS-CoV-2 infection, or PASC — has been documented in millions of people, across age groups and across severities of the initial infection. The clinical picture is broad: persistent fatigue, post-exertional malaise, cognitive symptoms, dysautonomia, exercise intolerance, sensory changes. Visual symptoms appear in many patient inventories — light feels harsher, screens feel heavier, faces in dim restaurants are work in a way they weren't before, eyes feel tired by lunchtime. They are commonly reported, commonly under-investigated, and the standard eye chart rarely picks them up.

This post is about a specific sliver of the research effort: what is being measured about contrast sensitivity — the part of vision that registers faint patterns — in people with Long COVID. The peer-reviewed literature here is small and early. We are at the case-series and small-cohort stage. What follows is what those early reports show, why the mechanism is plausible, what an at-home measurement can and cannot do for a Long COVID patient, and the open questions still waiting on a few good years of follow-up.

If you have Long COVID and you've been told the chart looks fine but something is still off about how the world looks to you, this post is for you. The community has been dismissed enough that we'd rather say up front: we read your reports as credible data. The measurement is not going to settle anything about your diagnosis. It may give you one more thread to hold next to the others.

What Long COVID is

Long COVID is the umbrella term for symptoms that persist beyond the acute phase of a SARS-CoV-2 infection — usually defined as continuing or appearing more than three months after the initial illness and lasting at least two months, without another explanation. The WHO arrived at this clinical definition through a Delphi-consensus process in 2022 (Soriano, Murthy, Marshall, Relan & Diaz, 2022).¹ The definition deliberately does not require laboratory confirmation of the acute infection — many mild infections were never tested, and a confirmation-required definition would exclude a meaningful share of the population the condition affects.

The clinical picture is multi-system. The international cohort study by Davis and colleagues, surveying 3,762 people with confirmed or suspected Long COVID across 56 countries, catalogued more than 200 reported symptoms across body systems at once — most commonly fatigue, post-exertional malaise, and cognitive dysfunction, but also dysautonomic features, sleep disruption, sensory changes, and visual symptoms (Davis, Assaf, McCorkell et al., 2021).² The cluster persists past six months in a meaningful fraction of patients.

The point worth holding: Long COVID is a clinical syndrome diagnosed by history, not a single-system disease defined by a test. There is no biomarker that confirms it. Whatever a contrast sensitivity reading shows, it does not settle the diagnostic question — that determination lives with a clinician who can take the history, examine you, and consider alternatives.

Visual symptoms in Long COVID patient inventories

When Long COVID cohorts are surveyed about their full symptom list, visual complaints appear in a noticeable share of respondents. The reported symptoms include blurred vision, visual fatigue (especially with screens), light sensitivity, slower visual processing, after-images that linger longer than they should, perceived changes in color saturation, and difficulty with tasks that depend on faint contrast — dim restaurant menus, low-light driving, faces across a room. The exact share varies between cohorts and instruments, but it is consistently large enough to be a routine line in the symptom list rather than a fringe report.

Two patterns are worth knowing. First, the standard eye examination is usually unremarkable in this group. Acuity is typically intact; structural ocular health is typically clean. Yet patients report symptoms the chart does not capture, which is exactly what one would expect from a condition that, if it affects vision, likely does so through the visual processing pipeline rather than through gross damage to the eye. Second, there is overlap with the convergence-insufficiency and accommodation-fatigue patterns seen after concussion — eyes pulling apart with screen reading, near-vision tasks feeling more effortful, words seeming to drift. These sit in the wheelhouse of neuro-optometric rehabilitation rather than primary optometry.

The clinical implication, even before any contrast measurement enters: when a Long COVID patient reports visual symptoms and the standard exam is clean, the question is not whether the symptoms are real but where in the visual pipeline they live.

What's been measured about contrast sensitivity specifically

The peer-reviewed contrast-sensitivity literature in Long COVID is small. What exists is mostly case series and small-cohort studies, not large adequately-powered trials. Treat none of the findings below as load-bearing on its own.

Coco-Martín and colleagues published one of the first dedicated studies (Coco-Martín, Leal-Vega, Alcoceba-Herrero et al., 2023).³ They recruited 25 hospitalised COVID-19 patients (mean age about 54) and 16 age- and sex-matched controls, measuring both achromatic (luminance-based) and chromatic (color-based) contrast sensitivity with the Optopad test. The COVID-19 group showed higher contrast thresholds — reduced sensitivity — at all spatial frequencies on the achromatic test and at most on the chromatic test. At a follow-up at least six months later, most of the differences persisted despite recovery from the acute infection. The cohort was hospital-recruited rather than community-recruited Long COVID, which is why we treat this as suggestive rather than definitive.

Some reports describe reductions concentrated in the mid spatial-frequency band — roughly the 3-to-12 cycles-per-degree region most relevant to face perception and screen reading at conversational distance — though this specific pattern is not yet firmly established in the COVID literature, and we flag it as tentative rather than settled. It is also the band most often reported as affected after concussion, which is interesting but not yet evidence of a shared mechanism. The pattern is, at most, consistent enough to motivate larger work; it is not specific enough to call diagnostic.

The plausibility argument runs through neuroinflammation. The hypothesis is that Long COVID is, at least in some patients, sustained by ongoing inflammation in the central nervous system — microglial activation, cytokine signalling, downstream effects on synaptic plasticity and myelin maintenance. A narrative review in Oxford Open Immunology lays out the case that the cognitive features of Long COVID look mechanistically like a neuroinflammatory phenomenon (Kavanagh, 2022).⁴ The visual system occupies a large share of the cortex, with metabolic demands hard to camouflage when the system is fighting a chronic inflammatory load. If you accept that hypothesis tentatively, predicting subtle changes in contrast processing is not a leap. The "if" remains open.

