Intravitreal Injection Therapy: The Procedure Behind the Molecule

Fifteen years ago, a diagnosis of wet macular degeneration meant a slow, predictable slide toward legal blindness.

Today the same diagnosis is met with a needle – and, for most patients, a stable retina for years afterward.

Intravitreal injection is the delivery route that made anti-VEGF therapy possible, and it is now one of the most frequently performed procedures in medicine.

The pharmacology gets the attention, but the outcome in any individual patient is decided just as much by the injection itself – the sterile technique, the timing, the choice between fixed dosing and treat-and-extend.

A well-executed intravitreal injection service is what turns a molecule with excellent trial data into a real-world result.

What Is Intravitreal Injection Therapy?

Intravitreal injection is the direct delivery of a drug into the vitreous cavity through the pars plana, bypassing the blood-retinal barrier that limits systemic or topical drugs from reaching the retina in useful concentration.

The agents delivered this way fall into several classes:

Anti-VEGF agents – ranibizumab, aflibercept, bevacizumab (used off-label), brolucizumab and the bispecific agent faricimab, which additionally blocks angiopoietin-2
Corticosteroids – dexamethasone and fluocinolone acetonide implants, for inflammatory and refractory oedema
Complement inhibitors – pegcetacoplan and avacincaptad pegol, for geographic atrophy
Antimicrobials – for infectious endophthalmitis

The injection technique itself is essentially identical across all of these; what differs is the frequency and the disease being treated.

Indications

Intravitreal therapy has become first-line treatment across a wide range of retinal disease.

Neovascular age-related macular degeneration – the original and still the highest-volume indication
Diabetic macular oedema
Macular oedema following retinal vein occlusion, both branch and central
Myopic choroidal neovascularisation
Geographic atrophy, with complement inhibitors aimed at slowing lesion growth rather than restoring vision
Endophthalmitis, treated with intravitreal antibiotics as an emergency

The sheer breadth of this list is why injection clinics have become one of the largest single workloads in modern ophthalmology.

Anti-VEGF Pharmacology

Understanding why the drug is injected, rather than given systemically, explains most of the practical decisions around treatment.

Vascular endothelial growth factor drives the abnormal vessel growth and vascular leakage underlying wet AMD, diabetic macular oedema and venous occlusion
Anti-VEGF agents bind and neutralise VEGF, reducing leakage and, in many eyes, causing regression of neovascular complexes
Bevacizumab, ranibizumab and brolucizumab are single-target anti-VEGF-A agents; aflibercept also binds placental growth factor
Faricimab is a bispecific antibody blocking both VEGF-A and angiopoietin-2, targeting vessel stabilisation as well as VEGF suppression
Effect duration varies by molecule, which is what allows some newer agents to be dosed less frequently in treat-and-extend regimens

The clinically important point is that these are not cures – they suppress an active disease process, and most patients need sustained treatment to keep the gains they achieve.

Injection Technique

Pre-Injection Preparation

Topical antisepsis with povidone-iodine applied to the ocular surface and periocular skin immediately before injection – the single intervention with the strongest evidence for reducing endophthalmitis risk
Topical anaesthesia, and a lid speculum to keep the lashes clear of the injection site
Marking or estimating the injection site, typically 3.5 to 4 mm posterior to the limbus in a phakic eye and 3.5 mm in a pseudophakic eye

The Injection

A fine-gauge needle is passed through the pars plana, aimed toward the mid-vitreous cavity away from the lens and macula
The drug is delivered as a fixed-volume bolus appropriate to the agent
The needle is withdrawn and the injection site checked for reflux or a wick of vitreous

Post-Injection Care

A check of light perception or hand movement vision immediately after injection to exclude central retinal artery occlusion from a pressure spike
Patient counselling on red-flag symptoms – worsening pain, redness or vision loss – that should prompt urgent same-day review
No routine post-injection topical antibiotics; evidence does not support their use and they add cost without benefit

Because the whole procedure takes only a few minutes, technique variation between injectors is easy to overlook – yet it is precisely what separates a low-complication clinic from a high one.

Treatment Regimens

Fixed Dosing

Monthly or bimonthly injections given regardless of disease activity
Predictable and simple to plan, but the highest treatment burden for patients and clinics

Pro Re Nata (PRN)

Injection only when disease activity is detected on monthly monitoring
Reduces injection number but requires frequent visits regardless, and can allow disease to reactivate between assessments

Treat and Extend

Injections given at every visit, with the interval between visits progressively lengthened when the retina remains dry and shortened if fluid recurs
Aims to maintain continuous disease control while minimising visit frequency, and has become the dominant real-world strategy

Treat and extend has largely displaced strict PRN in practice because it avoids the monitoring-only visits where disease is found to have already reactivated.

Complications

Injection-Related

Endophthalmitis – rare but sight-threatening, presenting with pain, redness and vision loss usually within days of injection, requiring emergency vitreous tap and intravitreal antibiotics
Transient intraocular pressure spike, usually self-limiting but relevant in patients with pre-existing glaucoma
Subconjunctival haemorrhage – common, cosmetically alarming, but clinically benign
Retinal detachment or vitreous haemorrhage from inadvertent globe or retinal trauma – rare with correct technique
Traumatic cataract from lens touch, largely avoidable with correct needle angle and pars plana measurement

Drug-Related

Sterile intraocular inflammation, reported with some agents more than others, notably brolucizumab
Rare arterial thromboembolic events with systemic absorption, a theoretical concern that has driven long-term safety monitoring
Geographic atrophy progression has been reported with long-term anti-VEGF use in some studies, though causation remains debated

Endophthalmitis is the complication that defines injection-day protocol, because outcome depends almost entirely on how quickly it is recognised and treated once it occurs.

Patient Counselling

Realistic expectation-setting – most patients achieve stabilisation or modest improvement, not full restoration of lost vision
The long-term nature of treatment, often extending over years, and the impact of appointment burden on adherence
Clear red-flag symptoms and a direct route to urgent review if they occur
The rationale for treat-and-extend, since patients who understand why intervals change are more likely to tolerate a shortened interval without alarm

Adherence is, in practice, the single largest determinant of long-term visual outcome across every indication treated this way.

Prognosis

Outcomes depend heavily on the disease being treated and on how consistently therapy is delivered.

Neovascular AMD treated promptly and consistently maintains vision in the large majority of patients, with meaningful gains in many
Diabetic macular oedema responds well to anti-VEGF therapy, though a subset of eyes are relatively resistant and may benefit from steroid implants instead
Delayed or interrupted treatment – common in real-world practice compared with trial conditions – is consistently associated with worse visual outcomes than in clinical trials
Geographic atrophy treatment slows anatomical progression but has not been shown to improve visual acuity

The gap between trial results and real-world outcomes in this field is almost entirely explained by treatment persistence – the drugs work when they are actually given on schedule.

References

Rosenfeld PJ, Brown DM, Heier JS, et al. Ranibizumab for neovascular age-related macular degeneration. New England Journal of Medicine. 2006.
Heier JS, Khanani AM, Quezada Ruiz C, et al. Efficacy, durability, and safety of faricimab: TENAYA and LUCERNE trials. The Lancet. 2022.
Royal College of Ophthalmologists. Intravitreal injection therapy guidelines.
Anti-VEGF Intravitreal Therapy. EyeWiki, American Academy of Ophthalmology.
Intravitreal Injections. StatPearls, NCBI Bookshelf.