Background: The red eye can occur as a symptom of many diseases relevant to general medicine and ophthalmology.Often the ophthalmologist is not the primary physician contact.Assessing the urgency is extremely important for further diagnostics and therapy.Method: Selective literature search in August 2016 in PubMed as well as own clinical and scientific experiences.Results: In primary care, approximately 4-10 patients per week can be expected to present because of ocular symptoms.Clinically, most patients have a red eye.In addition to the detailed medical history, accompanying symptoms and simple basic ophthalmological examinations can narrow down the possible differential diagnoses.Duration of findings, laterality and pain intensity are the cornerstones of the diagnosis.In this way, a distinction can be made between less critical changes that require general medical care, diseases that require a planned ophthalmological presentation and emergencies with an ophthalmic surgical indication.Conclusion: The diagnosis of red eye can be quickly narrowed down with simple basic examinations and specific questions.In emergencies or when there is uncertainty about the correct diagnosis, each patient should be presented to an ophthalmologist immediately.Every doctor has been confronted with the symptom of the "red eye" (1–3).According to a study, eye problems account for 2–3% of the reasons for consultation in primary care and emergency care (1).This corresponds to around 4–10 patients per week (2).In most of these cases, a red eye is clinically impressive (3).In general emergency departments or in clinics without a corresponding consultant, non-ophthalmologists are faced with the difficult decision of whether further evaluation can be carried out electively or whether emergency evaluation is necessary (1, 2).In the event of a foreign body event or other injury as well as the key symptoms of pain, reduced visual acuity, a rock-hard bulb and/or involvement of the cornea, an immediate visit to an ophthalmologist should be made for every red eye.An Australian study was able to determine that 64% of red eyes treated primarily in general medicine were misdiagnosed;about 10% of these cases had a serious clinical course (4)."There is a shortage of specialist literature for non-ophthalmologists," concludes another study (5).After reading this article, the reader shouldNormally, the conjunctiva shows very fine vascular markings and the underlying sclera is white.As a result, hyperaemia, vascular ectasia or bleeding (hyposphagma) are immediately noticeable.Pain, extent, unilateral/bilateral nature, localization and duration, progression of the reddening and accompanying symptoms should be recorded.Diagrams 1, 2 and Table 1 as well as eTables 1 and 2 provide diagnostic assistance.The diagnostic procedure when describing other symptoms begins with determining whether an acute or chronic red eye is present.“Acute” is understood to mean a finding that lasts for a maximum of 7 days (Figure 1) .Both eyes should always be examined.Unilateral or bilateral findings allow for differential diagnostic considerations.Pain sensitivity should also be queried.Duration of findings, laterality and pain history are the cornerstones of the diagnosis.In the case of an acute, unilateral, painful red eye, the impact of a foreign body should always be ruled out.If there is a positive foreign body/trauma history, the patient should be presented to an eye clinic as an emergency.If there is no suspicion of a foreign body, the cornea should be examined more closely.Superficial changes such as erosio corneae or deeper structural defects such as a corneal ulcer can often be seen with the naked eye.If no changes are noticed here, the intraocular pressure should be checked by palpation in a lateral comparison in order to rule out an acute glaucoma attack (emergency).If the intraocular pressure is normal, inflammation of the sclera, uvea or iris should also be considered in the case of an acute, unilateral, painful red eye.Contact lens wearers with an acute, unilateral, painful red eye with diffuse ciliary injection and/or corneal opacity need to be admitted as an emergency.In the case of an acute, bilateral, painful red eye, there is often a viral conjunctivitis.As a rule, the patients show a serous/clear secretion (tearing of the eyes = epiphora), photophobia and blepharospasm.This is also an ophthalmological emergency.If an acute, unilateral, but painless red eye occurs, a quick visit to the