Introduction
The challenges of the adult strabismus population differ from those of the pediatric population. Strabismus related to medical, ocular, or neurological disease is more prevalent in adults, adding to the complexity of the evaluation process, the differential diagnosis, and management strategies. In addition, avoiding or eliminating diplopia is a very important element in both the assessment and the management of adult strabismus. An orthoptist can play an integral part in the workup and management of adult strabismus.
In the evaluation of adult strabismus, the orthoptist can assess the sensory status and provide accurate measurements in relevant fields of gaze. Most importantly, the orthoptist can interpret the findings, determine a differential diagnosis, and perform appropriate additional clinical tests to aid in arriving at the correct diagnosis. Additionally, with an understanding of the various causes of adult strabismus, the orthoptist is able to identify specific information in the patient’s medical history that could be relevant to the oculomotor system and apply that information to the evaluation strategy.
In the management of adult strabismus, the detailed sensory assessment provided by an orthoptist is crucial in determining the prognosis for achieving or maintaining single vision. The orthoptist can also assist in determining prism power when treatment with prisms is indicated, can offer eye exercises to build fusional amplitudes when appropriate, and can provide accurate strabismus measurements for surgical planning when muscle surgery is indicated. The orthoptist further assists in management by following patients for stability of the deviation preoperatively or for recovery of acquired cranial nerve palsies. The orthoptist can also follow patients for response to treatment when prisms have been prescribed or when surgery has been performed. In addition, the orthoptist can be a knowledgeable liaison between the physician and the patient for questions and in the discussion of diagnosis and management.
Sensory considerations in evaluation and management
The orthoptist utilizes prisms, the amblyoscope, and various other sensory tests to explore motor and sensory fusion. This includes, but is not limited to, measuring fusional amplitudes, determining retinal correspondence, and mapping suppression areas. In addition, various techniques can be utilized to map fields of single binocular vision.
Differentiating acquired from longstanding strabismus
Determining sensory status can help differentiate an acquired deviation from a longstanding childhood deviation. The presence of suppression or anomalous correspondence (ARC) suggests that the deviation has been present since early childhood. Abnormally large fusional amplitudes suggest that the deviation has been present for many years. Such findings can eliminate the need for extensive medical work-ups that might otherwise be associated with recently acquired deviations.
Determining prognosis for single vision
When adult strabismus surgery is a treatment option, it is important to preoperatively determine the risk of postoperative diplopia. This is especially important for adults with childhood strabismus, with macular disease, or with strabismus secondary to severe head trauma. In such cases, fusion ability can be impaired or absent. For a patient expecting single vision following surgery, obtaining excellent postoperative alignment will not be sufficient if a lack of fusion ability and absence of suppression results in new or persistent diplopia. Therefore, sensory testing and interpretation of the results is crucial in preoperative discussions with the patient regarding postoperative expectations for binocular vision.
Childhood strabismus
Most adults seeking correction of childhood strabismus are free from diplopia due to suppression. This is commonly seen in decompensated intermittent exotropia or decompensated fourth nerve palsy. When the adult presents with constant strabismus, preoperative sensory testing can determine whether fusion ability and adequate fusional amplitudes have been preserved. Sensory testing is essential to determine whether comfortable single vision can be obtained postoperatively and thus identify the patient as an excellent surgical candidate in terms of postoperative sensory outcome. If fusion ability is very weak, as in cases of decompensated microtropia, or when it is absent, the orthoptist can test for and map the suppression area to determine whether suppression might protect the patient from postoperative diplopia. In some cases, the suppression area does not include the fovea, putting the patient at risk of diplopia when the eyes are aligned. By mapping the suppression area, the orthoptist can help determine a surgical strategy that would minimize this risk. For example, in an esotropic subject, the suppression scotoma might extend from the nasal periphery to 3 degrees nasal of the fovea. In such a case, the risk of postoperative diplopia would be reduced if the goal of surgery were to leave the patient with a residual esodeviation greater than 6 prism diopters, rather than attempt to correct the entire deviation.
Preoperative sensory testing would include tests for ARC with evaluation for the subsequent risk of paradoxical diplopia if the visual axes are surgically aligned. When ARC is diagnosed, mapping the suppression area can again aid surgical planning to minimize the risk of paradoxical diplopia.
