Dissociated Vertical Deviation

Etiology

The etiology of dissociated vertical deviation is not known. Few of the important points to consider include:

1. An elastic preponderance of the elevator or depressor muscles, which imbalances the amount of innervation from each vestibular organ
2. Abnormalities of the visual pathway
3. Imbalance of binocular stimulation

Epidemiology

In the United States, about 3% to 5% of children have strabismus. The prevalence of dissociated vertical deviation among the diagnosed cases of strabismus was estimated at 1.9% in the United States of America.[3] It was found to be more frequently associated with sensory esotropia than exotropia.[4] 

Kutluk et al. reported a prevalence of DVD to be 12.5% in patients with sensory hypertropia.[4] Cherfan et al. reported that the common strabismus subtypes associated with DVD were congenital esotropia (53%), esotropia with a developmental deficit (25%), and accommodative esotropia (3.4%) in declining order.[3]

Pathophysiology

The exact cause of dissociated vertical deviation is not well understood. However, a few of the important theories explaining the underlying mechanism include:

1. Bielschowsky’s theory – He proposed alternating and intermittent excitation of subcortical vertical divergence centers as the cause of DVD.
2. Later it was thought of as a dorsal light reflex that modulates the central vestibular tone. Any fluctuations in the binocular visual input were supposed to stimulate the visual-vestibular reflex.[5] 
3. Guyton described it as a nystagmus blocking mechanism.[6]
4. Spielmann suggested an imbalance of binocular stimulation.
5. Few authors have also postulated an imbalance between the cortical input and subcortical pathways as the underlying pathology.[7]

History and Physical

Dissociated vertical deviation rarely presents in infancy. It usually presents around the age of 2 to 5 years. It can present as an isolated finding or associated with other forms of strabismus like infantile esotropia most commonly, or rarely infantile exotropia, or Duane’s retraction syndrome.[8] It is usually bilateral and asymmetrical. The deviation is often more prominent in the amblyopic eye. The clinical features can be broadly divided into:

• Deviation – This is defined as the upward drifting of the non-fixating eye when the patient fixates on the target with the other eye. This vertical deviation is often associated with extortion and slight abducting movement of the deviated eye. There is no movement seen in the contralateral eye, as the uncovered eye takes up fixation. Thus, DVD is said to disobey the Herring law of ocular motility.
• Head Posture – An abnormal head posture can be associated with one-third of the patients with DVDs. This is a motor adaptation to maintain binocular single vision. Patients presenting with this adaptation have do not complain of double vision and tend to have good vision in both eyes.
• Sensory Adaptations – Patients have binocular vision in the normal state, but fusion is lost when one eye deviates. The image from the deviated eye is suppressed, thus eliminating diplopia.

Types of DVD

1. Comitant: The vertical deviation is the same in abduction, primary position, and adduction. 
2. Incomitant: There is a significant difference in the deviation in abduction, primary position, and adduction. 

Types Based on Degrees of Deviation

This is defined based on the degrees of deviation measured in prism diopters (PD).

1. Mild (0 to 9 PD)
2. Moderate (10 to 19 PD)
3. Severe (>20PD)

Evaluation

The diagnosis of dissociated vertical deviation is based on clinical examination alone. A few of the important tests include:

• Cover/Uncover Test – Spielmann’s translucent occluder is used. a) Manifest DVD – Once the fixating eye is covered, the deviated eye shows a downward movement to take up the fixation. b) Latent DVD – The deviation manifests only once the eyes are dissociated. In this, no movement of the uncovered eye occurs. The eye undercover is elevated and moves down once it is uncovered. 
• Head tilt test – The deviation of the eye increases on contralateral head tilt.
• Bielschowsky’s graded density filter test – In this test, when one eye is covered, it elevates under the cover, and the other eye takes up the fixation. The filter bar consisting of increasing density of neutral density filters is placed before the fixing eye. As the filter of gradually greater density is placed before the fixing eye, the elevated eye starts moving down. When the filters of lesser density are placed in front of the fixing eye, the other eye undercover will start elevating again.
• Red filter test – This is also a dissociation test. Torchlight is shown as a target, and red glass is placed in front of one eye. The red glass dissociated the eyes. The eye behind the filter drifts up, and the patient localizes the red image the fixation light. In DVD, the red light is always seen below the fixation light irrespective of whichever eye the filter is placed. This helps differentiate DVD from other forms of cyclo-vertical deviations, in which red light position changes relative to the fixation light based on which eye fixates.
• Measurement of DVD – This is done using a base down prism. A prism is placed in front of the deviated eye first, and the patient is asked to fixate at a target located at a distance of 6 meters. The occluder is then shifted to the fixating eye allowing the deviating eye to take up fixation. The alternate cover test is repeated with prisms of greater power until no downward movement of the deviated eye is noted. In patients with bilateral DVDs, this test needs to be repeated for each eye separately.
• Grading of DVD

1+ deviation = a slight deviation

2+ deviation = a small deviation

3+ deviation = a moderate deviation

4+ deviation = a large deviation

Treatment / Management

Treatment of dissociated vertical deviation can vary from observation, non-surgical or surgical.

