The near work hypothesis, also referred to as the "use-abuse theory" states that spending time involved in near work strains the intraocular and extraocular muscles. Some studies support the hypothesis, while other studies do not. While an association is present, it is not clearly causal. Nearsightedness is also more common in children with diabetes, Childhood arthritis, uveitis, and systemic lupus erythematosus. Because myopia is a refractive error, the physical cause of myopia is comparable to any optical system that is out of focus. Borish and Duke-Elder classified myopia by these physical causes:  As with any optical system experiencing a defocus aberration, the effect can be exaggerated or masked by changing the aperture size. In the case of the eye, a large pupil emphasizes refractive error and a small pupil masks it. This phenomenon can cause a condition in which an individual has a greater difficulty seeing in low-illumination areas, even though there are no symptoms in bright light, such as daylight. Under rare conditions, edema of the ciliary body can cause an anterior displacement of the lens, inducing a myopia shift in refractive error. A diagnosis of myopia is typically made by an eye care professional, usually an optometrist or ophthalmologist. During a refraction, an autorefractor or retinoscope is used to give an initial objective assessment of the refractive status of each eye, then a phoropter is used to subjectively refine the patient's eyeglass prescription. Other types of refractive error are hyperopia, astigmatism, and presbyopia. Various forms of myopia have been described by their clinical appearance:     The degree of myopia is described in terms of the power of the ideal correction, which is measured in diopters:  Myopia is sometimes classified by the age at onset:   Some suggest that more time spent outdoors during childhood is effective for prevention. Various methods have been employed in an attempt to decrease the progression of myopia, although studies show mixed results. Many myopia treatment studies have a number of design drawbacks: small numbers, lack of adequate control group, and failure to mask examiners from knowledge of treatments used. Among myopia specialists, mydriatic eyedrops are the most favored approach, applied by almost 75% in North America and more than 80% in Australia. Behavioral intervention (counseling to spend more time outdoors and less time with near-work) is favored by 25% of specialists, usually in addition to the medications.