Vision loss may occur from an abnormality in the visual system from the tear film to the occipital cortex. The most important factor to determine initially is the rapidity of symptom onset. Early assessment of the presence or absence of associated symptoms such as pain, double vision, and flashes and/or floaters is also important. This helps to identify patients with potentially life-threatening disease, and highlights those patients who require prompt ophthalmology opinion  or surgical intervention. 
Acute vision loss that occurs suddenly or over the course of several minutes to hours usually requires urgent ophthalmic opinion. Sub-acute or chronic cases (where vision loss has developed over weeks, months, or years) may still need specialist input but usually on a non-urgent basis.
Any significant vision loss justifies a call to the ophthalmologist for advice on referral timing.
Vision loss may also be the initial manifestation of a number of systemic diseases for which separate work-up is required. 
Retinal or optic nerve disease usually presents acutely. Other acute causes of vision loss include acute angle-closure glaucoma, retinal vascular occlusion, and trauma.
The patient's age and determination of the medical history assists in diagnosis.
Patients may recognise symptoms of chronic deterioration 'acutely', meaning chronic vision loss may present in the ER, making diagnosis difficult.
Associated symptom review makes it possible to differentiate between urgent and non-urgent cases and arrange for appropriate ophthalmic consultation.
It is essential to take thorough histories, focusing on vascular and neurological processes as well as visual processes.
Diabetic retinopathies may develop before systemic diabetes is diagnosed, making it important to measure blood glucose levels in all patients presenting with acute vision loss and to perform ophthalmological examination of all newly diagnosed adults with diabetes.
Other serious systemic diseases (including granulomatosis with polyangiitis [Wegener's granulomatosis], bacterial meningitis, or disseminated malignancies) may need to be addressed in the emergency department or by hospital admission. 
Department of Ophthalmology
University of Colorado School of Medicine
JRS declares that he has no competing interests.
Dr Jeffrey R. SooHoo would like to gratefully acknowledge Dr Prem S. Subramanian, the previous contributor to this monograph. PSS declares that he has no competing interests.
The H. Stanley Neuro-ophthalmology Professor of Ophthalmology, Neurology, and Neurosurgery
The University of Iowa Hospitals and Clinics
AGL declares that he has no competing interests.
Professor of Ophthalmology
University of New Mexico Medical School
RA declares that he has no competing interests.
Clinical Senior Lecturer
Health Services Research Unit
University of Aberdeen
Honorary Consultant Ophthalmologist
AA-B declares that he has no competing interests.
Special Professor of Ophthalmology
University of Nottingham
Consultant Ophthalmic Surgeon
Nottingham University Hospitals
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