Best Disease (Best vitelliform macular dystrophy)

Best disease, also known as Best Vitelliform Macular Dystrophy, is a rare genetic eye condition that affects the macula, the central part of the retina responsible for sharp, detailed vision. It is caused by a mutation in a gene called BEST1, which is usually passed down from one parent. In this condition, a yellow, fatty substance called lipofuscin builds up underneath the retina, often looking like an egg yolk when an eye doctor examines the back of the eye. In the early stages, which often begin in childhood or adolescence, people with Best disease usually have normal or near-normal vision despite the noticeable changes in the eye. Over many years, the "egg yolk" lesion eventually breaks down and can cause scarring or thinning of the macula. This progression leads to a gradual loss of central vision, making tasks like reading, recognizing faces, or driving more difficult. However, peripheral (side) vision is typically preserved, and completely losing all sight is very rare. Currently, there is no cure for Best disease, but regular monitoring by an eye specialist is important. Doctors can check for complications, such as abnormal blood vessel growth (choroidal neovascularization), which can be treated to prevent sudden vision loss. Researchers are actively studying new treatments, including gene therapies, which offer hope for preserving vision in the future. Patients and families are encouraged to undergo genetic testing to confirm the diagnosis and understand how the condition might affect other family members.

Condition category: Macular Dystrophy

Prevalence: 1 in 10,000

Inheritance patterns: Autosomal Dominant

Age of onset: First two decades of life (typically 3 to 15 years, average 6 years), though detection is often incidental and can occur as late as 75 years of age.

Clinical overview: Best Vitelliform Macular Dystrophy (BVMD), also known as Best disease, is a rare, inherited macular dystrophy characterized by the bilateral accumulation of lipofuscin-like material within and beneath the retinal pigment epithelium (RPE). It is the most common of the bestrophinopathies, a group of clinically distinct but genetically related disorders. The condition classically presents with a striking "egg-yolk" (vitelliform) macular lesion, which undergoes a series of morphological changes over decades, eventually leading to macular atrophy and central vision loss. The OMIM phenotype number for BVMD is 153700, and the causative gene, BEST1, is OMIM 607854. The Orphanet identifier is ORPHA1243. Clinically, BVMD is unique because the profound structural changes seen in the macula during the early stages often do not correlate with visual acuity, which can remain surprisingly good for many years. The disease is a prime example of a channelopathy affecting the RPE, highlighting the critical role of ionic balance in maintaining the health of the RPE-photoreceptor complex. The diagnosis and classification of BVMD rely heavily on electrophysiology, specifically the electro-oculogram (EOG), which is universally abnormal in affected individuals and gene carriers, even before visible fundus changes occur. While currently incurable, BVMD is the subject of active research, including gene therapy trials, making accurate diagnosis and genetic characterization essential for patient management and future therapeutic interventions.

Patient and family guide: Best disease, also known as Best Vitelliform Macular Dystrophy, is a rare genetic eye condition that affects the macula, the central part of the retina responsible for sharp, detailed vision. It is caused by a mutation in a gene called BEST1, which is usually passed down from one parent. In this condition, a yellow, fatty substance called lipofuscin builds up underneath the retina, often looking like an egg yolk when an eye doctor examines the back of the eye. In the early stages, which often begin in childhood or adolescence, people with Best disease usually have normal or near-normal vision despite the noticeable changes in the eye. Over many years, the "egg yolk" lesion eventually breaks down and can cause scarring or thinning of the macula. This progression leads to a gradual loss of central vision, making tasks like reading, recognizing faces, or driving more difficult. However, peripheral (side) vision is typically preserved, and completely losing all sight is very rare. Currently, there is no cure for Best disease, but regular monitoring by an eye specialist is important. Doctors can check for complications, such as abnormal blood vessel growth (choroidal neovascularization), which can be treated to prevent sudden vision loss. Researchers are actively studying new treatments, including gene therapies, which offer hope for preserving vision in the future. Patients and families are encouraged to undergo genetic testing to confirm the diagnosis and understand how the condition might affect other family members.

