The choroid is a thin, blood-vessel-rich layer that sits between the retina and the tough outer wall of the eye (the sclera). Its main job is feeding the outer retina with oxygen and nutrients, but it also acts as a heat sink, helps regulate the eye’s focal length, and even serves as a drainage pathway. Because the choroid is so densely vascular and sits directly behind the light-sensing tissue, problems here can quietly erode vision long before a person notices symptoms.
What the Choroid Looks Like Up Close
If you could peel back the retina and peer behind it, you would find the choroid as a dark, spongy sheet roughly 200 to 400 micrometers thick at its thickest point beneath the center of the macula. Its color comes from the enormous concentration of blood vessels and the pigment melanin woven through the tissue. Anatomists divide it into distinct layers, starting from the retina side and moving outward toward the sclera.
The innermost boundary is Bruch’s membrane, a paper-thin barrier that separates the retinal pigment epithelium from choroidal blood flow. Just outside Bruch’s membrane sits the choriocapillaris, a dense mesh of tiny capillaries arranged in a single, nearly flat sheet. This is where the actual nutrient and oxygen exchange with the retina happens. Farther out are two layers of progressively larger blood vessels named after the anatomists who described them: Sattler’s layer (medium-sized vessels) and Haller’s layer (the large arteries and veins). Finally, the suprachoroid is the outermost zone, a transitional space where the choroid meets the sclera.
1PubMed. The structure and function of the human choroidIn birds, the choroid contains large fluid-filled spaces (lacunae) that can swell or shrink dramatically, changing choroidal thickness by up to four-fold over a matter of days. Human choroids are less dramatic, but the principle holds: the choroid is not a fixed structure. It has non-vascular smooth muscle cells, concentrated behind the fovea, whose contraction can thin the tissue. It also has its own intrinsic neurons that receive input from the sympathetic and parasympathetic nervous systems, giving it a surprising degree of local control over blood flow and thickness.
2PubMed Central. The multifunctional choroidHow the Choroid Keeps the Retina Alive
The retina has two separate blood supplies. The inner layers get blood from the retinal arteries, but the outer layers, including the photoreceptors that actually detect light, depend entirely on the choroid. Classic experiments in cats and monkeys showed this elegantly: when the retinal arteries were blocked, having the animal breathe pure oxygen could still return inner-retina oxygen levels to normal, because oxygen diffused inward from the choroidal circulation. But when intraocular pressure was raised high enough to also crush choroidal blood flow, no amount of supplemental oxygen could compensate.
3PubMed Central. Retinal oxygenation via the choroidal circulationThis dual supply system means the choroid is not just “extra.” It is essential backup plumbing for the inner retina and the sole supply line for the outer retina. The photoreceptors are among the most metabolically demanding cells in your body. They burn through oxygen at a rate that would leave most tissues starved, and the choriocapillaris is engineered to keep up.
The Choroid as a Cooling System
The eye focuses incoming light onto the macula, and all that concentrated radiant energy generates heat. Without a way to carry that heat away, the delicate photoreceptors and retinal pigment epithelium could overheat. The choroid’s high blood flow serves exactly this purpose. Research in monkey eyes showed that when ambient illumination was increased, temperature at the retina climbed, but the choroidal circulation dissipated much of that heat. The researchers suggested this cooling function may actually be one of the primary reasons choroidal blood flow is so much higher than what you would expect from metabolic demand alone.
4American Journal of Ophthalmology. Choroidal blood flow as a heat dissipating mechanism in the maculaA more recent thermal model of the human retina confirmed this picture, finding that conduction through tissue and choroidal blood perfusion together dominate heat removal from the retina under normal conditions. Radiation and convection contribute almost nothing by comparison. In this model, heat conducts through the retinal layers and then gets carried away systemically by the choroidal blood flow, essentially making the choroid a heat sink for the most light-exposed part of the eye.
5Physical Biology. Steady-state thermal homeostasis model of heat generated and dissipated in the human retinaThe Choroid Changes Thickness Throughout the Day
Your choroid is not the same thickness at 8 a.m. and 8 p.m. Studies using high-resolution imaging have shown a consistent daily rhythm. One study of healthy subjects found mean choroidal thickness swung between a maximum of about 372 micrometers and a minimum of about 341 micrometers across the day, an amplitude of roughly 34 micrometers.
