Kelvin Z. Li

Topographic variation of choroidal and retinal thicknesses at the macula in healthy adults.

Background/aims To determine the topographic variation of macular choroidal and retinal thicknesses (RTs) in normal eyes and their relationship with refractive error. Methods Spectral domain optical coherence tomography with enhanced depth imaging was performed on 124 healthy participants using a standardised imaging protocol. Manual segmentation of choroidal boundaries was performed by trained graders, and mean choroidal thickness (CT) was compared with mean RT in corresponding sectors of the Early Treatment Diabetic Retinopathy Study (ETDRS) grid. Results Mean central subfield CT was 322.2 µm. The choroid was thickest at the temporal and superior sectors (323.1–338.1 µm), followed by inferior sectors (314.0–321.8 µm), and thinnest at the nasal sectors (232.8–287.8 µm). In contrast, the retina was thicker nasally (343.4 µm) and thinner temporally (287.1 µm). CT was thickest among emmetropes in all ETDRS subfields and became thinner progressively among low, moderate and high myopes (p<0.001). The variation of both choroidal and RTs among refractive error groups resulted in different topographic patterns at the macula. Conclusion There is significant topographic variation of choroidal and RTs at different regions of the macula, with progressive change of choroidal thickness in all sectors based on the refractive status of the eye.

A novel technique of adjusting segmentation boundary layers to achieve comparability of retinal thickness and volumes between spectral domain and time domain optical coherence tomography.

PURPOSE The quantitative assessment of retinal thickness and volume varies according to the optical coherence tomography (OCT) machine used due to differences in segmentation lines. We describe a novel method of adjusting the segmentation lines of spectral-domain OCT (SD-OCT) to enable comparison with time-domain OCT (TD-OCT), and assess factors affecting its accuracy. METHODS In a prospective study, SD-OCT (Spectralis OCT) and TD-OCT (Stratus OCT) were sequentially performed on 200 eyes of 100 healthy individuals. Central retinal thickness (CRT), central point thickness (CPT), and 1-mm volume of the Early Treatment Diabetic Retinopathy Study grid were compared between the two machines. The segmentation lines on SD-OCT were manually adjusted by a trained operator and the parameters compared again with TD-OCT. RESULTS The mean CRTs of Spectralis and Stratus were significantly different (268.2 μm vs. 193.9 μm, P < 0.001). After adjustment of segmentation lines, the mean adjusted Spectralis CRT was 197.3 μm, with the difference between SD-OCT and TD-OCT measurements decreasing from 74.3 μm to 3.4 μm (P < 0.001). The difference between the adjusted Spectralis and Stratus CRTs was smallest for high myopes (≤ -6.0 diopters [D]) compared with those with moderate and low myopia (1.5 μm vs. 3.5 μm and 4.6 μm, respectively; P < 0.001). Similar trends were obtained for central 1-mm volumes and CPT. Interoperator and intraoperator repeatability for adjustment of the segmentation lines were good, with an intraclass correlation of 0.99 for both. CONCLUSIONS Manual adjustment of SD-OCT segmentation lines reliably achieves retinal thickness and volume measurements that are comparable to that of TD-OCT. This is valuable to allow comparisons in multicenter clinical trials where different OCT machines may be used.

Diurnal variation of retinal thickness measured by optical coherence tomography in normal adults.