ophthalmologist should be made within 24 hours to rule out a carotid-sinus-cavernosus fistula.In the case of a chronic, one-sided, pain-free red eye with lid closure deficit, an urgent ophthalmological consultation is advisable.In addition, every patient with red eyes after recent eye surgery should be presented immediately to an ophthalmologist.Conjunctivitis is inflammation of the conjunctiva and keratoconjunctivitis is inflammation of the cornea and conjunctiva.In conjunctivitis and keratoconjunctivitis, there is an inflammatory process on the ocular surface with vasodilatation (conjunctival injection), cellular infiltration, and watery, mucous, or purulent exudation (6).A red eye and sticky lids in the morning are typical, and the eye may show glassy edema (chemosis) (Figure 1).The foreign body and pressure sensation are particularly disturbing for the patient, often in combination with photophobia and epiphora.If there is also eyelid spasm (blepharospasm), this indicates involvement of the cornea.The causes of conjunctivitis are varied (6).This is how infectious conjunctivitis is distinguished from non-infectious conjunctivitis.The latter are caused by external stimuli (smoke, dust, etc.), reduced tear production, anomalies in the position of the lids with mechanical irritation or uncorrected refractive errors.The most important therapeutic measure in non-infectious conjunctivitis begins with avoidance of the trigger.Vasoconstrictive eye drops (active ingredient: tetrazoline) should be prescribed for support for a few days at most, since they only set in symptomatically and can lead to tachyphylaxis, which was described more than 20 years ago in a case series of 11 patients (7).An ophthalmological co-assessment should take place in the course.Infectious conjunctivitis are usually bacterial or viral.In German-speaking countries, the most common pathogens causing bacterial conjunctivitis are staphylococci, streptococci, or pneumococci (6, 8, 9).Pain, severe reddening of the eye and mucopurulent secretion with yellowish crust formation are typical signs.In principle, all available antibiotics can be considered for the topical therapy of acute bacterial conjunctivitis, as similar therapeutic successes have been demonstrated in randomized controlled studies and meta-analyses (3, e1–e4).Non-proprietary names of commonly used topical antibiotics are gentamycin (aminoglycoside antibiotic) or ofloxacin (fluoroquinolone antibiotic).The application should not be less than 3 × daily for 5 days.If the findings do not improve after five days at the latest, the patient should be examined promptly by an ophthalmologist.A special form of bacterial (kerato) conjunctivitis often occurs in contact lens wearers.Here, for example, infections with acanthamoeba or Pseudomonas aeruginosa occur (10, 11).A red eye with a corneal ulcer can occur peracutely, particularly in the case of pseudomonas.This typically shows a greenish-yellow mucus formation.The pathogens are often found in the storage liquids/containers of the contact lenses, and there is often resistance to common antibiotics.It is therefore important to ensure a timely diagnosis that the uncleaned contact lens case, including the liquid and lens, is made available and that the ophthalmologist takes a direct smear test from the focus of inflammation for microbiological pathogen detection as part of an immediate ophthalmological visit.Microbial keratitis can lead to perforation of the cornea and permanent visual loss or even morphological loss of the eye after just a few hours (10).A keratoplasty à chaud with a reduced vision prognosis is then often the only treatment option, as confirmed by a large retrospective case-control study published in 2015 (11).If a corneal perforation is suspected, the application of a non-compressive protective bandage with a compress fixed to the forehead and cheek is recommended.In contrast to bacterial conjunctivitis, there is always a suspicion of a viral agent or an allergy if the eyes are red on both sides with watery-mucous secretion and itching.Viral conjunctivitis initially begins unilaterally.In the case of a viral cause (most frequently due to the herpes simplex virus [12]), typical periocular, herpetiform grouped vesicles appear on a red background.In contrast to a varicella-zoster virus-associated infection, the dermatological changes in herpes simplex are not limited to the dermatome.These changes are almost always a recurrence – the primary infection is often