1. Observation – This might be sufficient in patients with latent DVD, especially more than 8 years of age, when the risk of amblyopia is not of concern, small degrees of anomalous head postures, not a significant cosmetic blemish.
2. Non-surgical – Conservative therapy, in the form of changing the fixation pattern or encouraging fusion, can be useful for small deviations. A slight optical blur induced in the fixating eye example by adding a +2D lens shifts the fixation preference to the other eye, and thus, DVD is avoided.[9] Injection of Botulinum toxin into the superior rectus has also been described.[10]
3. Surgical – This is indicated when there is greater than +2 deviation, large DVD frequently manifests, anomalous head posture, or a significant cosmetic blemish. Several surgical options for DVD have been described in the literature. Important ones among these include:

• Inferior oblique weakening[11]
• Anteriorization of the inferior oblique[12] 
• Anteriorization of inferior oblique with resection[13] 
• Superior rectus large recession (7 to 10 mm)[14][15][14] 
• Faden operation with superior rectus recession of 3 to 5 mm[16] 
• Inferior rectus resection – 4 mm for small deviations, 6 mm for intermediate deviations, and 8mm for large deviations[17] 
• Anteronasal transposition of inferior oblique[18]
• Inferior rectus tucking[19]
• Four muscle oblique surgery[20]
(B2)

The surgical treatment can be planned based on the grade of the DVD and the presence or absence of inferior oblique (IO) overaction.

1. DVD with no IO overaction - Superior rectus recession + inferior rectus resection
2. Moderate DVD with IO overaction - Recession with anteropositioning of IO
3. Severe DVD with IO overaction - Recession with anteropositioning of IO + superior rectus recession (7 to 10 mm)[21]

Differential Diagnosis

The closest and most important differential to be considered are inferior oblique overaction (IOOA) and acquired skew deviations. It is important to differentiate as the surgical approach differs for these conditions.

The important differentiating points between IOOA and DVD include:

• Hyperdeviation maximum in adduction in IOOA, while it is almost of similar degrees in primary, adducted, and abducted positions in DVD
• V pattern is associated with IOOA, while absent in DVD
• Superior oblique under action is usually associated with IOOA
• An extorted macula can be seen in IOOA, while it is absent in patients with DVD
• Bielschowsky phenomenon is present in DVD, while it is absent in DVD.[22]

The important differentiating points between acquired skew deviation and DVD include:

• Skew deviations can present at any age, while the most common age of presentations is around 2-4 years among DVD patients
• In skew deviations, there is intortion of the higher eye and extortion of the lower eye, while it is vice versa in DVD
• Brainstem or cerebellar region lesion can be associated with skew deviation
• Vertical diplopia is associated with skew deviation, while DVD is not associated with similar complaints
• Seesaw or Hemi-seesaw nystagmus is associated with skew deviation, while DVD is associated with latent nystagmus[23]

Prognosis

Depending on the frequency or degrees of deviation, the dissociated vertical deviation can be observed or might need non-surgical or surgical treatment. No major studies have compared non-surgical versus surgical management options. The most frequently performed procedures include inferior oblique anteriorization or superior rectus weakening procedures. The choice of surgical procedure depends on the degrees of deviation, degree of anomalous head posture, and surgeon's preference.[22]

Complications

The complications can be divided into related to the disease, anesthesia, or related to the surgical process (intraoperative or post-operative)

• Related to disease - amblyopia, torticollis, contracture of the sternocleidomastoid muscle.
• Anesthesia-related – Oculocardiac reflex, malignant hyperthermia, cardiac arrest, hepatic porphyria, or succinylcholine–induced apnoea
• Surgical complications:
  • Intraoperative – Hemorrhage, Lost or slipped muscle, eyeball perforation, inadvertent injury to surrounding structures/muscles, operation of wrong muscle or eye
  • Postoperative – Suture reaction, conjunctival granuloma, anterior segment ischemia, diplopia, retinal detachment, under or overcorrections, adhesive syndrome.[24]