Sorsby Fundus Dystrophy

Sorsby Fundus Dystrophy is a rare, inherited eye condition that causes a gradual loss of central vision. It usually begins to affect people when they are between 30 and 50 years old. The disease damages the macula, which is the part of the retina at the back of the eye responsible for sharp, detailed vision needed for reading, driving, and recognizing faces. Early signs often include difficulty seeing at night or noticing that straight lines appear wavy or distorted. The condition is caused by a genetic change that leads to the buildup of abnormal material under the retina. This buildup can cause the light-sensing cells to slowly die off, or it can trigger the growth of abnormal, leaky blood vessels (a complication known as choroidal neovascularization). Because it is an inherited condition, it can be passed down from parent to child. While there is currently no cure for Sorsby Fundus Dystrophy, treatments are available to help manage the symptoms. If abnormal blood vessels develop, doctors can use specific eye injections to stop them from leaking and preserve vision for as long as possible. Regular eye exams are crucial so that any changes can be caught and treated early. Patients are also advised not to smoke, as smoking can worsen the condition.

Condition category: Macular Dystrophy

Prevalence: Extremely rare

Inheritance patterns: Autosomal Dominant

Age of onset: Typically between the second and sixth decades of life, with an average onset during the fourth to fifth decade (ages 30-50).

Clinical overview: Sorsby Fundus Dystrophy (SFD) is a rare, progressive, autosomal dominant inherited macular dystrophy characterized by the loss of central vision. First described by Arnold Sorsby in 1949, the condition typically presents in adulthood, distinguishing it from many other early-onset inherited retinal diseases. It shares significant phenotypic overlap with age-related macular degeneration (AMD), including the presence of drusen, geographic atrophy, and a high risk of choroidal neovascularization (CNV). The clinical significance of SFD lies in its high penetrance and the severe visual impairment it causes if left untreated. The disease primarily affects the macula, the central part of the retina responsible for sharp, detailed vision, but can progress to involve the peripheral retina in later stages. The hallmark of the disease is a thickened Bruch's membrane due to the accumulation of abnormal protein deposits. SFD is classified under OMIM #136900 (the disease phenotype) and OMIM #602194 (the causative TIMP3 gene). It is also recognized in the Orphanet database under ORPHA59181. Due to its rarity and phenotypic similarities to AMD, accurate diagnosis relies heavily on clinical suspicion based on family history, early age of onset, and confirmatory genetic testing.

Patient and family guide: Sorsby Fundus Dystrophy is a rare, inherited eye condition that causes a gradual loss of central vision. It usually begins to affect people when they are between 30 and 50 years old. The disease damages the macula, which is the part of the retina at the back of the eye responsible for sharp, detailed vision needed for reading, driving, and recognizing faces. Early signs often include difficulty seeing at night or noticing that straight lines appear wavy or distorted. The condition is caused by a genetic change that leads to the buildup of abnormal material under the retina. This buildup can cause the light-sensing cells to slowly die off, or it can trigger the growth of abnormal, leaky blood vessels (a complication known as choroidal neovascularization). Because it is an inherited condition, it can be passed down from parent to child. While there is currently no cure for Sorsby Fundus Dystrophy, treatments are available to help manage the symptoms. If abnormal blood vessels develop, doctors can use specific eye injections to stop them from leaking and preserve vision for as long as possible. Regular eye exams are crucial so that any changes can be caught and treated early. Patients are also advised not to smoke, as smoking can worsen the condition.

Symptoms and clinical features: In the early stages of Sorsby Fundus Dystrophy, patients are often asymptomatic, though clinical examination may reveal drusen-like deposits in the retina. The first noticeable symptoms typically emerge between the ages of 30 and 50. Patients frequently report delayed dark adaptation (nyctalopia), making it difficult to see in low-light conditions. They may also experience mild visual distortion (metamorphopsia) or a gradual decrease in central visual acuity. As the disease progresses to the intermediate stage, symptoms become more pronounced. The most significant threat to vision during this phase is the development of choroidal neovascularization (CNV). If CNV occurs, patients experience sudden, severe central vision loss, accompanied by significant distortion and central blind spots (scotomas). Even in the absence of CNV, progressive geographic atrophy of the macula leads to a steady decline in central vision, affecting reading, driving, and facial recognition. In the advanced stages, the disease is characterized by widespread chorioretinal atrophy and fibrosis that extends beyond the central macula into the peripheral retina. This results in profound bilateral vision loss. Patients may lose ambulatory vision and become legally blind, with significant peripheral visual field defects in addition to the complete loss of central vision.