inapparent.Keratitis caused by fungi (keratomycosis) can only be diagnosed differentially by an ophthalmological and microbiological examination (e5).Viral eye diseases such as epidemic keratoconjunctivitis are highly contagious (13, 14).It occurs seasonally due to a smear infection with adenoviruses and begins after an incubation period of 8 to 10 days (15).In the acute full picture, there are severely painful, itchy, light-sensitive, red and watery eyes on both sides.Healing usually takes place after 2 weeks, mostly without consequences (15).The most important thing is dealing with those affected: no shaking hands, no sharing of towels and strict compliance with hygienic measures.The areas of the medical practice with which the patient was in contact must be decontaminated according to special hygiene/disinfection specifications.For prophylaxis, it is advisable to have the affected person wait separately if possible and to hand out disposable gloves in the case of red eyes.The patient must strictly abstain from contact until the ophthalmologist confirms the diagnosis immediately and is unable to work.If patients are referred for confirmation of the diagnosis, a short prior telephone call should be given.According to Section 7, paragraph 1 of the Infection Protection Act, the public health department is notified by name of the direct detection of adenoviruses in the conjunctival smear no later than 24 hours after it has been obtained (e6).While epidemic keratoconjunctivitis requires topical symptomatic treatment with wetting agents (15), herpetic (kerato)conjunctivitis is treated topically and/or orally with virostatics (e7–e10).Even with a non-infectious anamnesis, there is an acute need for action.Typical examples of non-infectious keratopathy are neurotrophic keratopathy and keratopathia e lagophthalmo (see section lid malpositions).In neurotrophic keratopathy there is a greatly reduced/eliminated corneal sensitivity.These patients therefore often present with a discrepancy between actual findings and subjective perception of pain (16).The typical anamnesis includes previous operations in the head area (trigeminal neuralgia, acoustic neuroma), diseases that compress the trigeminal nerve (tumor, brainstem hemorrhage) or previous varicella-zoster virus infection.Diabetes mellitus or multiple sclerosis can also be causes.Therapeutically, sufficient wetting of the corneal surface with tear substitutes and eye ointments (e.g. dexpanthenol) must be ensured in order to avoid exposure keratopathy.Furthermore, the application of a sealing watch glass bandage at night with the formation of a "moist chamber" is recommended.Potential new forms of therapy, for example in the form of nerve growth factors to be applied topically, are currently being investigated in randomized clinical studies (e11, e12).In most cases, allergic conjunctivitis is acutely triggered by immunoglobulin E-reactive allergens (type I: immediate-type allergy; classic “hay fever”).The usually more serious chronic courses are not IgE-mediated, but caused by T-lymphocytes, which is why they are also called "cell-mediated reactions" (type IV: delayed-type allergy).Ophthalmological examples are vernal or atopic keratoconjunctivitis.Acute symptoms are bilateral eyelid swelling, epiphora, itching and conjunctival conjunctival hyperemia with chemosis.In addition to allergen avoidance, acute local and systemic antihistamines are used, and mast cell stabilizers (chromoglycic acid, lodoxamide) or second- and third-generation H1 blockers (e.g., olopatadine) can be prescribed for long-term prophylaxis, as recent reviews have shown (17, e13).If the symptoms are more severe, local steroids (fluorometholone, loteprednol) can be applied for a short time and desensitization can be initiated (18, 19).In the case of clinically relevant type IV sensitization after long-term use of eye drops containing preservatives or to ingredients in facial care and cosmetic products, avoidance of allergens is obligatory.For the most severe courses, systemic steroid application or immunosuppressants are available as therapy.Patch testing should be initiated to identify the causative agent.Therapy is usually interdisciplinary (18).For keratoconjunctivitis sicca, see the detailed cme article by EM Messmer from January 2015 (20).A hyposphagma is a sudden, subconjunctival hemorrhage (Figure 2) .Hyposphagmata occur more frequently, especially under oral anticoagulation, poorly controlled or untreated arterial