Acute red eye is a common presenting complaint to primary care physicians.  A detailed history of the presenting symptoms and previous ophthalmic and medical history can narrow the differential diagnosis and aid in the interpretation of key examination findings. The lack of specialist equipment in the primary care setting, along with a very broad differential diagnosis, can cause difficulty in establishing the correct diagnosis, and in such cases a specialist ophthalmic opinion should be sought.   Serious vision-threatening conditions that present as red eye are rare and can occasionally be overshadowed by associated systemic symptoms; in light of this they should always be considered within the differential and excluded on examination.  http://www.rwjuh.edu/health_information/adult_eye_illustra.html [Robert Wood Johnson University Hospital: anatomy of the eye]
Similar conditions to acute red eye include orbital cellulitis and thyroid eye disease.
Well-recognized complications of acute red eye are dependent on the underlying etiology. Conditions affecting the cornea, trauma, anterior uveitis, Image 16 and angle-closure glaucoma Image 17 can lead to impaired visual acuity. Scleritis, Image 10 corneal ulceration, Image 11 high-velocity foreign bodies, and trauma can lead to perforation of the eye. Image 18