We read with interest the article by Jo et al. in the August 2011 issue describing the diurnal variation of retinal thickness in normal subjects measured by time-domain (TD) and spectraldomain (SD) optical coherence tomography (OCT). The authors concluded that the diurnal variation in the retinal thickness observed with TD-OCT was caused by limitations in the repeatability of the OCT device rather than by actual variation of retinal thickness. Although this was a well-designed study, one limitation was that the retinal thickness was assessed at only two time points, which provides limited information on the pattern of diurnal change. A difference between two time points is less compelling than if the retinal thickness measurements differ across several additional points during the day. We would like to highlight two other studies in normal subjects that provide additional evidence to support the findings of Jo et al. A significant strength of both of the additional studies is that the retinal thickness was measured at five time points during a single day, thus providing more comprehensive and robust data for the analysis of diurnal variation compared with the two time points that were used in the present study. In addition, in both studies, the examination sequence (five OCT scans during the day) was repeated on a separate day to confirm the consistency of the results. In a study of 12 normal individuals, sequential SD-OCT scans were performed with the Spectralis OCT (Heidelberg Engineering, Heidelberg, Germany). The authors found no significant variation in the macular thickness over the five time points during the day, with a maximum amplitude (the difference between the maximum and minimum retinal thickness) of 0.9 m. This result is comparable to the difference of 0.3 m reported by Jo et al. for central macular thickness measured with the Cirrus OCT (Carl Zeiss Meditec, Dublin, CA). In addition, this study used the tracking function of the Spectralis OCT to reduce the chance of a small displacement of the foveal B-scan position between sequential OCT scans and to ensure that the same point on the retina was measured for all OCT scans. We feel that this is a useful and important advantage, conferred by the tracking function of the Spectralis OCT. In another study, Chakraborty et al. 3 used an optical biometer instead of OCT to measure retinal thickness. They also reported no significant diurnal variation in retinal thickness (mean amplitude, 8 m) over the 2 days of examination. Another interesting point that merits discussion is that, in Jo et al., in the sectors that showed a significant difference in retinal thickness on TD-OCT, the mean difference ranged from 2.5 to 3.5 m. Although this difference may be statistically significant, it is unlikely to be of clinical relevance. In addition, the coefficient of repeatability of Stratus OCT retinal thickness in normal individuals has been reported to be approximately 17 m, which is much greater than the difference between the two time points in this study. Jo et al. also cited earlier studies that have reported diurnal variation in retinal thickness in patients with macular diseases. In these studies, various versions of TD-OCT were used to perform the OCT scans. Although it is certainly possible that the diurnal variation in retinal thickness may be due to the underlying disease process, it is also possible that at least part of it is due to motion artifact resulting from poor fixation as a result of the underlying disease process. It would be interesting to know whether a similar degree of diurnal change in retinal thickness in diseased eyes would be observed with tracking-capable SD-OCT devices. In summary, in confirming this finding using SD-OCT with tracking, we agree with Jo et al. that there does not appear to be significant diurnal variation in retinal thickness in normal individuals. Colin S. H. Tan Wei Kiong Ngo Milton C. Chew Kelvin Z. Li Louis W. Lim Srinivas R. Sadda

Abdominal schwannomas: case report with literature review.

Schwannomas are rare tumors that arise from Schwann cells in neural sheaths. They are commonly found in the central nervous system, spinal cord, or peripheral nerves of the body. Occasionally, they occur in the gastrointestinal tract, with the stomach being the most common site. However, colorectal and retroperitoneal schwannomas are very rare. Preoperative diagnosis is often difficult and definitive treatment entails surgical excision. We herein present 3 cases of intraabdominal schwannomas.

Equations to calculate central subfield thickness on optical coherence tomography

