negative relative accommodation
The maximum amount of accommodation that can be relaxed while maintaining clear, single binocular vision at a fixed distance, typically 40 cm. It is measured clinically by adding plus lenses binocularly until the patient reports the first sustained blur. NRA assesses the flexibility of the accommodative system relative to vergence demand. Low NRA findings may indicate accommodative excess or latent hyperopia, while high values suggest uncorrected hyperopia or over-minused refraction.
neutral density filter test
This test differentiates functional (reversible) from organic (pathological) amblyopia. Visual acuity is measured with and without neutral density filters that reduce luminance equally across wavelengths. In organic amblyopia, acuity significantly decreases with filters, while in functional amblyopia, acuity is minimally affected. This test helps determine if reduced acuity is due to retinal/optic nerve pathology or suppression. Filters of increasing density are used until acuity diminishes.
nystagmus
An involuntary, repetitive, and rhythmic movement of the eyes characterized by alternating slow and fast phases. It can occur in a horizontal, vertical, rotary, or combined pattern. Nystagmus may be congenital, appearing in infancy, or acquired later in life due to vestibular disorders, neurological conditions, medications, or toxins. It can affect visual acuity, depth perception, and balance. Treatment depends on the underlying cause and may include corrective lenses, medications, or surgery in some cases.