Molecular pathology: Sorsby Fundus Dystrophy is caused by mutations in the TIMP3 gene, which encodes the tissue inhibitor of metalloproteinases-3. TIMP3 is normally expressed and secreted by the retinal pigment epithelium (RPE) and is a crucial component of Bruch's membrane. Its primary physiological functions include regulating extracellular matrix (ECM) turnover by inhibiting matrix metalloproteinases (MMPs), modulating inflammation, and exhibiting anti-angiogenic properties by blocking the interaction between vascular endothelial growth factor (VEGF) and its receptor, VEGFR2. Pathogenic mutations in TIMP3, most of which introduce an unpaired cysteine residue, lead to the abnormal accumulation and dimerization of the mutant protein within Bruch's membrane. This accumulation disrupts the normal homeostasis of ECM remodeling, resulting in a thickened and structurally abnormal Bruch's membrane. The compromised integrity of Bruch's membrane interferes with its physiological functions, impairing the transport of nutrients and waste products between the choroid and the RPE. This dysfunction ultimately leads to RPE cell death, photoreceptor degeneration, and the development of choroidal neovascularization (CNV), likely exacerbated by the loss of TIMP3's normal anti-angiogenic activity.

Genetics: Sorsby Fundus Dystrophy is inherited in an autosomal dominant pattern with complete penetrance. It is caused by heterozygous mutations in the TIMP3 (tissue inhibitor of metalloproteinases 3) gene, located on chromosome 22q12.3. To date, numerous pathogenic variants in TIMP3 have been identified, including Ser156Cys, Ser179Cys, Tyr174Cys, Tyr177Cys, Tyr182Cys, and Ser204Cys, among others. The Ser204Cys mutation is particularly common among affected individuals in the United Kingdom. Most of the known mutations involve the substitution of a cysteine residue, which is thought to lead to abnormal dimerization of the TIMP3 protein. While the clinical endpoint of widespread chorioretinal atrophy is relatively uniform, there is significant phenotypic variability in the age of onset and specific disease course, even among individuals with the same mutation. Some genotype-phenotype correlations have been suggested; for instance, carriers of the Tyr174Cys mutation may experience symptom onset up to 20 years earlier than those with Tyr182Cys or Tyr177Cys mutations. However, the exact mechanisms driving this variability remain incompletely understood and may involve other genetic or environmental modifiers.

Diagnostic evaluation: Clinical diagnosis of Sorsby Fundus Dystrophy (SFD) involves a comprehensive ophthalmic workup. Fundoscopy typically reveals multiple yellowish drusen-like deposits at the posterior pole and along the vascular arcades, often accompanied by reticular pseudodrusen. As the disease progresses, signs of choroidal neovascularization (CNV) or geographic atrophy become apparent. Fundus autofluorescence (FAF) imaging is particularly useful for visualizing reticular pseudodrusen and areas of macular atrophy. Optical coherence tomography (OCT) provides high-resolution cross-sectional images, demonstrating a thickened Bruch's membrane, subretinal drusenoid deposits, retinal pigment epithelium (RPE) disruption, and subretinal fluid if active CNV is present. Fundus fluorescein angiography (FFA) and indocyanine green angiography (ICG-A) are employed to detect and assess the leakage pattern of CNV. Notably, reduced late-phase fluorescence on ICG-A has been identified as a potential early subclinical sign of SFD. Electrophysiological testing, such as electroretinography (ERG), often shows abnormal responses, particularly in scotopic (low-light) conditions, reflecting widespread retinal dysfunction. Visual field testing frequently demonstrates central scotomas corresponding to macular involvement, and peripheral visual field defects may develop later. The differential diagnosis primarily includes age-related macular degeneration (AMD), which shares phenotypic similarities like drusen and CNV but typically presents later in life. Other inherited macular dystrophies must also be considered. Definitive diagnosis is confirmed through molecular genetic testing identifying pathogenic heterozygous mutations in the TIMP3 gene.

Differential diagnosis: Differential diagnosis of Sorsby fundus dystrophy includes: (1) Age-related macular degeneration (wet) — older onset, no family history of autosomal dominant pattern, no TIMP3 mutations. (2) Stargardt disease — flecks, younger onset, ABCA4 mutations. (3) Best disease — vitelliform lesion, abnormal EOG. (4) Pattern dystrophy — characteristic pigment patterns, PRPH2 mutations. (5) Doyne honeycomb retinal dystrophy — radial drusen pattern, EFEMP1 mutations. (6) Central areolar choroidal dystrophy — geographic atrophy without choroidal neovascularization initially.

Natural history: The natural history of Sorsby Fundus Dystrophy typically begins with an asymptomatic phase characterized by the accumulation of drusen-like deposits at the posterior pole. Patients usually become symptomatic between the third and sixth decades of life, most commonly in their 40s or 50s. Initial symptoms often include delayed dark adaptation (nyctalopia) or sudden central vision loss due to the development of choroidal neovascularization (CNV). Without treatment, the disease progresses rapidly. The development of CNV leads to subretinal hemorrhage, fluid accumulation, and subsequent fibrotic scarring, causing severe and irreversible loss of central vision. In cases where CNV does not occur, patients experience progressive geographic atrophy of the macula, which also results in significant visual impairment. In the late stages of the disease, typically by the seventh decade of life, patients develop widespread chorioretinal atrophy that extends beyond the vascular arcades, accompanied by choroidal thinning and extensive fibrosis. This advanced stage often leads to the loss of ambulatory vision and legal blindness.