Dear Editor, We read with interest the article by Dr Abedi and co-authors comparing the measurements of central subfield macular thickness (CSMT) between the Cirrus and Stratus optical coherence tomography (OCT) machines [1]. The authors reported a mean difference of 58.91 μm between the Cirrus and Stratus CSMT [1], which is due to the differences in position of the segmentation lines drawn by the software of the respective OCT machines [1–3]. The authors then proposed an equation for the conversion of Cirrus OCT measurements to an equivalent Stratus OCT CSMT. This equation was derived from the linear transformation of eyes with signal strength 6 or more that maximizes the agreement between 2 OCTmeasurements [1]. The results calculated from this equation were then compared using Lin’s concordance coefficients and Bland–Altman plots. Since the equation was originally derived from the authors’ data using linear transformation, applying this equation to the same group of subjects would be expected to yield transformed Cirrus CSMT values that have good correlations. Instead, we suggest that it would have been more robust to have used one group of subjects to derive the equation, then apply this equation on an unrelated set of Cirrus OCT measurements (a group that was not used to derive the equation) and evaluate the outcomes in this group instead. We performed a similar study where sequential timedomain and spectral-domain OCTs were performed on both eyes of 80 healthy volunteers (160 eyes). We randomly selected 80 eyes of 40 subjects (the equation group) and used linear regression to derive a similar transformation equation. This equation was then used to calculate the equivalent CSMT on both the original equation group and the remaining 40 subjects (the test group). The mean difference between the Stratus and transformed CSMT was 6.0 μm for the equation group, and 6.1 μm for the test group. Performing intra-class correlations (ICC), we found that while the agreement between the Stratus and transformed CSMT was good for both groups, the ICC for the equation group was higher than that of the test group (0.904 vs 0.886). Based on these results, we suggest that it is useful to test an equation derived from a specific group on a separate test group to ensure its reliability. In our study, there was a small drop in comparability when the equation was applied to a separate group, although the overall reliability was still very good. It is also important to test this equation on other populations of normal individuals, and, as the authors have suggested, patients with retinal pathology, to determine whether this can be applied to a wider population. Nevertheless, the authors’ results are an important step in determining ways to achieve comparability between time-domain and spectraldomain OCT machines. The authors have no financial or proprietary interests in the subject of this correspondence

Charles Bonnet syndrome and Terson’s syndrome from subarachnoid hemorrhage: good news from bad news

Dear Editor, We read with interest the report by Cebulla et al. [1] reporting visual hallucinations associated with Charles Bonnet syndrome (CBS) in a patient with bilateral dense vitreous hemorrhage from subarachnoid hemorrhage. The authors report rapid improvement in her cognition and cessation of the hallucinations after pars plana vitrectomy was performed in one eye. This is consistent with earlier reports where improvement of visual acuity leads to a reduction or cessation of the hallucinations in CBS [2–4]. The authors referred to the “commonly limited prognosis for hallucinations” in CBS[1]. While it is true that the hallucinations do persist in some patients, in others these hallucinations may cease spontaneously after a period of time, even if the visual loss is irreversible. In fact, it has been suggested that an acute change in visual acuity may be the trigger for the onset of visual hallucinations in CBS [5, 6] and the hallucinations cease once the visual acuity has stabilized. Patients with CBS have been treated with a variety of drugs to control the hallucinations. However, in our experience, reassurance that the hallucinations are normal, and that they are not “mad” or suffering from a psychiatric disease, are a source of great relief to patients [3]. We would also like to point out that although older age is a risk factor for CBS, this condition can occur in patients of any age group [3], even in the pediatric age group [7, 8]. Therefore, it is important to consider this diagnosis even in younger patients who present with visual hallucinations without other neurological causes.

Evaluation of choroidal thickness in psoriasis using spectral-domain optical coherence tomography

We congratulate Türkcü et al. [1] for their paper evaluating the choroidal thickness in patients with psoriasis using spectral-domain optical coherence tomography (OCT). The authors reported that the choroid is thicker in patients with psoriasis compared to normal controls. When comparing choroidal thicknesses among various groups, it is important to consider the ocular and systemic factors that affect choroidal thickness. Studies have reported that choroidal thickness is influenced by a variety of factors, including age, axial length and refractive error [2]. While extremes of refractive error were excluded in the current study, we are curious to know whether the two groups were comparable in terms of spherical equivalent and axial length. Earlier studies have shown that choroidal thickness varies by 6.205–25.4 lm per dioptre and -17.031 to -58.2 lm per mm of axial length [2]. Hence, any significant difference in ocular parameters may potentially affect the choroidal thickness. In the current paper, the authors reported a single measurement for choroidal thickness, which we assume is the subfoveal choroidal thickness. Studies have shown, however, that the choroid exhibits considerable topographic variation throughout the macula [2, 3]. In normal eyes, for example, the choroid is thickest at the temporal and superior sectors and thinnest at the nasal sectors. It would be interesting to know whether patients with psoriasis exhibit any differences in the topography of the choroid compared to healthy controls. It is possible that these differences may manifest in other regions of the macula. In the methods section, the authors reported that two observers measured choroidal thickness and measurements were excluded if the difference between observers was higher than 15%. This highlights the variability of choroidal thickness measurements [3], which further emphasizes the importance of comparing thicknesses at different locations of the macula, rather than at a single point. We are curious to know how many eyes were excluded as a result of a discrepancy in measurements exceeding 15%. It is significant that the authors performed the OCT scans during a specific time period (between 9 a.m. and 12 p.m.). Studies have reported significant diurnal variation of choroidal thickness throughout the day [4], with amplitudes ranging from 3 to 67 lm (mean 33.7 lm). If the timing of the OCT scans had been performed at random times, then differences in K. Z. Li C. S. Tan (&) National Healthcare Group Eye Institute, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore e-mail: Colintan_eye@yahoo.com.sg

