Dissociated vertical deviation (DVD) is an eye movement disorder where one or both eyes move or deviate upward when not in use. The upward movement may be occasional or regular, and the shift can vary at different times of the day. The magnitude of the deviation may also be disproportionate between the eyes where both eyes are affected. The affected eye(s) may also experience a rotation or outward drift. DVDs usually affect children between 18 months and three years old and are often linked to a squint. The condition can affect vision because the patient's brain blocks or suppresses the image from the affected eye(s), a form of reduced visual acuity called amblyopia. Eye drops, patching, or eyeglasses can treat amblyopia.
DVD may be manifest or latent. Manifest DVD occurs spontaneously, while latent DVD only arises when one eye is obscured/blocked.
DVD may also be comitant or incomitant. In comitant DVD, the amount of drift is the same in abduction, while in incomitant DVD, there is a variation in the deviation.
Causes & Risk Factors
DVD is thought to occur due to faulty eye muscles' innervation and is usually associated with infantile esotropia/exotropia, latent nystagmus (jiggling of the eyes), oblique muscle disorders and Duane's retraction syndrome. However, the exact cause of the condition hasn't been determined.
Children with early onset of misalignment of the eyes (strabismus) are at the highest risk of developing DVD. Other risk factors include a poorly seeing eye, decreased use of the eyes together (poor fusion) and eye muscle surgery for strabismus.
Signs & Symptoms
DVD usually affects both eyes and is often unsymmetrical or uneven. Periods of visual inattention may aggravate the condition, with or without fatigue or daydreaming. With simultaneous extortion, either eye slowly drifts upwards and outwards, and at times a horizontal motion occurs in the exotropic direction.
The brain usually suppresses the affected eye, so visual symptoms rarely arise, but some patients may have double vision or eye strain.
The eye specialist will conduct the Bielschowsky Darkening Wedge Test to diagnose DVD. S/he will cover one of the patients' eyes and ask the patient to look at a light through a filter placed in front of the other eye. S/he will gradually increase the density of the filter and observe the behavior of the covered eye. The doctor may also use other tests such as Hirshberg's test, prism bar under cover test and red glass test.
There are several different non-surgical and surgical DVD treatment options.
Non-surgical therapy may include observation, encouraging fusion of bifixation and switching fixation. The goal of DVD non-surgical treatment is to enhance the patient's fusional mechanisms, eliminate simultaneous strabismus and vision optimization through correct refractive prescription and amblyopia treatment.
Eye muscle surgery is the standard DVD treatment. It is recommended in cases with visual symptoms, disfigurement, large deviation causing physical discomfort, or where DVD is regular. The treatment aims to minimize the frequency of the upward drift. Surgery may be done only on the opposite eye if one eye is usually used for fixation. If either eye is used for fixation, it may be appropriate to operate on both eyes, but asymmetrically. However, DVD may still occur in some cases after the surgical intervention.
Prevention & Follow Up
DVDs aren't preventable. The doctor can follow up to assess the progress and effectiveness of the treatment.