6PubMed. Diurnal variation of choroidal thickness in normal, healthy subjects measured by spectral domain optical coherence tomographyA separate study tracking choroidal thickness over a full 24-hour period in healthy young adults found a similar pattern, with the thickest readings occurring between roughly 2 and 4 a.m. and the thinnest during the daytime.
7Scientific Reports. Diurnal changes in choroidal optical coherence tomography angiography indices over 24 hours in healthy young adultsThis matters clinically because if a doctor is measuring your choroid to track a disease, the time of day the scan is taken can shift the number by enough to mimic or mask a real change. Age and the length of the eyeball also influence how much the choroid swings, so these factors need to be accounted for when comparing measurements across visits.
How the Choroid Responds to Focus and Myopia Signals
One of the more surprising discoveries in recent decades is that the choroid actively responds to whether the eye is in focus. When researchers imposed optical blur on the eyes of schoolchildren, the choroid thickened or thinned within hours depending on the type of blur. Myopic defocus (where the image falls in front of the retina, as when you wear reading glasses that are too strong) caused relative thickening of the choroid, while hyperopic defocus (image behind the retina) caused thinning.
8PLOS ONE. Optical Defocus Rapidly Changes Choroidal Thickness in SchoolchildrenAdult eyes show the same pattern. In one experiment, two diopters of myopic defocus produced a rapid increase in subfoveal choroidal thickness, reaching roughly 20 micrometers of thickening within an hour. Hyperopic defocus caused a comparable thinning over the same period.
9PubMed. Effect of retinal image defocus on the thickness of the human choroidThis thickening-and-thinning response is now being studied as a possible marker for how well myopia-control treatments are working. Some researchers are tracking choroidal thickness changes in children wearing multifocal contact lenses to see if short-term choroidal thickening predicts long-term slowing of eyeball growth.
10PubMed Central. Choroidal response to optical defocus as a potential surrogate marker for myopia control effectThe underlying logic is straightforward: the choroid sits between the sclera and the retina, so changing its thickness literally shifts the retina forward or backward by a tiny amount, nudging the focal plane. It is a fine-tuning mechanism layered on top of the more permanent changes in eye length that drive myopia. Whether the choroid’s rapid adjustments are a cause or merely a marker of myopia-related changes is still debated, but the speed of the response is striking.
How Doctors See the Choroid
For a long time, the choroid was difficult to image in living patients. Standard optical coherence tomography (OCT) scans focused on the retina, and the choroid appeared as a blurry shadow beneath it. That changed with the development of enhanced depth imaging OCT (EDI-OCT), a technique that repositions the imaging device to bring the deeper choroidal tissue into sharper focus. A pilot study showed that this approach could reliably measure macular choroidal thickness in normal eyes and detect its relationship to age.
11PubMed. A pilot study of enhanced depth imaging optical coherence tomography of the choroid in normal eyesSubsequent work confirmed that EDI scans significantly increased imaging depth compared with conventional scans, making it possible to visualize structures deeper within the choroid.
12PubMed Central. Optimizing visualization in enhanced depth imaging OCT in healthy subjects and patients with retinal pigment epithelial detachmentEDI-OCT has become a standard clinical tool, but it is not perfect for every scenario. In cases of choroidal inflammation, for example, indocyanine green angiography (ICGA), which uses a fluorescent dye that highlights choroidal blood flow, can detect disease recurrences and treatment responses that EDI-OCT misses entirely.
13PubMed. Enhanced depth imaging is less suited than indocyanine green angiography for close monitoring of primary stromal choroiditisEDI-OCT is also being used to study choroidal changes in inflammatory eye disease, giving clinicians an objective way to track whether the choroid is swollen or thinning over time.
14PubMed Central. Examining the choroid in ocular inflammation: a focus on enhanced depth imagingIncreasingly, artificial intelligence is being applied to automate choroid measurements from OCT scans. Deep-learning algorithms can segment the choroid’s boundaries with an average error in the range of 7 to 14 micrometers compared to manual tracing, and the automated measurements reliably reproduce the known finding that choroidal thickness is reduced in highly myopic eyes.
15PubMed Central. Application of Artificial Intelligence and Deep Learning for Choroid Segmentation in Myopia 16Scientific Reports. Automatic choroidal segmentation in OCT images using supervised deep learning methods
Conditions That Affect the Choroid
Wet Age-Related Macular Degeneration
Wet (neovascular) AMD is one of the most serious choroidal conditions. In this disease, abnormal new blood vessels grow from the choroid through Bruch’s membrane and into the space beneath or within the retina. These fragile vessels leak fluid and blood, which detaches and damages the retinal pigment epithelium and photoreceptors, leading to progressive central vision loss.