Choroidal thinning in Fuchs Uveitis Syndrome

Dear Editor, We read with interest the article by Cerquaglia et al. [1] describing thinning of the choroid in eyes with Fuchs Uveitis Syndrome compared to the uninvolved fellow eyes. The authors reported differences in choroidal thickness when assessing the overall choroidal thickness, as well as individual layers such as the Haller’s Layer and the Sattler’s-choriocapillaris layer. This paper illustrates the important role played by newer imaging technologies such as optical coherence tomography (OCT) in the evaluation and monitoring of ocular diseases [2–5]. Choroidal thickness has been reported to vary with different retinal conditions [5–7] and may be a marker of disease and its progression. The authors reported that only patients without any sign of active inflammation in the anterior chamber over the last 6 months were enrolled. We are curious whether the authors determined when the diagnosis of Fuchs Uveitis Syndrome was first made, and whether the authors correlated the duration of the disease with the differences in choroidal thickness between contralateral eyes. As the authors pointed out, during the active phase of some inflammatory conditions, patients may manifest with thickening of the choroid [1]. In contrast, choroidal thinning observed during the chronic phase may be the result of ischemic changes and atrophy, which occur during and following the course of the inflammation. It would be interesting, therefore, to know how the choroidal thickness in the affected eye varied through the various stages of the disease. In this study, the relative deviation of choroidal thickness varied with location, especially for the Sattler’schoriocapillaris layer, where it varied from 33.7% to 43.3% [1]. In addition, the difference in subfoveal Haller’s layer choroidal thickness between groups was not statistically significant. This highlights the importance of considering spatial variation of the choroid when evaluating its thickness. Studies have reported considerable topographic variation of the choroid in normal eyes [8–10]. For example, the choroid is thinnest in the nasal sectors, and thickest in the superior and temporal sectors [8, 10]. The current study provides some indication that differences in choroidal thickness between diseased and normal eyes may vary with the region where the choroid is measured, and it would be interesting in future studies to evaluate the differences in choroidal thickness in other regions of the macula, such as superiorly and inferiorly. In summary, we congratulate the authors on their study, which provides insights into the effects of Fuchs Uveitis Syndrome on the choroid, and look forward to follow-on studies elucidating the variation of choroidal thickness in different regions of the eye.

Comment on visual impact of sub-Tenon anesthesia during combined phacoemulsification and vitrectomy surgery.