Symptoms and clinical features: The clinical presentation of Best Vitelliform Macular Dystrophy is characterized by a slow, progressive evolution of macular changes, often divided into several stages. In the early or previtelliform stage, patients are typically asymptomatic with normal visual acuity. The fundus may appear entirely normal or show only subtle retinal pigment epithelium (RPE) mottling, but the electro-oculogram (EOG) is already abnormal. As the disease enters the classic vitelliform stage (usually in childhood or adolescence), a distinct, well-circumscribed, yellow, egg-yolk-like lesion appears in the macula. Remarkably, despite this dramatic clinical appearance, patients often maintain excellent visual acuity (20/20 to 20/40) and may only be diagnosed incidentally during a routine eye exam. Some patients may report mild symptoms such as slight dimness of vision or mild metamorphopsia (distortion of vision). In the intermediate and advanced stages, the lesion undergoes structural changes. It progresses to the pseudohypopyon stage, where the yellow material layers inferiorly within the lesion, and then to the vitelliruptive stage, where the material breaks up, giving a "scrambled egg" appearance. During these stages, visual acuity begins to decline more noticeably. Finally, the disease reaches the atrophic or cicatricial stage, characterized by RPE atrophy or subretinal scarring. In this advanced stage, patients experience significant central vision loss and central scotomas, making reading and recognizing faces difficult. A sudden drop in vision at any stage may indicate the development of a choroidal neovascular membrane (CNV), a complication that can cause subretinal hemorrhage and fluid accumulation.

Molecular pathology: Best Vitelliform Macular Dystrophy is caused by mutations in the BEST1 gene, which encodes the protein bestrophin-1. Bestrophin-1 is a 585-amino acid integral transmembrane protein that localizes predominantly to the basolateral plasma membrane of the retinal pigment epithelium (RPE). The protein forms a homo-pentameric structure with a central pore, functioning primarily as a calcium-activated chloride channel. It also acts as a channel for other large anions, including bicarbonate, and plays a critical role in regulating intracellular calcium signaling by interacting with voltage-dependent calcium channels. Mutations in BEST1 disrupt the normal ion channel function of bestrophin-1. In BVMD, these mutations often lead to a dominant-negative effect, where the mutant protein incorporates into the pentameric channel and abolishes or severely attenuates the calcium-activated anion current. This disruption in ionic transport across the RPE alters the fluid and ionic homeostasis in the subretinal space. The impaired RPE function leads to a failure in the normal phagocytosis and processing of photoreceptor outer segments. Consequently, there is an abnormal accumulation of lipofuscin and related fluorophores (such as A2E), fluid, and unphagocytosed debris in the subretinal space and within the RPE cells themselves. This accumulation forms the characteristic vitelliform (egg-yolk) lesions. Over time, the toxic accumulation of these materials and the chronic disruption of the RPE-photoreceptor interface lead to RPE cell death, secondary photoreceptor degeneration, and subsequent central vision loss.

Genetics: Best Vitelliform Macular Dystrophy is primarily inherited in an autosomal dominant pattern with incomplete penetrance and variable expressivity. This means that not all individuals carrying a disease-causing mutation will develop clinical symptoms, and the severity can vary significantly even among family members with the same mutation. The disease is caused by heterozygous mutations in the BEST1 gene (formerly known as VMD2), located on chromosome 11q12-q13. To date, over 250 different mutations in the BEST1 gene have been identified across the spectrum of bestrophinopathies. In classical BVMD, most mutations are missense mutations that exert a dominant-negative effect, leading to an absent or severely reduced chloride current in the retinal pigment epithelium. Less commonly, haploinsufficiency mechanisms may be involved. While BVMD is classically autosomal dominant, rare cases of autosomal recessive inheritance (often termed autosomal recessive bestrophinopathy or ARB) have been reported, typically involving homozygous or compound heterozygous mutations. Genotype-phenotype correlations in BVMD are generally weak; individuals with the exact same BEST1 mutation can present with vastly different clinical courses, ranging from severe early-onset macular degeneration to lifelong asymptomatic carriage with only EOG abnormalities. This variability suggests that other genetic modifiers, environmental factors, or epigenetic influences may play a role in disease expression.