Management and treatment research: ### Current management and monitoring Sorsby fundus dystrophy (SFD) is an inherited retinal disease usually caused by disease-causing changes in the **TIMP3** gene. It affects the macula, the central part of the retina responsible for sharp, detailed vision. Vision loss can also occur when **choroidal neovascularization (CNV)** develops. CNV involves abnormal blood vessels growing beneath the retina, where they may leak fluid or blood. Current care focuses on monitoring the retina, recognizing CNV promptly, treating active CNV, and supporting remaining vision. - **Regular retinal monitoring:** Eye care may include dilated examinations and retinal imaging, such as optical coherence tomography (OCT). Fluorescein angiography or OCT angiography may be used when abnormal blood vessels are suspected. - **Prompt assessment of vision changes:** New distortion of straight lines, a blurred or dark spot in central vision, reduced detail vision, or a sudden change in vision should be assessed urgently. An Amsler grid may help some people notice new distortion between visits. - **Treatment of active CNV:** Intravitreal anti-VEGF injections may be used when CNV occurs. These medicines reduce the activity of vascular endothelial growth factor (VEGF), a signal that promotes abnormal blood-vessel growth and leakage. Bevacizumab, ranibizumab, or aflibercept may be used off label for CNV associated with SFD. The injection schedule is individualized based on retinal imaging and treatment response. - **Low-vision rehabilitation:** Magnifiers, improved lighting, electronic reading aids, screen readers, accessibility settings, and orientation and mobility training can help people use their remaining vision and maintain independence. - **Genetic counseling:** SFD is often inherited in an **autosomal dominant** pattern. A person with a disease-causing TIMP3 variant may have a 50% chance of passing it to each child. Genetic testing and counseling can help clarify the diagnosis, inheritance, and family-planning options. ### Approved therapies There are currently **no treatments approved specifically for Sorsby fundus dystrophy** and no approved gene therapy for TIMP3-related SFD. Anti-VEGF medicines are established treatments for CNV in several retinal conditions. In SFD, their use for CNV is generally off label. They can treat the abnormal blood-vessel complication but do not correct the underlying TIMP3-related disease process. ### Investigational therapies There are no condition-specific investigational treatment programs currently listed for Sorsby fundus dystrophy in the available treatment pipeline. Research continues to examine how TIMP3-related changes affect the retinal pigment epithelium, Bruch’s membrane, and the macula. This work may help identify useful measures of disease progression and potential targets for future therapies. ### Clinical trial participation A recruiting registry for inherited retinal degenerative diseases is available: - **NCT02435940 — Inherited Retinal Degenerative Disease Registry** (recruiting), sponsored by the Foundation Fighting Blindness This registry does not test a treatment. It collects health, genetic, and vision information from people with inherited retinal diseases to improve understanding of these conditions and may help identify individuals for future research opportunities. Eligibility and participation requirements can be discussed with the study team.

Outlook: The visual prognosis for patients with Sorsby Fundus Dystrophy is generally poor without intervention, as the disease inevitably leads to severe central vision loss and, in late stages, potential loss of ambulatory vision. The development of choroidal neovascularization (CNV) is a major prognostic factor; untreated CNV results in rapid and irreversible macular scarring. However, the prognosis has improved significantly with the advent of anti-VEGF therapies. If CNV is detected early and treated promptly with intravitreal injections, patients have a good chance of maintaining functional central vision for an extended period. Despite treatment for CNV, underlying geographic atrophy may still progress slowly. The condition significantly impacts quality of life, requiring low vision aids, lifestyle adjustments, and psychological support to cope with progressive sight loss.

Epidemiology: Sorsby Fundus Dystrophy is an extremely rare condition. The estimated prevalence is approximately 1 in 220,000 individuals. As an autosomal dominant disorder with complete penetrance, it affects males and females equally, with no specific geographic or ethnic predilection reported.

Selected references: 1. Sorsby A, Mason ME, Gardener N. A fundus dystrophy with unusual features. Br J Ophthalmol. 1949;33(2):67-97. PMID: 18111560 2. Weber BH, Vogt G, Pruett RC, Stöhr H, Felbor U. Mutations in the tissue inhibitor of metalloproteinases-3 (TIMP3) in patients with Sorsby's fundus dystrophy. Nat Genet. 1994;8(4):352-356. PMID: 7894485 3. Christensen DR, Brown FE, Cree AJ, Ratnayaka JA, Lotery AJ. Sorsby fundus dystrophy - A review of pathology and disease mechanisms. Exp Eye Res. 2017;165:35-46. PMID: 28847738 4. Gliem M, Müller PL, Mangold E, et al. Sorsby Fundus Dystrophy: Novel Mutations, Novel Phenotypic Characteristics, and Treatment Outcomes. Invest Ophthalmol Vis Sci. 2015;56(4):2664-2676. PMID: 25813993 5. Tsokolas G. Sorsby fundus dystrophy (SFD): A narrative review. Medicine (Baltimore). 2022;101(38):e30595. PMID: 36197222 6. Anand-Apte B, Chao JR, Singh R, Stöhr H. Sorsby fundus dystrophy: Insights from the past and looking to the future. J Neurosci Res. 2019;97(1):88-97. PMID: 30003585