Dear Editor, W e read with interest the article by Chen [1] on the visual impact of sub-Tenon anesthesia during combined phacoemulsification and vitrectomy surgery. The authors reported that 93.3% of patients in the study experienced loss of perception of light at least during one of the surgical steps after the contralateral eye was covered intraoperatively[1]. Most of the patients (81%) reported losing light perception between 6min and 30min from the commencement of the surgery. While it is not unusual for patients to experience loss of light perception during vitrectomy, Chen [1] have reported a higher incidence of loss of light perception compared to other similar papers. An earlier study of patients undergoing vitreous surgery performed under regional (peribulbar or retrobulbar) anesthesia reported that 29.2% of patients experience transient loss of light perception, while 24.6% of patients experienced no light perception throughout the entire duration of the surgery[2]. Among studies of patients undergoing phacoemulsification, the rate of loss of light perception varied from 0 to 20%[3]. The loss of light perception in ophthalmic surgery is not only limited to phacoemulsification and vitrectomy surgery. Among patients who underwent laser keratomileusis (LASIK) surgery, 75.6% also experienced loss of light perception during corneal flap fashioning [4]. This was higher in the eyes where the Zyoptix XP microkeratome was used, compared to those who underwent intralase during corneal flap fashioning (90.2% 61.0%, =0.004). In that study, 62% of patients also reported lost of light perception during vacuum suctioning. The higher incidence of loss of light perception reported by Chen [1] may be due to various factors such as differences in demographics, or disease severity in this cohort. Regardless of the reason, an important consideration is that many patients may not expect to experience no light perception during an ophthalmic surgery. In a study by Ang [5] on patients expectation and experience of visual sensations during phacoemulsification, 36.7% of patients expected to experience at least light perception during the surgery, while 24.5% of them were unsure what to expect. Among patients expecting to see light, the loss of light perception may be frightening , as these patients may assume that complications have occurred during the surgery. Fear experienced by patients is undesirable as it may cause patients to be uncooperative intraoperatively, and may also trigger a sympathetic stress response [7]. This highlights the importance of counseling the patients pre-operatively on what to expect. In addition to no light perception, many patients also experienced various other visual sensations such as color, movement, flashes and instruments . As many as 19.4% of the patients found their visual experience frightening[5]. It will be equally important to highlight these to patients during their pre-operative counseling. In summary, we congratulate the authors on their paper, and suggest that patients undergoing ocular surgery should be counseled pre-operatively on the possibility of intra-operative visual sensations, including loss of light perception. ACKNOWLEDGEMENTS Foundation: Supported by the National Healthcare Group Clinician Scientist Career Scheme Grant CSCS/12005. Conflicts of Interest: Li KZ, None; Tan CS, receives travel support from Bayer, Heidelberg Engineering and Novartis. REFERENCES

Changes in choroidal thickness after photodynamic therapy for Sturge–Weber syndrome

We read with interest the article by Cacciamani et al. on choroidal thickness (CT) changes with photodynamic therapy (PDT) for a diffuse choroidal hemangioma in Stuge–Weber syndrome (SWS) [1]. The authors reported a reduction in CT after PDT and recommend that CT changes should be noted in patients undergoing PDT for the treatment of choroidal hemangioma. While it is useful to describe the CT in patients with choroidal haemangioma, it is also important to consider the timing when the choroid is imaged. Several studies have previously described the diurnal variation of CT in normal, healthy subjects measured by spectral domain optical coherence tomography [2], where it has been reported that the choroid was thickest in the morning at 9 am and progressively thinned throughout the day. The mean amplitude (difference between the thickest and thinnest value) was 33.7 lm, but could be as large as 43.1 lm [2]. In Cacciamani’s study, it is unclear if the repeated choroid measurements were taken at approximately the same time of day. If not, this may have an effect on the observed difference in choroidal thickness. The authors described an initial change in CT from 251 to 236 lm following PDT and the difference of 15 lm could partly be explained by the effects of diurnal variation. In addition, measurement of CT is often only performed at the fovea. However, the choroid is a three-dimensional vascular structure with significant topographic variation [3]. Measurement of the central point thickness, therefore, provides limited information on general changes of the choroid, and it may be more informative to assess mean CT within various sectors of the early treatment diabetic retinopathy study (ETDRS) grid. There have also been previous reports that CT decreases after PDT. Interestingly, Sonoda et al. showed that both the luminal and interstitial components of choroid decrease after PDT, but the luminal area decreased more than the interstitial area [4]. This has implications on choroidal hypoperfusion and hence careful follow-up after PDT is necessary to determine the long-term effects of PDT. Other than the use of spectral domain OCT, recent advances in imaging such as swept source (SS) OCT now enable ophthalmologists to image deeper structures such as the choroid with unprecedented clarity. K. Z. Li C. S. Tan National Healthcare Group Eye Institute, Tan Tock Seng Hospital, Singapore 308433, Singapore