Section 14.3: Troubleshooting Visual Complaints
Key Takeaways
- Many visual complaints, such as peripheral distortion or narrow reading fields, are caused by frame misalignment rather than incorrect prescriptions.
- The 'fishbowl' effect is caused by a significant change in base curve and is minimized by decreasing vertex distance and adding positive face form wrap.
- A narrow reading zone in progressive lenses is resolved by decreasing vertex distance, increasing pantoscopic tilt, or adjusting segment height.
- The progressive 'swim' effect is reduced by moving the lenses closer to the eyes and ensuring proper face form wrap.
- A patient tilting their head back to read indicates the segment height is too low; tilting the head forward indicates the segment height is too high.
Section 14.3: Troubleshooting Visual Complaints
Quick Summary: Visual complaints from spectacle wearers are often caused by physical frame misalignment rather than incorrect lens prescriptions. Opticians can resolve issues like the "fishbowl" effect, narrow reading zones in progressive addition lenses, peripheral swim, and postural head tilt by adjusting the frame's vertex distance, pantoscopic tilt, face form wrap, and vertical positioning.
The Diagnostic Approach to Dispensing Complaints
When a patient returns to an optical dispensary complaining of visual discomfort or distortion in their new spectacles, the optician must follow a systematic troubleshooting protocol. A common mistake is assuming that the lens prescription is incorrect and immediately requesting a doctor's recheck. In a high percentage of cases, the visual complaint is caused by a physical misalignment of the frame on the patient's face.
Before checking the prescription on a lensmeter, the optician must inspect the frame's fit while the patient is wearing it. The optician should observe the frame's vertical height, horizontal centration, pantoscopic tilt, vertex distance, and face form wrap. By modifying these physical parameters, the optician can alter how light passes through the lenses, correcting the visual distortion without altering the lens prescription itself.
The "Fishbowl" Effect and Oblique Aberrations
The "fishbowl" effect is a visual complaint where the patient describes a sensation of peripheral distortion, feeling as though they are looking through a curved glass bowl. Straight lines (such as doorways, walls, or computer screens) appear to bow or curve in their peripheral vision, and the patient may experience mild dizziness or motion sickness when moving their head.
Understanding Peripheral Distortion
The fishbowl effect is primarily caused by changes in the base curve of the new lenses compared to the patient's previous glasses. The base curve is the manufacturer's front surface curve of the lens. If a patient was accustomed to a flat base curve and receives new lenses with a steeper base curve (or vice versa), the peripheral magnification changes significantly. This difference in magnification across the lens surface causes the brain to perceive straight lines as curved.
Role of Face Form (Wrap) and Tilt
If the frame has neutral or negative face form (meaning the frame front is perfectly flat or curves backward away from the face), the patient's line of sight will strike the outer portions of the lens at an oblique angle during lateral gaze. This oblique angle introduces unwanted astigmatism and distortion.
- Adjustment Action:
- Decrease Vertex Distance: Bringing the lenses closer to the eyes (decreasing vertex distance) allows the eyes to utilize a wider area of the lens and reduces peripheral magnification differences, minimizing the fishbowl sensation.
- Add Positive Face Form Wrap: Bending the frame bridge slightly so that the frame front contours gently around the patient's face (positive face form) aligns the peripheral areas of the lenses more perpendicular to the eyes' visual axis during lateral gaze, reducing distortion.
Progressive Addition Lens Corridor Adjustments
Progressive addition lenses (PALs) provide a seamless progression of power from distance to intermediate and near vision. However, because of the physics of progressive lens design, the narrow corridor of clear vision is bordered by areas of unwanted peripheral astigmatism. A frequent patient complaint is that the "reading area is too narrow," forcing them to turn their head excessively to keep text in focus.
PROPER ALIGNMENT VERTEX TOO FAR
(Wide Reading Field) (Narrow Reading Field)
|---| |----------|
| | | |
( o ) Lens closer ( o ) Lens farther
\\ /
Eye Eye
Vertex Distance and Pantoscopic Tilt Effects
A narrow reading field is often caused by an excessive vertex distance. If the lenses sit too far from the patient's eyes, the field of view through the progressive corridor is narrowed. This is analogous to looking through a keyhole: the closer the eye is to the keyhole, the wider the view of the room on the other side.