What has not yet been done: a large, prospectively-recruited Long COVID cohort with serial contrast-sensitivity measurements paired with the rest of the clinical phenotype. We are early.

What an online CSF test can do for someone with Long COVID

Given that framing — small literature, plausible mechanism, real but inconclusive patient reports — what is reasonable to ask of an at-home measurement?

A baseline. A single careful reading on a calibrated setup gives you a number to anchor against. It does not, on its own, mean anything in particular about Long COVID; it establishes a reference point future readings can be compared against.

A trend over time. Long COVID is variable; symptoms fluctuate week to week, sometimes day to day. A curve assembled over months — same device, same lighting, same approximate time of day — is far more informative than a snapshot. The longer the series, the more the day-to-day noise washes out and the signal you care about — the trend — becomes legible.

One thread to bring to your care team. If you have access to a Long COVID clinic, a neurologist, or a neuro-optometric rehabilitation specialist, a curve over time is something a clinical conversation can be built on. If you don't have that access, sharing it with your primary care clinician is reasonable. Asking for a referral to neuro-optometry for vision symptoms specifically — the Neuro-Optometric Rehabilitation Association maintains a referral directory — is often more productive than a generic optometry follow-up, because neuro-optometric specialists routinely work with patients whose chart looks fine and whose visual function does not.

You are collecting data alongside your experience, not asking the data to settle whether the experience is real.

What it cannot tell you

Note. An online contrast sensitivity test is a screening and tracking measurement. It is not a diagnostic instrument and it is not, in particular, a Long COVID test.

It does not diagnose Long COVID. The diagnosis is a clinical one based on history and the exclusion of alternatives, and no home test can substitute for that.

A reduced contrast sensitivity reading does not confirm Long COVID. The same pattern of reduction can come from refractive error, dry eye, an outdated glasses prescription, fatigue, certain medications, early cataract, post-concussion changes, multiple sclerosis, glaucoma, age-related macular degeneration, diabetic retinopathy, and a long list of other conditions. The test cannot disambiguate. A clinician can.

A normal contrast sensitivity reading does not rule out Long COVID. The condition is multi-system; visual symptoms, even when present, may live in oculomotor or accommodation function rather than contrast — and a CSF measurement does not probe those. Many people with Long COVID will have unremarkable contrast sensitivity, and that finding is consistent with the diagnosis.

A single result is a snapshot. Test-retest variation is real even on the most carefully validated clinical instruments⁵ — and a consumer-screen test sits in a noisier regime than the clinical ones do. A trend over multiple sessions on the same setup is far more informative than any single reading.

The measurement is here to give you data alongside your experience. It is not here to adjudicate whether your experience is valid. It is — because you are experiencing it.

Practical

A few concrete things you can do this month.

Take the test on a stable day. Long COVID has crash days and stable days; the difference can dominate the measurement noise. For a baseline, pick a representative stable day — not your worst, not your best — and test in the lighting and at the device you typically use. If you keep a post-exertional malaise or pacing diary, note the day's status alongside the result.

Retake monthly, and on crash days if you can. A monthly cadence is enough to surface trend without becoming a chore. Adding occasional "I'm having a worse week" readings — annotated as such — sometimes reveals whether your visual layer tracks your overall picture or runs independently.

Pair the curve with the rest of your record. A small line in your symptom journal — "took CSF today, log CS ≈ X, brain fog 6/10, slept 7 hours" — is enough. The journal is the context the curve lives inside. Bring both to appointments.

Pace, don't push. This is the rule across Long COVID and it applies to the test too. If a measurement session would push you into a crash, it is not worth the data point. Reschedule.

Read the neighbours. For the conceptual background, the primer on what contrast sensitivity measures is the place to start. The post-concussion vision changes piece covers the same mid-band pattern in a better-studied condition, and vision and brain fog sits closest to the Long COVID symptom picture.

What we don't yet know

The honest list, written as questions because that is what they still are.

Is mid-frequency contrast sensitivity loss specific to Long COVID, or is it a general post-viral fingerprint? The pattern overlaps with what is reported after concussion, in some MS cohorts, and possibly in other post-viral states. Specificity is what a larger comparative study would establish.

Does contrast sensitivity recovery track symptom recovery? Anecdotally, some patients report their visual fog easing as their broader picture improves. Whether the curve confirms that — and how tightly the two are coupled in time — is open.

Are there Long COVID subgroups with different visual phenotypes? Long COVID is increasingly understood as several overlapping syndromes. People with predominantly neurological features may show different visual patterns than people with predominantly cardiopulmonary or dysautonomic features. The published cohorts are small enough that subgroup analysis is mostly speculative.

Do any treatments that help Long COVID also move the contrast curve? If pacing, rehabilitation, or autonomic-stabilising medication reliably moves the curve, the curve becomes a useful objective endpoint. If nothing does, it remains a descriptive measurement.

How much of the visual symptom load lives in contrast versus elsewhere in the visual pipeline? Convergence, accommodation, saccadic control, vestibular-visual integration — none of those show up cleanly on a CSF measurement, and several are routinely affected after viral and inflammatory insults. Comprehensive neuro-optometric examination remains the gold standard; CSF is one slice.

None of these questions invalidates taking the test. They define what the test is and what it isn't, and place it inside a research picture that is still being assembled.

Take the test

Take a baseline. Retake monthly. Build a curve. Bring the trend, your symptom journal, and your questions to your care team.

Take the test now. Three minutes today, three minutes next month. The first is a snapshot. The second is the start of a comparison only you can make.