Diagnostic evaluation: The diagnosis of Best Vitelliform Macular Dystrophy (BVMD) relies on a combination of clinical findings, electrophysiology, and genetic testing. Fundoscopy typically reveals a bilateral, symmetric, egg-yolk-like (vitelliform) subretinal deposit at the macula. As the disease progresses, this lesion may undergo changes, appearing as a pseudohypopyon, vitelliruptive (scrambled egg), or atrophic lesion. Fundus autofluorescence is highly characteristic, showing brilliant hyper-autofluorescence of the vitelliform material, which corresponds to lipofuscin accumulation. Optical coherence tomography (OCT) is crucial for staging and monitoring. In the vitelliform stage, OCT shows a homogenous hyperreflective material between the retinal pigment epithelium (RPE) and the photoreceptor layer. In later stages, OCT may reveal subretinal fluid, disruption of the ellipsoid zone, and thinning of the outer nuclear layer. Fundus fluorescein angiography (FFA) is used primarily to rule out choroidal neovascularization (CNV), a known complication. The vitelliform deposit typically causes blocked fluorescence (hypofluorescence) without the "dark choroid" sign seen in Stargardt disease. Electrophysiology is a hallmark of BVMD diagnosis. The electro-oculogram (EOG) is characteristically abnormal, showing a reduced Arden ratio (light peak to dark trough ratio typically less than 1.5) in both affected individuals and asymptomatic carriers. In contrast, the full-field electroretinogram (ERG) is usually normal, which helps differentiate BVMD from more generalized retinal dystrophies. Genetic testing confirms the diagnosis by identifying heterozygous mutations in the BEST1 gene. Differential diagnosis includes adult-onset foveomacular vitelliform dystrophy (which typically has a normal or mildly subnormal EOG and later onset), Stargardt disease, age-related macular degeneration, central serous chorioretinopathy, and other bestrophinopathies such as autosomal recessive bestrophinopathy (ARB).

Differential diagnosis: Differential diagnosis of Best vitelliform macular dystrophy includes: (1) Adult-onset foveomacular vitelliform dystrophy — later onset (30-50 years), smaller lesion, normal EOG, PRPH2 mutations in some cases. (2) Stargardt disease — flecks, dark choroid, ABCA4 mutations. (3) Pattern dystrophy — butterfly or reticular pattern, PRPH2 mutations. (4) Central serous chorioretinopathy — serous detachment, no vitelliform material, resolves spontaneously. (5) Batten disease (CLN3) — vitelliform-like changes with neurodegeneration and seizures. (6) Acute exudative polymorphous vitelliform maculopathy — acquired, often associated with systemic illness.

Natural history: The natural history of Best Vitelliform Macular Dystrophy typically follows a progression through several distinct clinical stages, though the timeline is highly variable. The disease usually begins in the first or second decade of life (often between ages 3 and 15) with the previtelliform stage, where the fundus may appear normal but the EOG is already abnormal. This is followed by the classic vitelliform stage, characterized by the appearance of a yellow, egg-yolk-like macular lesion. During these early stages, visual acuity often remains normal or only mildly reduced. As the patient ages, usually into their 30s or 40s, the lesion progresses to the pseudohypopyon stage, where the yellow material layers inferiorly, and then to the vitelliruptive (scrambled egg) stage, where the lesion breaks up and becomes irregular. Visual acuity typically begins to decline more noticeably during the vitelliruptive stage due to disruption of the photoreceptor layer. In the advanced stages, the lesion evolves into an atrophic or cicatricial (scarring) phase, leading to more severe central vision loss. A significant complication that can alter the natural history is the development of choroidal neovascularization (CNV), which occurs in a subset of patients and can cause sudden, severe visual decline. Despite this progression, many patients retain reading and driving vision in at least one eye well into adulthood.