17PubMed Central. Choroidal Neovascularization: Mechanisms of Endothelial Dysfunction 18PubMed Central. Neovascular Macular Degeneration: A Review of Etiology, Risk Factors, and Recent Advances in Research and Therapy
The standard treatment involves injections of anti-VEGF drugs directly into the eye, which block the growth signal driving the abnormal vessels. Research is ongoing into additional pathways, including work in mouse models suggesting that thrombin signaling, through factors like CTGF and VEGF, contributes to choroidal neovascularization and that targeting this pathway can accelerate lesion repair.
19PubMed Central. Dabigatran and Wet AMD, Results From Retinal Pigment Epithelial Cell Monolayers, the Mouse Model of Choroidal Neovascularization, and Patients From the Medicare Data BaseCentral Serous Chorioretinopathy
Central serous chorioretinopathy (CSCR) is a condition in which fluid accumulates beneath the retina, causing a blister-like detachment that distorts central vision. It tends to affect younger adults, and stress and corticosteroid use are recognized triggers. Despite decades of study, the precise chain of events remains unclear. What researchers agree on is that choroidal hyperpermeability (excessive leakage from choroidal vessels) and dysfunction of the retinal pigment epithelium are the two cornerstones.
20PubMed Central. Pathophysiology of central serous chorioretinopathy: a literature review with quality assessmentA recent proposal suggests that the thickened choroid in CSCR creates increased hydrostatic pressure against the retinal pigment epithelium, eventually overwhelming and reversing the RPE’s normal pump function so that fluid is actively driven into the subretinal space rather than removed from it. This “pump reversal” model distinguishes acute CSCR, where focal leaks actively push fluid outward, from chronic CSCR, where degenerative RPE changes allow fluid to ooze through more passively.
21PubMed. Pathogenesis of Central Serous Chorioretinopathy and the Link Between Choroidal Hyperpermeability and Retinal Pigment Epithelium Pump ReversalPachychoroid Spectrum Disorders
CSCR is now understood to be part of a broader family called the pachychoroid spectrum, where “pachychoroid” simply means an abnormally thick choroid with dilated large vessels. The spectrum includes several related conditions ranging from subtle pigment epithelium changes (pachychoroid pigment epitheliopathy) to polypoidal choroidal vasculopathy, in which balloon-like outpouchings of choroidal vessels cause bleeding and exudation. Newer additions to the group include peripapillary pachychoroid syndrome and focal choroidal excavation.
22PubMed Central. Pachychoroid Spectrum Disorders: An Updated ReviewThe practical significance is that these conditions share overlapping risk factors and imaging features. A patient diagnosed with one pachychoroid entity is likely being watched for signs of progression into another.
Choroidal Thinning in Pathologic Myopia
While the pachychoroid conditions involve a choroid that is too thick, pathologic myopia is associated with one that is dangerously thin. In highly myopic eyes, the eyeball is elongated, which stretches and thins the choroid. When a posterior staphyloma (an outward bulging of the back wall of the eye) is also present, choroidal thinning is dramatically worse. One study found that highly myopic eyes with a staphyloma had a mean subfoveal choroidal thickness of only about 55 micrometers, compared to roughly 158 micrometers in highly myopic eyes without one.
23Scientific Reports. The relationship between scleral staphyloma and choroidal thinning in highly myopic eyes: The Beijing Eye StudyThis extreme thinning starves the outer retina of its blood supply and is a key feature of pathologic myopia, which is defined not by the degree of refractive error but by the presence of structural complications in the back of the eye, including diffuse choroidal atrophy.
24PubMed Central. IMI Pathologic Myopia 25Scientific Reports. Decreased choroidal and scleral thicknesses in highly myopic eyes with posterior staphyloma
Choroidal Melanoma
The choroid is the most common site for uveal melanoma, the most common primary cancer inside the eye in adults. Across Europe and the United States, about six cases are diagnosed per million people per year. Unlike skin melanomas, which typically carry BRAF or NRAS mutations, uveal melanomas usually begin with a mutation in the GNAQ or GNA11 genes. Risk factors include fair skin, light-colored eyes, and certain inherited conditions. Symptoms tend to be vague: flashes of light, floaters, visual field loss, or blurry vision.