- Adjustment Action:
- Decrease Vertex Distance: The optician should bring the frame closer to the face by widening the nosepads, bending the pad arms back, or increasing the temple spread. This significantly widens the horizontal field of view through both the intermediate corridor and the near-vision segment.
- Increase Pantoscopic Tilt: Adding pantoscopic tilt rotates the bottom of the lens closer to the eye, bringing the near-vision portion of the progressive channel into a closer and more optimal position for reading.
Swim Effect and Lateral Sway
The swim effect is a sensation of movement or swaying in the peripheral vision when the wearer turns their head. It is closely related to the fishbowl effect but is specific to progressive lens wearers due to the lateral astigmatic corridors.
- Adjustment Action: To resolve swim, the optician must decrease the vertex distance, bringing the progressive lenses as close to the eyes as possible. This causes the peripheral astigmatism to shift farther out into the extreme periphery, where the patient's brain can more easily ignore it. Additionally, ensuring proper pantoscopic tilt (8-12 degrees) and positive face form wrap will align the optical corridor with the natural arc of the eyes' rotation, smoothing the visual transition. If physical adjustments fail, the patient may require a digital free-form progressive lens, which calculates customized surfacing on the back of the lens to minimize peripheral aberrations.
Postural Compensation and Segment Height Errors
If a patient must alter their natural head posture to read or see distance clearly, the segment height (or progressive fitting height) is misaligned.
Head Tilt Back (Chin Up)
If a patient must tilt their head backward to read, the reading segment of the bifocal or the progressive corridor is sitting too low relative to their pupils. The patient is forced to raise their chin to look through the lower, stronger near-vision zone of the lens.
- Adjustment Action: The optician must raise the frame vertically on the patient's face. This is achieved by:
- Narrowing the nosepads to squeeze the bridge opening.
- Bending the pad arms downward relative to the frame front.
- Decreasing the pantoscopic tilt (which raises the lower rim of the frame).
Head Tilt Forward (Chin Down)
If a patient must tilt their head forward to see distance clearly, or if they complain that the reading segment interferes with their distance vision, the segment height is too high.
- Adjustment Action: The optician must lower the frame vertically on the patient's face. This is achieved by:
- Widening the nosepads to widen the bridge opening.
- Bending the pad arms upward relative to the frame front.
- Increasing the pantoscopic tilt (which lowers the lower rim of the frame).
Troubleshooting Symptom-Action Matrix
The following matrix outlines common dispensing complaints, their underlying physical causes, and the corrective adjustments:
| Patient Complaint | Potential Physical/Optical Cause | Corrective Adjustment Action |
|---|---|---|
| Must tilt head back (chin up) to read | Segment height or progressive fitting cross is too low | Narrow nosepads, bend pad arms downward, or decrease pantoscopic tilt |
| Must tilt head forward (chin down) to read / distance blur | Segment height or progressive fitting cross is too high | Widen nosepads, bend pad arms upward, or increase pantoscopic tilt |
| Reading field feels too narrow (PAL) | Vertex distance is too far, or pantoscopic tilt is insufficient | Decrease vertex distance (widen pads/bend pad arms back), and increase pantoscopic tilt |
| "Fishbowl" effect (peripheral distortion) | Change in base curve, or flat face form wrap | Decrease vertex distance, and add positive face form wrap to the frame front |
| Swim effect (peripheral movement) | Unwanted progressive astigmatism in primary field of view | Decrease vertex distance, increase pantoscopic tilt, and add positive face form wrap |
| Lashes touching the back of the lenses | Vertex distance is too short | Increase vertex distance by narrowing nosepads, bending pad arms forward, or decreasing temple spread |
| One side of distance vision is blurry | Frame sits crookedly on the face, causing one lens to be misaligned | Adjust temples (bend temple on high side up, or low side down) to level the frame front |
A patient wearing new progressive addition lenses complains that they must tilt their head back excessively in order to read fine print. What is the most likely cause, and how can the optician temporarily adjust the frame to relieve this?
Which adjustment will help widen the perceived reading field of view for a patient wearing progressive addition lenses?
A patient describes a 'fishbowl' effect where straight walls appear to curve or bend in their peripheral vision when wearing new single vision lenses. What is the most likely optical cause?