The outcomes of this study indicate that the 3D CISS MRI scanning demonstrated that there is a larger structure free space at orbital depths of 12.5 than 25mm which was useful as a non-invasive sequence to confirm the presumed clinical findings that there was greater free space for the important orbital structures at 12.5mm compared to the 25mm depth. 3D CISS MRI with multi-planar reconstructions in axial, coronal and sagittal planes provides an excellent method to demonstrate the orbital anatomy in relation to ophthalmic anesthesia and to characterize the relationship between the adjacent orbital structures with high spatial intrinsic resolution.
The orbit can be divided into 3 anatomical spaces (anterior, mid and posterior) for a better appreciation of the relationship of the injection site. The mid-orbit ends posteriorly about 10–12mm from the back surface of the globe. It contains primarily muscle bellies and adipose connective tissue. The posterior orbit ends at the optic canal and consists mainly of muscle origins and a collection of neurovascular bundles.
A previous study of 150 patients reported that a 15mm needle with digital pressure (with thumb and index finger around the needle hub during injection) gives comparable results to a 25mm needle.
Liu et al. investigated the MRI appearance of the optic nerve in extremes of gaze with implications for the position of the globe for retrobulbar anesthesia and showed that the safe locations for needle insertion are at the extreme inferotemporal corner of the orbit and in the medial area due to a compartment with larger volumes of fat containing adipose connective tissue. Our results using the CISS sequence concur with Liu et al. For example, we found that the inferotemporal fat area is statistically significantly larger at the 12.5mm plane (P=0.033). At deeper planes, the structures are more tightly packed together and the posterior orbital fat is smaller in comparison to the size of the inferior and lateral recti muscle bellies which are closer to each other with little intervening fat at the orbital floor and lateral orbital wall respectively.
There is a larger structure-free space at a depth of 12.5 than at 25mm. Therefore, the inference is that, a needle inserted in the infero-temporal zone to a depth of 12.5mm is less likely to injure the eyeball or extra-ocular muscles than one advanced to 25mm.
Conflict of interest
Trauma with foreign bodies in the eye are not uncommon and may trigger a wide range of complications, including hyphema, cataract, vitreous hemorrhage, and retinal tears and detachment. Missed IOFBs may present with different clinical aspects that may limit their detection and symptoms may only become apparent after a prolonged time period. Certain metallic foreign bodies within the eye may produce retinotoxic ions. Ferrous ions can destroy retinal photoreceptors and pigment epithelial dub ubiquitin leading to siderosis, on the other hand copper containing intraocular foreign body can induce chalcosis. Thus, most metallic IOFBs should be removed promptly to prevent these reactions and minimize intraocular inflammation. Other indication to remove intraocular foreign body is to prevent endophthalmitis, which commonly causes a destructive fibro-vascular response that may ultimately result in blindness. A good patient history and a thorough ocular examination are still the most important factors for diagnosing IOFB. Radiological investigations such as plain X-rays, ocular ultrasonography, computed tomography and magnetic resonance imaging can be used to detect and localize IOFBs. Most intraocular metallic foreign bodies are composed of iron, steel or one of their alloys. We report on a case of a thread-like IOFB in the AC of the right eye that was observed over a five-year period in which the patient first presented with decreased vision.
In January 2007, a seven-year-old boy presented with decreased vision in his right eye after sustaining a trauma while playing with fireworks 2months prior to presentation. He was managed in a suburban hospital with a topical antibiotic and a topical steroid. No surgical intervention was performed at that time. In an examination of the right eye: the un-aided visual acuity was 6/60 on a Snellen chart. A slit lamp examination revealed central horizontal corneal scaring approximately 6mm in length, with the iris adherent to the nasal edge of the scar, which caused a slight irregularity in the AC. A whitish thread that was approximately 5mm in length, was lying obliquely over the iris superiorly at 11 o’clock, and its superior end was hidden at an angle. The inferior end of the thread was embedded within the iris away from the pupil, without touching the endothelium, non-mobile, and with a quiet AC (Fig. 1). It seemed that the trauma caused a full thickness corneal laceration nasally and allowed the foreign body to enter into the AC, which was then sealed by the iris. The iris was adherent to the corneal laceration and caused a slight irregularity of the pupil but maintained the AC form. The pupil was reactive and the intra-ocular pressure was 16mmHg. The lens was clear. Examination of the left eye was normal. A fundus examination showed flat retinas in both eyes. The initial management plan was to remove the IOFB under general anesthesia. The patient did not appear for his appointment and was lost to follow-up. He returned in January 2011, and at that time, the examination of the right eye was as follows: the un-aided visual acuity of 6/12, un-changed findings on the slit lamp examination. A cycloplegic refraction was performed which returned the following results: right eye: +1.75/−2.25×30, left eye was: +0.50/−0.50×165. The patient’s vision improved in the right eye to 6/9 with +1.00/−2.25×30. The un-aided visual acuity of the left eye was 6/6 with normal anterior and posterior segments. The patient was orthophoric and had full extra-ocular muscle movement in both eyes. No further investigations were requested. The patient was again lost to follow-up but returned in September 2012 complaining of headaches after lengthy reading. The right eye vision was 6/6 without correction with the same slit lamp examination. The right eye had an IOP of 16mmHg with a normal fundus, and the left eye was normal and had an IOP of 16mmHg. The cycloplegic refraction of the right eye was: +0.25/−4.00×15, the left eye was: +0.50/−0.75×175. By subjective refraction, the vision of the right eye was 6/6 with −3.25×15, the left eye was 6/6 with-0.50×175. The patient’s sight was clear with spectacles.