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Basic Vertical Imbalance

Karlen McLean, ABOC, NCLC

I felt off-kilter when I learned about vertical imbalance in opticianry school, so I thought I 'd attempt to explain it.

Simply put, vertical imbalance occurs when gaze drops below the optical center (OC) in lenses that are of unequal value (typically 1.50D or higher). The vertical prismatic effect is called vertical imbalance.

CAUSE AND EFFECT

At the OC, there should be zero prismatic effect. But when the gaze lowers below the OC, vertical imbalance can result. This can primarily depend on the prescription, yet frame fit and adjustment, eye movement versus head movement, and personal tolerance also come into play.

When there's a power difference between lenses, different prismatic effects occur at varying positions of gaze. Anisometropic patients (those requiring different lens powers) are typically prone to vertical imbalance.

Any difference in Rx powers eye-to-eye is the cause of vertical imbalance. As a rule, a 1.50D or higher power difference between the right and left lenses will result in vertical imbalance. Looking 10mm below the OC will induce vertical imbalance.

Vertical imbalance can cause visual discomfort and fatigue, and can affect both single vision wearers and multifocal/PAL wearers. By dropping their heads—not just their gaze—below the OC, many single vision wearers can circumvent vertical imbalance. But for multifocal/ PAL wearers, positioning of the segment is crucial.

ASSESS AND ADDRESS

To assess vertical imbalance, check for imbalance in the 90-degree meridian, check the prescription (old against new), and resolve the total lens power at 90 degrees. Also, observe patients as they read; those who drop their heads when reading may not be affected or mildly affected, while those who only drop their eyes may require compensation.

The three most successful compensations for vertical imbalance, as defined by the late Michael DiSanto, ABOM, independent industry trainer, are: 1. Prescribe near-power-only glasses for reading tasks; 2. Make slab-off lenses, or; 3. Make reverse slab-off lenses.

Slab-off is bicentric grinding that adds a second OC, used to create vertical prism in the lower portion of a lens, and is placed on the most minus (least plus) lens. Reverse slab-off places the prism base down on the most plus (least minus) lens. EB

Senior editor Karlen McLean has been in the optical industry for 35 years. She has spent half of that time in the dispensary. In this monthly column, Karlen shares her own experiences, and hopes you'll share yours. Contact her via email at karlen.mclean@wolterskluwer.com.