2.2 Sub-Chart Acuity Protocols: CF, HM, LP, and NLP

Sub-Chart Acuity: CF, HM, LP, and NLP

Visual acuity is most comparable when measured with calibrated optotypes. If the patient cannot identify the largest symbol at the standard distance, first follow the chart's instructions for moving closer or using a low-vision chart. Only then move to qualitative or semiquantitative responses. The goal is to document what the patient actually demonstrates, not to force every patient through a rigid ritual.

Prepare the test

Confirm the eye, habitual distance correction, testing distance, chart illumination, and whether a pinhole or trial lens is being used. Test monocularly unless binocular function is the stated task. Use an opaque occluder that does not press on a recently operated eye, and watch for peeking around its edge. Explain the task in language and a format the patient can understand.

Poor responses can reflect hearing, language, cognition, fatigue, photophobia, poor fixation, or an inaccessible test—not only ocular disease. Record these limitations.

Counting fingers

Present a small, changing number of fingers against a plain, contrasting background at a measured distance. Avoid giving rhythm, voice, reflection, or shadow cues. Repeat enough trials to establish that responses are reproducible. If the patient succeeds, record the tested eye and distance, for example CF at 1 m or CF at 2 ft, according to local notation.

The exact finger choices and starting distance are protocol-dependent. What matters is a consistent method, measured distance, adequate lighting, complete fellow-eye occlusion, and reproducible identification. A bare entry of CF omits important information.

Hand motion

If fingers cannot be counted at the closest approved distance, test whether the patient can detect a moving hand under controlled illumination. Keep the test position and distance consistent and vary movement with nonmovement so the patient cannot answer from timing alone. Some clinics ask the patient to identify direction; others document detection of motion. Use the clinic's defined method and record the conditions.

Hand motion is not a precise Snellen equivalent. Do not convert HM into an invented chart fraction or compare it as though it were a linear acuity value.

Light perception and localization

If hand motion is not reproducibly detected, test whether the patient can distinguish the light source on from off. Reduce ambient light enough to control stray cues and use the clinic-approved light source. Shield the fellow eye and change the timing of presentations. A reliable on/off response is documented as LP.

Light localization or projection asks where the light originates, often from several directions. It may add information, but it is not the same as visual acuity and should be recorded separately. Do not tell a patient that one localization result determines surgical prognosis.

No light perception

Record NLP only after repeated controlled presentations produce no reliable light detection. Recheck eye identity, pupil exposure, battery and light output, occlusion, understanding, and response reliability. Follow facility policy for confirmation, especially when the result is new, unexpected, inconsistent with pupillary findings, or has major management implications.

NLP describes the response during that examination. It does not by itself establish cause, duration, permanence, legal blindness status, or whether urgent treatment is indicated. The licensed clinician integrates the result with pupils, anatomy, imaging, and history.

Documentation table

Use neutral wording. If responses vary, document the range and reason rather than selecting the best trial without explanation.

Documentation and repeatability controls

Record the exact chart or stimulus, test distance, correction, lighting, eye, occlusion method, and patient response. For counting fingers, document the distance and field used; for hand motion, document the distance and direction when the protocol requires it. For light perception, identify whether projection was tested and how. A bare entry such as "CF" cannot be compared reliably with a later "CF at 1 foot."

Before accepting a major decline, check identity, laterality, habitual correction, occlusion, room setup, and comprehension. Reposition the patient and repeat only enough to confirm a reproducible response without coaching. If the response changes, document both the initial and repeat result and what changed. Escalate a new or unexplained reduction promptly. Do not infer irreversible blindness from NLP, and do not promise prognosis from any sub-chart category; media opacity, severe retinal disease, optic-nerve disease, neurologic factors, and testing conditions all require clinician assessment.