Management and treatment research: ### Current Management and Monitoring There is no cure that corrects the underlying genetic cause of Best disease, also called Best vitelliform macular dystrophy (BVMD). Care focuses on monitoring retinal changes, preserving useful vision, and treating complications when they occur. Follow-up with an ophthalmologist or retina specialist may include: - Visual-acuity testing and dilated eye examinations - Optical coherence tomography (OCT), a noninvasive scan that shows retinal layers and can detect fluid beneath or within the retina - Fundus photographs and, when needed, fluorescein angiography or OCT angiography to evaluate abnormal blood vessels A possible complication is **choroidal neovascularization (CNV)**. In CNV, abnormal blood vessels grow beneath the retina and may leak fluid or blood, causing distortion or more rapid central vision loss. If CNV develops, a retina specialist may recommend injections into the eye of anti-vascular endothelial growth factor (**anti-VEGF**) medicines. VEGF is a protein that promotes blood-vessel growth. Anti-VEGF medicines, such as bevacizumab, ranibizumab, or aflibercept, can reduce leakage and bleeding and may stabilize or improve vision affected by CNV. These treatments address CNV; they do not correct the underlying **BEST1**-related disease. Supportive care may include: - Low-vision evaluations and devices, including magnifiers, electronic reading aids, high-contrast settings, and screen-reader software - Vision rehabilitation or occupational therapy for daily activities, school, and work - Educational accommodations for children and students - Genetic counseling and genetic testing for the affected person and, when appropriate, relatives ### Approved Disease-Modifying Therapies No disease-modifying treatment is currently approved specifically for Best disease or other **BEST1**-related retinal diseases. ### Investigational Therapies and Research #### Gene Therapy A recruiting early-stage study is evaluating an investigational gene therapy for people with BVMD or autosomal-recessive bestrophinopathy, another condition caused by changes in **BEST1**: - **OPGx-BEST1** is delivered by subretinal injection, meaning it is placed beneath the retina. The Phase 1/2 study is evaluating safety and tolerability. Early-phase studies primarily assess safety and help determine dosing; they are not designed to establish effectiveness. **NCT07185256** BEST1-related conditions affect the retinal pigment epithelium (RPE), a cell layer that supports the light-sensing retina. Research is examining how disease-causing **BEST1** variants disrupt RPE function, contribute to vitelliform material buildup, and may lead to retinal damage or CNV. #### Observational and Natural-History Studies Observational studies do not test a treatment. They can help researchers understand how Best disease changes over time, identify useful imaging and vision measures, and support the design of future clinical trials. Recruiting research studies include: - Study of BEST1 Vitelliform Macular Dystrophy: **NCT05809635** - Inherited Retinal Degenerative Disease Registry: **NCT02435940** - National Eye Institute cell-collection study for eye diseases: **NCT01432847** - National Ophthalmic Genotyping and Phenotyping Network (eyeGENE): **NCT06491615** A study of wide-field OCT angiography (OCTA)—an imaging method that maps blood flow in the eye without dye injection—is listed as not yet recruiting and may include people with selected ocular diseases. Eligibility should be confirmed with the study team. **NCT07298174** ### Clinical Trial Participation Each study has specific eligibility requirements, which may include genetic test results, age, vision level, retinal findings, and prior treatment history. A retina specialist or inherited retinal disease clinic can help determine whether a gene therapy study, natural-history study, registry, or genetic research study may be appropriate.

Outlook: The visual prognosis for Best Vitelliform Macular Dystrophy is generally favorable compared to many other inherited macular dystrophies. Most patients maintain good central vision (often 20/40 or better) through the vitelliform and pseudohypopyon stages, which can last for decades. Significant visual decline typically occurs later in life when the disease progresses to the atrophic or cicatricial stages, or if complications such as choroidal neovascularization (CNV) develop. Even in advanced stages, peripheral vision remains intact, and total blindness does not occur. Quality of life is usually well-preserved during childhood and early adulthood. As central vision declines in later stages, patients may require low vision aids and accommodations for reading and driving. The variable expressivity of the disease means that prognosis must be individualized; some patients may retain driving vision in at least one eye throughout their lives, while others may experience more severe central visual impairment. Regular monitoring is essential to manage treatable complications like CNV promptly.

Epidemiology: Best Vitelliform Macular Dystrophy is a rare inherited retinal disease. The prevalence is estimated to be between 1 in 16,500 and 1 in 21,000 individuals, based on population studies such as those conducted in Olmsted County, Minnesota. The condition affects both males and females equally, with no significant sex predilection. While it is found worldwide across various ethnic groups, specific founder mutations have been identified in certain populations. The true prevalence may be higher than reported due to the variable expressivity and incomplete penetrance of the disease, leading to underdiagnosis in mildly affected or asymptomatic individuals.

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