hypertension.Those affected often report previous Valsalva maneuvers (going to the toilet, gardening, etc.).With every “simple” hyposphagma, blood pressure should be measured in the practice to rule out acute blood pressure crises.The patient should be referred to an internist because a large population-based study reported an increased risk (hazard ratio: 1.33) of (late) cardiovascular complications after hyposphagia (e14).The hyposphagma resorbs spontaneously after 2 to 4 weeks.At the slightest suspicion of blunt or penetrating trauma, surgical exploration must be performed at an ophthalmic surgical center.Erosio corneae is a superficial corneal defect affecting the epithelium.Pain, foreign body sensation, photophobia, and epiphora with blepharospasm are typical symptoms.In most cases, patients can define the cause well.If these symptoms occur after drilling or flexing, a metallic foreign body is often the cause of the symptoms.When these foreign bodies are in contact with the cornea, they cause a halo of rust after just a few hours, which must be promptly removed by an ophthalmologist.If there is no history of a foreign body, treatment with an antibiotic eye ointment (e.g. chloramphenicol) can optionally be carried out until an ophthalmologist is consulted (e15).In case of doubt, an ophthalmological assessment should be carried out immediately to rule out a penetrating injury.Red eyes after operations under intubation anesthesia or in patients on long-term ventilation in the intensive care unit are a special case. Erosio corneae with conjunctival reddening, usually in the lower third of the cornea, is caused by “drying out” of the corneal surface due to insufficient lid closure (21).Deeper inflammation of the cornea can progress to keratitis or an ulcer (Figure 3) .At this stage there is a risk of perforation of the cornea.Keratoplasty à chaud is often necessary.Overall, the visual prognosis is unfavorable.As the ulcer heals, extensive corneal scarring and irregular astigmatism often occur.In photoelectric keratitis, the cornea is inflamed by “flashing”.The epithelium of the exposed ocular surface dies off acutely due to UV-C radiation.Typical examples are welding without goggles or spending time in high mountains without appropriate UV protection (e16, e17).Most of those affected present themselves about 6-8 hours after the flash has been applied, because the initially moderate pain and symptoms (almost always bilateral) (sensation of a foreign body, photophobia, epiphora) become unbearable and - due to the blepharospasm triggered - transient, up to 48-hour lasting functional blindness.Despite a clear anamnesis, an ophthalmological presentation should be made promptly.The dominant, bilateral blepharospasm can be briefly interrupted by a single application of a topical local anesthetic (e.g. eye drops containing 0.4% oxybuprocaine HCl).Studies with higher levels of evidence do not exist, but oral analgesics should be given at the point of initial consultation, the patient should avoid rubbing the eyes, and vitamin A eye ointment should be used.The regeneration time of the damaged corneal epithelium is 24-48 hours.Topical local anesthetics should not be prescribed.Episcleritis is a moderately painful inflammation of the connective tissue between the sclera and conjunctiva, with erythema usually confined to one sector and injection predominantly conjunctival (Figure 4) .In contrast to episcleritis, scleritis involves pronounced bulbar pain, often with reduced vision (22).In addition, the hyperaemia associated with scleritis cannot be suppressed by the diagnostic (!) application of a vasoconstrictive agent (Figure 5).While episcleritis is usually triggered by exogenous factors (22), up to 50% of those affected with scleritis have a systemic underlying disease (e.g., rheumatoid arthritis, vasculitis), which is of infectious origin in up to 20% (e.g Example Varicella zoster virus) (22-24).In any case, an ophthalmological presentation should be made promptly.Episcleritis is usually self-limiting but can be recurrent.Systemic comorbidities are rare; treatment begins with lubricating eye drops and non-steroidal anti-inflammatory drugs ([NSAIDs], e.g. diclofenac) (e18, e19).The treatment of scleritis follows a step-by-step scheme (e18, e19).The underlying disease often has to be treated with anti-inflammatory and immunosuppressive therapy.New treatment options with biologics/biosimilars such as tocilizumab or rituximab can be used under certain conditions (25, 26).Delaying the start of therapy can be associated with a significantly increased mortality rate from the underlying systemic disease (e20, e21).Foster et al. reported a 10-year mortality rate of 30% in patients with rheumatoid arthritis (e22).Anterior uveitis includes iritis and iridocyclitis (inflammation of the iris/involving the ciliary body and the anterior vitreous body).Iritis is the most common form of uveitis.It is often the case that no clear cause can be found, but immunological causes are more common than infectious causes (e.g. syphilis or borrelia infection) (27, 28).In particular, diseases of the rheumatic type, such as Bechterew's or Behcet's disease, are associated with anterior uveitis (29).There is also an association with inflammatory multisystem diseases such as sarcoidosis or juvenile idiopathic arthritis (30).Cardinal symptoms of iritis are photophobia, ciliary injection, and the absence of secretion typical of primary conjunctivitis.In the case of a rheumatic disease or suspected anterior uveitis, the patient should be presented to the ophthalmologist within 24 hours to avoid complications from exudative fibrinous inflammatory reactions.Depending on the etiology of the anterior uveitis, different therapy regimens are pursued;Frequent topical therapy with steroid-based eye drops and the application of pupil-dilating eye drops 2 to 3 times a day is often sufficient (e23, e24).Acute angle-closure glaucoma (“glaucoma attack”) is one of the acute emergencies in ophthalmology, which is most commonly caused by pupillary block.This leads to attachment of the peripheral iris to the cornea and thus to obstruction of the drainage areas of the aqueous humor with a sharp increase in intraocular pressure (31).Pain can radiate to the head and teeth and can also result in secondary abdominal discomfort, cardiac arrhythmias and nausea via vagus irritation.The main symptoms are the one-sided "rock-hard" globe, which can be detected by palpation in a comparison of the sides: Using the index finger of one hand, the globe is fixed as far as possible from above with the patient looking down in the orbital fatty tissue and carefully the sclera with the index finger of the other hand identified.The deterioration in visual acuity and the perception of colored rings around light sources (“halos”) are caused by swelling of the cornea (32).The increase in intraocular pressure causes intraocular ischaemia and paralysis of the sphincter pupillae muscle with an unrounded, medium-sized and light-fixated pupil.The flattening of the anterior chamber of the eye with attachment of the iris to the cornea can be recognized with the help of a ward lamp in lateral illumination.Systemic and local therapy to reduce intraocular pressure should be initiated immediately (e25).Many cases require emergency surgical intervention to interrupt the angle block (e26).The hordeolum (= stye) is often found in all age groups (1, 33).This is usually an acute bacterial inflammation of the sebaceous or sweat glands on the edge of the lid, which appears as a red, raised nodule, usually painful (Figure 6).The precise incidence cannot be quantified (34).The recommended therapy is the application of dry heat (e.g. with an infrared light lamp) and antibiotic ointments (e.g. gentamycin or ofloxacin, 3 times a day for 1 week), although randomized clinical studies are lacking.If the findings do not improve, an ophthalmological presentation should be made to rule out further differential diagnoses (e.g. sebaceous gland carcinoma) (1, 33).Frequent malpositions of the lower eyelids are ectropion and entropion, which usually result from age-related relaxation of the eyelid holding apparatus (35).In entropion, the edge of the lid is inverted toward the bulb and the eyelashes rub against the surface of the eye (trichiasis).There is a risk here that defects in the corneal epithelium, including a corneal ulcer, can develop.Squeezing the eye tightly can be used to test whether an entropion position can be provoked.In ectropion (Figure 7), the lower eyelid rotates outwards, the tear fluid can no longer properly wet the surface of the eye, which also causes defects in the corneal surface.A typical trigger for non-infectious keratopathy is keratopathia e lagophthalmo.In this case, there is typically an incomplete or completely suspended eyelid closure.Paralysis of the orbicularis oculi muscle is most common in peripheral facial paralysis.An ophthalmological presentation should be made