RNFL thickness deviation map enhances assessment of glaucoma progression

Map generated with spectral-domain optical coherence tomography proves more sensitive than circumpapillary measurement with time-domain OCT.

Retinal nerve fiber layer thickness mapping proved more reliable than circumpapillary retinal nerve fiber layer assessment in detecting glaucomatous eye damage and disease progression, a study found.

Investigators performed circumpapillary retinal nerve fiber layer (RNFL) measurement with time-domain optical coherence tomography (Stratus OCT, Carl Zeiss Meditec) and generated an RNFL thickness deviation map with spectral-domain OCT (Cirrus HD OCT, Carl Zeiss Meditec).

“Analyzing the RNFL thickness map obtained with spectral-domain OCT imaging is a more sensitive approach to detect glaucomatous RNFL defects compared with circumpapillary RNFL measurement,” Christopher K.S. Leung, MD, the corresponding author, told Ocular Surgery News in an e-mail interview.

Advantages of spectral-domain OCT

Dr. Leung discussed the novelty of using the RNFL thickness deviation map to image anatomic features and gauge glaucoma progression.

“Analysis of the RNFL thickness map provides a novel approach to evaluate the pattern of RNFL defects and facilitates the detection of RNFL changes in disease progression,” he said.

Time-domain OCT is more time-consuming and less sensitive than spectral-domain OCT in mapping RNFL thickness, the authors said. Spectral-domain OCT offers rapid scanning and three-dimensional high resolution of 200 × 200 pixels.

“The standard circumpapillary RNFL measurement is derived from a circle scan with a diameter of 3.45 mm centered at the optic disc,” Dr. Leung said. “Analysis of RNFL defects is based on the RNFL profile displayed in a 2-D graph. In contrast, the RNFL thickness deviation map provides visualization of the spatial distribution of RNFL defects in the 6 mm² × 6 mm² optic disc region.”

Patients and methods

The prospective study, published in Ophthalmology, included 121 patients with glaucoma and 102 healthy control subjects. Inclusion criteria were visual acuity of 20/40 or better and spherical refractive error within +4 D and –8 D. Patients with macular disease, history of refractive or retinal surgery, neurologic disease or diabetes were excluded. Glaucoma was defined by visual field defects regardless of optic nerve head and RNFL appearance. Normal eyes had a normal visual field with no history of IOP exceeding 21 mm Hg.

Glaucoma patients had a mean age of 54 years and controls had a mean age of 50.3 years. The between-group difference in mean age was statistically significant (P = .031).

One eye of each patient was imaged with time-domain OCT and spectral-domain OCT. The Humphrey Field Analyzer II (Zeiss) was used to test visual fields.

A five-point scale was used to analyze RNFL thickness map with reference to the defect size, shape, depth, location and distance from disc margin; a score of 5 denoted glaucomatous RNFL defect and a score of 0 denoted no defect. A masked observer scored each feature independently.

Clock-hour and average RNFL thickness categorical classifications were used to analyze the diagnostic performance of circumpapillary RNFL measurement.

Primary outcome measures were diagnostic sensitivity and specificity. Map sensitivity and specificity were rated for scores of 3 or more, 4 or more, or 5.

Sensitivity and early detection

The RNFL thickness deviation map showed sensitivities of 92.1% to 98.3% for glaucoma detection. A map score of 3 or more had a specificity of 72.5%, a score of 4 or more had a specificity of 87.3%, and a score of 5 had a specificity of 95.1%. The differences were statistically significant (P = .014).

A map score of 5 correlated with greater diagnostic sensitivity (95%) than all other clock-hour or average RNFL thickness classifications obtained with Status OCT or Cirrus HD-OCT, the authors said.

“The RNFL thickness map can detect RNFL defects missed by the standard circumpapillary scan,” Dr. Leung said. “The difference in sensitivity between the two approaches is most obvious in patients with early glaucoma.”

Dr. Leung advised surgeons to thoroughly scrutinize spatial findings gleaned from analysis of the RNFL thickness deviation map, particularly the distance of defects from the optic disc margin.

“In analyzing RNFL damage in the OCT RNFL analysis printout, clinicians are advised to examine the RNFL thickness deviation map with reference to the size, shape, depth, location and distance from the disc margin of the defects,” he said. – by Matt Hasson

Reference:

• Leung CKS, Lam S, Weinreb RN, Liu S, Ye C, Liu L, et al. Retinal nerve fiber layer imaging with spectral-domain optical coherence tomography. Ophthalmology. 2010;117:1684-1691.

• Christopher K.S. Leung, MD, can be reached at Department of Ophthalmology and Visual Sciences, Chinese University of Hong Kong, Hong Kong, PRC; e-mail: tlims00@hotmail.com. Dr. Leung has received research support and honorariums for conference presentation from Carl Zeiss Meditec.