Keratoconus, the anterior chamber, is important in many fields

Keratoconus, an ectatic non-inflammatory
disorder of the eye in which corneal thinning and protrusion cause the cornea
to assume a conical shape, is most commonly bilateral and asymmetric, with no gender or race predilection. It typically begins
at puberty, becomes
apparent during the second decade of life, and typically progresses until the fourth decade of life when it
usually stabilizes. The
estimated prevalence of keratoconus is 54 per 100000 in the general population
and 26 per 1073 in the young population. Clinical and research applications in Keratoconus
require reliable and precise measurements of anterior segment parameters. Assessment
of anterior segment characteristics, particularly different parameters of the
anterior chamber, is important in many fields of ophthalmology, including
preoperative examination, planning and monitoring surgical methods of keratoconus
management, and advanced
intraocular lens (IOL) power calculation formulas. Anterior chamber depth (ACD)
is also a major and important risk factor for primary
angle closure (PAC) Anterior
chamber depth is the distance between the posterior vertex of the cornea and
the anterior surface of the crystalline lens measured along the optical axis. The mean anterior chamber depth is
approximately 3 mm. A chamber depth of 2 mm or less is considered shallow (Smith
1979) 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 to
affect the ACD. ACD measurement provides a careful
assessment of glaucoma, with the anterior chamber being shallower in patients
at risk. In performing refractive surgery, such as excimer laser
photorefractive keratectomy, the ACD is important to set a correct optical zone
ablation diameter. Therefore, the accuracy of the ACD measurement is becoming
more and more important in the clinical setting. Several
methods are available for the measurement of the anterior chamber depth (ACD) that
can be divided to 3 categories: photographic (based on the Scheimpflug
principle), ultrasonic (based on reflected sound waves), and optical (based on
the Jaeger principle). Ultrasound
biometry is the most common method used for the measurement of the anterior
chamber depth. The
Pentacam (Oculus OptikgeräteGmbH, Wetzlar, Germany) system uses a rotating
Scheimpflug camera and a monochromatic slit-light source that rotate together
around the optical axes of the eye for measuring the anterior segment
topography. Several reports have shown the high reproducibility and
repeatability of the Scheimpflug imaging system in the measurement of anterior
segment parameters. The
Orbscan II topography system was initially designed for corneal topography, and
has been demonstrated to be a useful tool in anterior segment biometry. The
IOLMaster (Carl Zeiss) measures the ACD based on the optical method. Anterior
segment 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 partial
coherence interferometry for axial length measurement; however, it measures the
corneal radius and ACD based on image analysis in which the distances between
light reflections on the cornea, iris, and lens are measured. Optical coherence tomography (OCT) uses low coherence
interferometry to obtain cross-sectional images of the ocular structures. A
wavelength (1,310nm) longer than what is used for the posterior segment is required
to provide images of the anterior segment. (McDonnell) Several studies have compared
the ACD in keratoconic and normal eyes and different results have been reported.
Due to inconsistencies in previous results and the importance of precise ACD values
in clinical assessment and treatment, this review was conducted to summarize
the results of ACD changes in keratoconus patients.