26Nature Reviews Disease Primers. Uveal melanoma 27PubMed Central. Choroidal Melanoma: A Mini Review
Treatment has shifted away from removing the entire eye (enucleation) toward various forms of radiotherapy, phototherapy, and local tumor resection, often in combination. Outcomes are best when tumors are small and not close to the optic disc or fovea. The major concern is metastasis: nearly half of patients eventually develop metastatic disease, which usually spreads to the liver and is typically fatal within a year once it does.
26Nature Reviews Disease Primers. Uveal melanomaThe Choroid as a Window Into the Rest of Your Body
Because the choroid is essentially a bed of blood vessels, diseases that affect blood vessels elsewhere in the body tend to leave a mark here too. A systematic review and meta-analysis found that people with systemic hypertension had significantly thinner subfoveal choroids compared with people with normal blood pressure.
28PubMed Central. Choroidal thickness in patients with systemic arterial hypertension: a systematic review and meta-analysisA study of eyes with diabetes found the same direction of effect: total choroidal area and the fluid-filled (luminal) portion of the choroid were both smaller in people with systemic hypertension, even after adjusting for other factors like age and the severity of diabetic retinopathy.
29Scientific Reports. Systemic and Ocular Determinants of Choroidal Structures on Optical Coherence Tomography of Eyes with Diabetes and Diabetic RetinopathyResearchers are now exploring a metric called the choroidal vascularity index (CVI), which captures the ratio of vascular space to total choroidal volume. A systematic review of 48 studies found that CVI is reduced in a range of systemic conditions, including diabetic retinopathy, autoimmune diseases, and neurodegenerative disorders, and that the reductions track with disease severity. The hope is that CVI could eventually serve as a non-invasive biomarker for systemic disease progression, detectable during a routine eye scan.
30PubMed Central. Choroidal vascularity index in health and systemic diseases: a systematic reviewWhy Drug Delivery to the Choroid Is So Difficult
If you want to treat a disease of the retina or choroid, getting the drug there is a genuine engineering challenge. Eye drops barely penetrate past the front of the eye. Oral medications are diluted by the entire body’s blood volume before a small fraction reaches the eye. The most reliable current approach is intravitreal injection, where a needle delivers the drug directly into the gel-filled center of the eye. But the choroid itself, with its high blood flow and rapid clearance, can sweep away drugs before they reach effective concentrations in the surrounding tissue.
Computer modeling of drug distribution in the posterior eye showed that the choroidal circulation acts as a clearance mechanism for drugs delivered through the sclera, though the loss to choroidal blood flow turned out to be smaller than the loss from the outer surface of the sclera.
31PubMed. Computer modeling of drug delivery to the posterior eye: effect of active transport and loss to choroidal blood flowResearch into transscleral delivery, where drugs pass through the white of the eye to reach the choroid and retina, has shown promise as a less invasive alternative to repeated injections. In both laboratory and animal studies, therapeutically useful concentrations of large molecules could reach the choroid and retina this way while retaining their biological activity.
32PubMed. Transscleral drug delivery to the retina and choroidNewer strategies being explored include nanoparticles, nanomicelles, liposomes, and microemulsions, all designed to slip past the eye’s physical and chemical barriers and deliver their payload more efficiently to the posterior segment.
33PubMed Central. Ocular drug deliveryHemorrhagic Choroidal Detachment
In rare and dramatic fashion, bleeding can separate the choroid from the sclera, creating a hemorrhagic choroidal detachment. This can happen during or after eye surgery, after trauma, or spontaneously in people on blood thinners. The blood pools between the choroid and sclera, pushing the choroid inward and compressing the vitreous cavity. It is a sight-threatening emergency that often requires surgical drainage.
A stepwise surgical approach has been described for managing these cases. The first step involves inserting a cannula directly into the highest peak of the blood collection, guided by ultrasound, while maintaining pressure in the front of the eye to prevent collapse. If initial drainage is insufficient, the surgeon progressively enlarges the incision. The procedure is delicate because the choroid is a thin, vascular tissue, and aggressive handling risks more bleeding.
34PubMed Central. A Stepwise Approach to the Surgical Management of Hemorrhagic Choroidal DetachmentsHemorrhagic choroidal detachments are uncommon enough that most people will never encounter one, but they illustrate just how central the choroid is to the eye’s structural integrity. When this layer separates from its foundation, everything it supports, from oxygen delivery to temperature regulation to the retina’s physical position, is disrupted at once.