Since the advent of genetic testing, the picture has become more complex. What was thought to be one disease is, in fact, probably at least three different diseases, each related to a different genetic change. Therefore it is currently a little confusing to define what Stargardts disease is. Stargardt disease (STGD1) is caused by bi-allelic ABCA4 gene variants (i.e., autosomal recessive). Importantly, the exact genotype (i.e., combinations of both ABCA4 variants) is highly prognostic for the age of onset and disease progression.Autosomal-dominant Stargardt-like diseases were linked to genes such as PROM1 (STGD3) or ELOVL4 (STGD4) missense mutations play a role remains to be seen.The carrier frequency in the general population of ABCA4 alleles is 5 to 10%. Different combinations of ABCA4 genes will result in widely different age of onset and retinal pathology. The severity of the disease is inversely proportional to ABCA4 function and it is thought that ABCA4 related disease has a role to play in other diseases such as retinitis pigmentosa, cone-rod dystrophies and age-related macular degeneration (AMD). STGD1: By far the most common form of Stargardt disease is the recessive form caused by mutations in the ABCA4 gene. STGD4:  A rare dominant defect in the PROM1 gene. STGD3:  A rare dominant form of Stargardt disease caused by mutations in the ELOVL4 gene. Late-onset Stargardt disease is associated with missense mutations outside known functional domains of ABCA4. Pathophysiology
In STGD1, the genetic defect causes malfunction of the ATP-binding cassette transporter (ABCA4) protein of the visual phototransduction cycle.
Defective ABCA4 leads to improper shuttling of vitamin A throughout the retina, and accelerated formation of toxic vitamin A dimers (also known as bisretinoids), and associated degradation byproducts. Vitamin A dimers and other byproducts are widely accepted as the cause of STGD1. As such, slowing the formation of vitamin A dimers might lead to a treatment for Stargardt. When vitamin A dimers and byproducts damage the retinal cells, fluorescent granules called lipofuscin in the retinal pigmented epithelium of the retina appear, as a reflecting such damage. In STGD4, a butterfly pattern of dystrophy is caused by mutations in a gene that encodes a membrane bound protein that is involved in the elongation of very long chain fatty acids (ELOVL4)

Diagnosis
Diagnosis is firstly clinical through history and examination usually with a Slit-lamp. If characteristic features are found the investigations undertaken will depend on locally available equipment and may include Scanning laser ophthalmoscopy which highlights areas of autofluorescence which are associated with retinal pathology. Spectral-domain optical coherence tomography, electroretinography and microperimetry are also useful for diagnostic and prognostic purposes. Fluorescein angiography is used less often than in the past. These investigations may be followed by genetic testing in order to avoid misdiagnosis. Other diseases may have overlapping phenotypic features with Stargardt Disease and the disease itself has multiple variants. In one study, 35% of patients diagnosed with Stargardt Disease through physical ophthalmic examination were found to be misdiagnosed when subsequent genetic testing was done.