Disc Damage Likelihood Score (DDLS)
Glaucoma is defined as a process wherein there is progressive loss of retinal ganglion cells manifest clinically as loss of neuroretinal rim tissue from the optic nerve. In order to detect this, a clinician must have a method to identify these changes and distinguish them from normal. There is also a need for a system to document any change in optic nerve appearance with time in order to determine progression. The concept of cup:disc ratio(CDR) was developed by Armaly in 1967 (1) as a standardised way of documenting disc appearance in order to address these issues. Whilst an enlarging cup:disc ratio is undoubtedly linked with glaucomatous loss, this system does not take into consideration the influence of optic disc size nor yet the focal changes seen in glaucomatous optic neuropathy. It is also well recognised that there is significant intra and inter observer error with this method.
The disc damage likelihood scale (DDLS) was devised by Spaeth et al in 2002 to incorporate the effect of disc size and focal rim width into a clinical grading scale(2). It is highly reproducible and does correlate strongly with the degree of field loss(3). The system categorises the disc as small (<1.5mm), medium(1.5-2.0mm) or large(>2.0mm). This ensures that the disc size is measured thereby reducing misclassification bias based on disc size. Disc size can be measured using a fundus lens at the slit-lamp. A slit beam is directed onto the disc and the graticule at the top used to reduce the height of the beam until it corresponds in size to the disc. The lens used will determine the correction factor.
A 66D gives the exact measure from the graticule.
Correction factors for the other lenses are -
The next stage is to measure the width of the thinnest part of the rim. This forces the examiner to evaluate the rim throughout its entire circumference in order to identify the area of greatest thinning. The measurement is expressed in rim:disc. Where there is no rim present at the thinnest point the value is 0. The circumferential extent of rim absence is then measured in degrees. Care must be taken when evaluating a sloping rim because a sloping rim is not an absent rim.
Whilst simply documenting a CDR is quick, in the main it is of little use in either the diagnosis or longitudinal monitoring of glaucoma. The DDLS not only forces the clinician to determine the size of the disc, which by itself already is alerting the observer to which discs are big and which are small but it also formalises the evaluation of the neuroretinal rim. Because each grade is assigned a numerical value the system can then be used in research settings to determine severity or degree of progression.
Optic nerve heads come in many shapes and forms. No method of classification will fit all of these different patterns and forms. The DDLS cannot be used to evaluate certain types of discs, such as those that are congenitally anomalous. Myopic discs may be difficult to grade. Probably one of the first mental steps one takes when considering the nature of the optic disc is, “Is this an anomalous disc? Is there any system that can be used to stage or characterize this disc?” There will be those situations which the answer to that question is, “No.” It is unwise in such situations to use any of the standard systems, such as cup/disc ratio, HRT evaluations, OCT evaluations, or the DDLS.
Another problem with the DDLS is that a disc may show progressive damage by having a continuing generalized narrowing of the neuroretinal rim, but not have an increase in the circumferential extent of rim absence. In such a situation the disc would unquestionably have become worse, but the DDLS score will not change. Fortunately this seems to be a rare occurrence.
It takes some effort to learn it and initially a copy of the table should always be to hand. However, given practice and used accordingly the DDLS is an excellent tool for classifying and monitoring the optic nerve in glaucoma.
The following references may be useful:
1. Armaly M. Genetic determination of cup/disc ratio of the optic nerve. Arch Ophthalmol 1967;78:35-43
2. Bayer A. Harasymowycz P. Henderer JD, et al. Validity of a new disc grading scale for estimating glaucomatous damage: correlation with visual field damage. Am J Ophthalmol 2002;133:758-763
3. Henderer JD, Liu C, Kesen M, et al. Reliability of the disc damage likelihood scale. Am J Ophthalmol 2003;135:44-8
4. Danesh-Meyer H, et al, Regional Correlation of Structure and Function in Glaucoma, Using the Disc Damage Likelihood Scale, Heidelberg Retina Tomograph, and Visual Fields, Ophthalmology 2006;113:603-611 © by the American Academy of Ophthalmology.
5. Bochmann F, Howell JP, Meier C, et al. The disc damage likelihood scale (DDLS): interobserver agreement of a new grading system to assess glaucomatous optic disc damage. Klin Monatsbl Bugenheilkd 2009;226:280-3.