Keratoconus, an ectatic non-inflammatorydisorder of the eye in which corneal thinning and protrusion cause the corneato assume a conical shape, is most commonly bilateral and asymmetric, with no gender or race predilection.
It typically beginsat puberty, becomesapparent during the second decade of life, and typically progresses until the fourth decade of life when itusually stabilizes. Theestimated prevalence of keratoconus is 54 per 100000 in the general populationand 26 per 1073 in the young population. Clinical and research applications in Keratoconusrequire reliable and precise measurements of anterior segment parameters. Assessmentof anterior segment characteristics, particularly different parameters of theanterior chamber, is important in many fields of ophthalmology, includingpreoperative examination, planning and monitoring surgical methods of keratoconusmanagement, and advancedintraocular lens (IOL) power calculation formulas. Anterior chamber depth (ACD)is also a major and important risk factor for primaryangle closure (PAC) Anteriorchamber depth is the distance between the posterior vertex of the cornea andthe anterior surface of the crystalline lens measured along the optical axis. The mean anterior chamber depth isapproximately 3 mm. A chamber depth of 2 mm or less is considered shallow (Smith1979) and its estimated prevalence in the general population is 54 per 100 000.
Age, gender, refractive error, body type, and cataract formation have been reported toaffect the ACD. ACD measurement provides a carefulassessment of glaucoma, with the anterior chamber being shallower in patientsat risk. In performing refractive surgery, such as excimer laserphotorefractive keratectomy, the ACD is important to set a correct optical zoneablation diameter. Therefore, the accuracy of the ACD measurement is becomingmore and more important in the clinical setting. Severalmethods are available for the measurement of the anterior chamber depth (ACD) thatcan be divided to 3 categories: photographic (based on the Scheimpflugprinciple), ultrasonic (based on reflected sound waves), and optical (based onthe Jaeger principle). Ultrasoundbiometry is the most common method used for the measurement of the anteriorchamber depth.
ThePentacam (Oculus OptikgeräteGmbH, Wetzlar, Germany) system uses a rotatingScheimpflug camera and a monochromatic slit-light source that rotate togetheraround the optical axes of the eye for measuring the anterior segmenttopography. Several reports have shown the high reproducibility andrepeatability of the Scheimpflug imaging system in the measurement of anteriorsegment parameters. TheOrbscan II topography system was initially designed for corneal topography, andhas been demonstrated to be a useful tool in anterior segment biometry. TheIOLMaster (Carl Zeiss) measures the ACD based on the optical method. Anteriorsegment biometry with such devices has been reported to have a high precision(_5 _m), high resolution (~12 _m), and good reliability.
The IOLMaster I (Carl Zeiss Jena GmbH, Jena, Germany) uses partialcoherence interferometry for axial length measurement; however, it measures thecorneal radius and ACD based on image analysis in which the distances betweenlight reflections on the cornea, iris, and lens are measured. Optical coherence tomography (OCT) uses low coherenceinterferometry to obtain cross-sectional images of the ocular structures. Awavelength (1,310nm) longer than what is used for the posterior segment is requiredto provide images of the anterior segment.
(McDonnell) Several studies have comparedthe ACD in keratoconic and normal eyes and different results have been reported.Due to inconsistencies in previous results and the importance of precise ACD valuesin clinical assessment and treatment, this review was conducted to summarizethe results of ACD changes in keratoconus patients.