The terminology used for regional orbital blocks is controversial. An intraconal (retrobulbar) block involves the injection of a local anesthetic agent into the muscle cone behind the globe formed by four recti muscles and the superior and inferior oblique muscles. An extraconal (peribulbar) block involves the injection of local anesthetic outside the muscle cone. Many studies confirm that there are multiple communications between two compartments, and it is sometimes difficult to differentiate if the injection is intraconal or extraconal. A combination of intraconal and extraconal blocks is described as combined retroperibulbar block.

The average distance between the orbital margin and the ciliary ganglion is approximately 38 mm (range 32–44 mm). If a 38-mm needle is used, it may reach the optic nerve in a large number of patients leading to serious catastrophic events. A needle <31 mm must be used to protect the globe, optic nerve, and other major structures behind the globe during both extraconal and intraconal blocks. In cadaver studies, Karampatiakis et al. found that when a retrobulbar block was carried out with a needle 40 mm long, the needle tip would reach the posterior optic nerve in 100 % of cases; even with needles 35 mm long, the covering of the optic nerve would be touched [2].

Average axial length (anteroposterior distance of the globe) of the eye varies from 22 to 24 mm. Eye with axial lengths >26 mm are more prone to globe damage. The axial length measurement is available during cataract surgery. If a patient is scheduled for non-cataract surgery, spherical power of the spectacles may offer some clue. If a patient has very thick prescription glasses, it indicates high myopia. The risk of damage to the globe and optic nerve is also greater when the globe is rotated during injection. Therefore, it is recommended that the globe should be in the neutral gaze during the injection. It is safer to introduce a needle as far lateral as possible because if it is introduced at the junction of medial 2/3rd and lateral 1/3rd of the inferior orbital rim, the inferior rectus and inferior oblique muscles and their nerves may be damaged.

In sub-Tenon block, local anesthetic agent is injected under the Tenon capsule. This block is also known as parabulbar block, pinpoint anesthesia, and medial episcleral block.

The physiological pressure (intraocular pressure) interior of the eye is between 10 and 20 mmHg. It is higher in patients with a large-diameter eyeball and in the recumbent position. It is higher in the morning than in the evening. It increases during coughing, physical exertions, and vomiting. An increase in the plasma concentration of carbon dioxide and decrease in oxygen concentration increase the intraocular pressure. Intraocular pressure increases after the needle technique, and this is due to increase in the volume behind the globe. There is little or no increase in intraocular pressure after sub-Tenon block. The anterior and posterior chambers of the eye contain 250 μl of an aqueous liquid (rate of synthesis ca. 2.5 μl/min).

Both intraocular procedures (cataract extraction, vitrectomy) and extraocular procedures (strabismus surgery, retinal detachment) can be carried out under local anesthesia in suitable patients. There are only a few absolute contraindications such as patient refusal to accept local anesthesia, allergy to local anesthetic agent, local infection, excessive abnormal body movements, severe breathlessness, psychiatric conditions, and in situations where it is difficult to establish a good communication between patients and health-care workers, but the list is shrinking and local anesthesia is increasingly used even in the above group.

Most ophthalmic surgical procedures are carried out as outpatient basis. There is debate as to the degree of preoperative assessment and investigation required and they vary worldwide. In the UK, the Joint Colleges Working Party Report recommended that routine investigations are unnecessary and the patients are not fasted [3]. Routine investigation of patients undergoing cataract surgery under regional anesthesia is not essential because it neither improves the health nor improves the outcome of surgery, but tests can be done to improve the general health of the patient if required [4].

The preoperative assessment should always include a specific enquiry about bleeding disorders and related drugs. There is an increased risk of hemorrhage, and this requires that a clotting profile is available (and recorded) prior to injection (see later) [5, 6]. Patients receiving anticoagulants are advised to continue their medications. Clotting results should be within the recommended therapeutic range. Currently, there is no recommendation for patients receiving antiplatelet agents. Blood pressure should be controlled in hypertensive patients. Diabetic patients are allowed to take their usual medications, and active control of blood sugar is not necessary. There is no need for antibiotic prophylaxis in patients with valvular heart disease, and surgery is not performed if patient has suffered myocardial infarction during the last 3 months [3]. The presence of a long eye, staphyloma, or enophthalmos, the use of faulty technique, a lack of appreciation of risk factors, an uncooperative patient, and the use of unnecessarily long needles are some of the contributing causes. Knowledge of axial length measurement is essential. A precise axial length measurement is usually available for intraocular lens diopter power calculation before cataract surgery [7]. If the block is performed for other surgery and the axial length measurement is not known, close attention to the diopter power of patients’ spectacles or contact lenses may provide valuable clues to globe dimension (patients usually provide this history).

The patient and the person performing the block must be involved in full discussion of the proposed technique. The anesthetic and surgical procedures are explained to the patients. Patient is placed in a comfortable position, and oxygen is administered via a suitable device. All monitoring and anesthetic equipments in the operating environments should be fully functional. Blood pressure, oxygen saturation, and ECG leads are connected, and baseline recordings are obtained. Insertion of an intravenous line is a prerequisite for needle block, but its use has been questioned during sub-Tenon injections [2]. It is not unusual to observe adverse medical events in elderly patient, and a working intravenous line could only be a good clinical practice.

The use of sedation varies in different parts of the world and may or may not be used. Sedation is used during block procedure, surgical procedure, or during block and surgery. However, sedation is common during topical anesthesia. Selected patients, in whom explanation and reassurance have proved inadequate, may benefit from sedation. Short-acting benzodiazepines, opioids, and small doses of intravenous anesthetic induction agents are favored, but the dosage must be minimal. An increased incidence of adverse intraoperative events is anticipated with sedation in elderly [6].

It is recommended that one of the following drugs is given immediately before carrying out regional anesthesia:

Deep sedation should not accompany regional anesthesia; the patient should be capable of cooperating.

The needle is deliberately directed toward the apex and within the muscle cone (see above) while the globe is in neutral gaze.