in each of these cases.Surgical correction of the lid misalignment is usually necessary.If an arteriovenous fistula forms spontaneously or traumatically between the internal carotid artery and the cavernous sinus, it is referred to as a carotid-cavernous sinus fistula.This can be accompanied by bilateral, massive, tortuous dilatation of the conjunctival and episceral vessels (Figure 8).Other findings may include: reduced visual acuity, pulsating exophthalmos, double vision, headache, and the perception of a flow noise in the orbit that can be auscultated (36).The ophthalmological diagnosis should be carried out quickly in an interdisciplinary center with modern imaging options (e.g. magnetic resonance angiography).Red eye in systemic diseasesA red eye often occurs, unilaterally or bilaterally, in connection with primarily systemic diseases.Among the primarily dermatological diseases, rosacea, mollusca contagiosa, but also severe atopy are common.Interdisciplinary care of these patients should not be neglected, since these forms can also progress to sterile corneal perforation (37).Incidence According to a study, eye problems account for 2-3% of the consultations in primary care and emergency care.Diagnosis Duration of findings, laterality and pain history are the cornerstones of the diagnosis.Conjunctivitis A red eye and sticky eyelids in the morning are typical.Vasoconstrictive eye drops These should be prescribed for a maximum of a few days as they only start symptomatically and can lead to tachyphylaxis.Contact lens wearers A special form of bacterial (kerato)conjunctivitis often occurs in contact lens wearers.To confirm the diagnosis, the uncleaned contact lens case, including the liquid and lens, is required for pathogen detection.Viral conjunctivitis Viral eye diseases, such as epidemic keratoconjunctivitis, are highly contagious.They occur seasonally due to a smear infection with adenoviruses and begin after an incubation period of 8–10 days.Infection prophylaxis in practice As a prophylactic measure in the case of red eyes, it is advisable to have the affected person wait separately if possible and to hand out disposable gloves.Topical therapy While symptomatic therapy is used for epidemic keratoconjunctivitis, herpetic (kerato)conjunctivitis is treated topically and/or orally with virostatic agents.Non-infectious (kerato)conjunctivitis Typical examples of non-infectious keratopathy are neurotrophic keratopathy and keratopathia e lagophthalmo.Allergic reactions Acute symptoms are bilateral eyelid swelling, epiphora, itching and conjunctival conjunctival hyperemia with chemosis.Hyposphagma Hyposphagmata occur frequently, particularly under oral anticoagulation, poorly controlled or untreated arterial hypertension.Those affected often report previous Valsalva maneuvers.Photoelectric keratitis Photoelectric keratitis causes the cornea to become inflamed as a result of “flashing”.The epithelium of the exposed ocular surface dies off acutely due to UV-C radiation.Episcleritis Episcleritis is a moderately painful inflammation of the connective tissue between the sclera and the conjunctiva, with redness usually limited to one sector and a predominantly conjunctival injection.Primary acute angle-closure Acute angle-closure glaucoma (“glaucoma attack”) is one of the acute emergencies in ophthalmology, which is most commonly caused by pupillary block.Lid misalignments In entropion, the lid edge is inverted toward the bulb and the lashes rub against the surface of the eye.There is a risk here that defects in the corneal epithelium, including a corneal ulcer, can develop.The authors declare no conflict of interest.Manuscript dates submitted: August 18, 2016, revised version accepted: December 29, 2016Address for the authors Dr.medicalAndreas Frings University Clinic for Ophthalmology at Heinrich Heine University Düsseldorf Moorenstraße 5 40225 Düsseldorf andi.frings@gmail.comHow to cite Frings A, Geerling G, Schargus M: Red eye—a guide for non-specialists.Dtsch Ärzteebl Int 2017;114:302-12.DOI: 10.3238/arztebl.2017.0302The German version of this article is available online: www.aerzteblatt-international.deAdditional material Literature marked with "e": www.aerzteblatt.de/lit1717 or via QR codeeTables: www.aerzteblatt.de/17m0302 or via QR codeYou must be registered to comment on articles, news or blogs.If you are already registered for the newsletter or the job market, you can register here 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