The amplitude of accommodation (AoA) test monocularly or binocularly measures the maximum increase in the eye’s obtainable dioptric power. The test determines the maximum optical power increase when the eye adjusts its focus from far to near. AoA is usually measured in routine clinical eye examinations.
Different methods are used to measure AoA, the oldest of which are the push-up method, originated by Donders, and the minus method. Research indicates that the push-up method works well for fully corrected ametropes or emmetropes.
An AoA test can be affected by target size, speed of target approach, and target illumination. Prescribed medications, some pathological conditions, refractive errors, sunlight adaptation, and race or ethnicity can influence the amplitude of accommodation. Other factors that may influence AoA include periocular temperature, intraocular pressure, Down syndrome, diabetes, dyslexia, urbanization, etc.
Also Known As
- Accommodative Response
- Minus lens
- Dynamic retinoscopy
- Push-down to recognition
AoA is a routine clinical examination used to detect and manage common refractive conditions that include latent hypermetropia and presbyopia. AoA constitutes part of a comprehensive eye exam for pediatric patients and can detect inefficiencies and accommodative deficiencies associated with poor academic performance. The test in these patients can help determine a systemic or neuropathy abnormality. Studies show that most learning disabilities are linked to accommodative disorders of amplitude.
Before the Procedure
AoA test is measured during routine ophthalmic or optometry examinations and is measured in daylight.
The test corrects for distance vision with the patient requested to view a detailed test object approaching the eye. For example, the doctor can slowly move a small letter of 0.4 m or 0.5m near the patient's eye until the letter blurs. The doctor measures the near point of accommodation, which is the distance or point where the letters became blurry. To arrive at AoA, the measurement is done in meters but converted to dioptres.
Here, the doctor moves the test object away from the eye with the patient requested to report when s/he first sights the object clearly.
This method is a modification of the push-up method, where the doctor measures the point at which the target is recognized when moved away from the eye.
The eye doctor keeps adding negative spherical lenses to the distance refractive correction until the patient cannot maintain the primary acuity at a preset viewing distance. The doctor determines the AoA by adding the maximum negative lens power as the patient maintains focus. The test is facilitated using a phoropter (refractive head).
The eye doctor starts with either a push-up or minus lens technique, then determines the endpoint by observing the retinoscopic reflex. This method is suitable for patients with visual impairment or communication challenges. However, it requires greater professional judgment by the eye doctor.
As noted with risk, results differ due to various factors such as the patient's psychology, age, boredom, fatigue, etc. Results may be overestimated or underestimated. Different methods produce different results.
Risks & Complications
The main risk is accuracy, where an inadequate method may produce errors in measurement and affect the outcome. Some of these risks may include:
- Depth of focus (DoF) which affects all AoA measurement techniques that require the patient’s recognition of blur. Recognizing blur depends on acuity and the awareness of blur. Recognizing blur also varies from patient to patient and with viewing conditions like luminance.
- Reaction time can result in an error that may influence any method involving target movement (push-up, push-down, and minus method). Reaction time sums up four consecutive reaction times when the object moves beyond the place where blur first occurred. These four points are the patient's time to register blur; the patient to vocalize it; the examiner to record that message; and stoppage of movement by the examiner. The error increases with target velocity when accommodation is measured on a distance scale.
- The reference point for measurements can affect those AoA tests where distance is measured. These points may include the corneal point, the spectacle plane, the eye, 14mm in front of the eye, and 7mm behind the anterior corneal pole. Different reference points produce a greater error at higher levels of the amplitude of accommodation.
- Instrumentation errors occasioned by factors specific to the Royal Air Force Rule (RAF) rule. These factors may include uncertainties surrounding the scale’s zero-point location and ambiguities about the slider’s index position on the scale.
- Bias by the eye doctor can produce error in measurements not fully automatic. The different techniques used by practitioners may impact how measurements are taken (subsequently influencing the outcome).
- Some errors are specific to dynamic retinoscopy. Because it is conducted at a closer working distance than static retinoscopy, this may reduce its precision, increasing scaling error. Besides, measuring away from the visual axis decreases the retinoscopy's reliability. Dynamic retinoscopy also produces glare, which can affect the outcome.
- Anomalous proximal cues can produce error. Psychological factors like awareness of the nearness of the test object can influence accommodation.