Macular disease
Disruption of the photoreceptors associated with macular disease can result in unstable fusion. Patients with macular edema, epiretinal membrane (cellophane maculopathy), macular degeneration, a macular hole, or bull’s eye maculopathy, or those with a history of a macula-off retinal detachment are at risk of disrupted central fusion. Any patient who is being considered for strabismus surgery with a history of any of these conditions should undergo careful sensory testing for fusion ability prior to surgery. Such testing will determine the prognosis for comfortable single binocular vision postoperatively. When fusion is disrupted, these patients likely will require prisms to aid in fusion since they often are not able to compensate for even a very small residual deviation. The orthoptist can assist the physician with prism management. The patient needs to be aware that even with prisms, the diplopia might not be entirely eliminated.
Strabismus associated with severe head trauma
Severe head trauma can result in the central loss of sensory fusion, motor fusion, or both. This is most commonly seen when the head trauma results in a prolonged period of unconsciousness. This loss of fusion ability is known as central disruption of fusion. Loss of central sensory fusion results in diplopia described as “horror fusionis.” This condition must be ruled out before surgical alignment is considered. If the eyes are aligned, this type of diplopia is especially bothersome to the patient. With horror fusionis, when the images are very close together, the diplopic image constantly moves around the other image but the images never fuse. This condition very rarely recovers.
Severe head trauma can be associated with bilateral fourth nerve palsies. The resulting diplopia can have a significant torsional component. If the patient is unable to fuse with prisms, reporting torsion of one or both images, testing with the amblyoscope enables the orthoptist to neutralize the torsion and assess fusion ability. It is important to determine whether the torsion is the barrier to fusion, or if there is a central disruption of fusion. Such testing enables one to determine the prognosis for single binocular vision postoperatively.
Oculomotor evaluation
Adult strabismus varies widely in terms of etiology and can be associated with other abnormalities of the oculomotor system. Not only must the deviation be carefully measured to determine a diagnosis, but thought must be given to the cause of the misalignment if it is acquired. Orthoptists are trained to consider the differential diagnosis of acquired adult strabismus while performing their assessment, and to perform additional clinical tests when indicated. They are also trained to identify congenital and acquired abnormalities of the eye movement system.
Measurement of deviations and interpretation of findings
The orthoptist is skilled at measuring deviations using a variety of methods including prism and cover testing, Maddox rod testing, corneal reflection methods, the Lancaster red/green test, and the Hess chart. Orthoptists can determine which gaze measurements are appropriate for diagnosis and management and which additional tests are indicated. For example, when a cyclovertical muscle palsy is suspected, measurements in the nine fields of gaze will be obtained, along with head tilt testing and double Maddox rod testing for a cyclodeviation.
With an understanding of the various causes of adult strabismus, the orthoptist is alert to and aware of the significance of secondary deviations and variable deviations. The presence of a secondary deviation will prompt the orthoptist to investigate possible causes such as a cranial nerve palsy or a mechanical restriction of movement causing fixation duress. Variability of a deviation will be noted by the orthoptist. If the variability involves an esodeviation, the orthoptist will apply testing strategies to evaluate for a sixth nerve palsy and for an accommodative component. The assessment of a variable esodeviation will include consideration of a possible spasm of the near reflex. The orthoptist will make note of any changes in pupil size, accommodation, and image clarity associated with changes in the deviation. Patients with variable deviations will also be tested for lid fatigue on prolonged upgaze as a sign of myasthenia gravis.
If the measurements suggest a third nerve palsy, the orthoptist will pay particular attention to the pupil exam and include evaluation of accommodation and lid position. The experienced orthoptist will also know how and when to test for aberrant regeneration of the third nerve by looking for lid elevation in adduction, lid elevation in downgaze, and for pupillary-gaze dyskinesis.
Assessing ocular rotations
Version testing will include identifying relative extraocular muscle overactions and underactions. Duction testing will identify any limitation of movement. While testing ductions and versions, the orthoptist will be alert to any retraction of the globe, gaze paretic nystagmus, myokymia, gaze dyskinesis, or gaze palsies. The near point of convergence is tested along with ocular rotations.
Limitation of movement
When there is a limitation of movement, the orthoptist will perform additional testing to help differentiate a mechanical restriction from a paresis. For example, a gross assessment of saccadic velocities will be obtained. Some orthoptists have experience with obtaining eye movement recordings for more accurate assessment saccadic velocities. In addition, the orthoptist can test for changes in intraocular pressure when a mechanical restriction is suspected. When thyroid ophthalmopathy is suspected, the orthoptist will assess lid position, look for chemosis, test color vision monocularly, and obtain exophthalmometry measurements.
Knowledge of special forms of strabismus enable the orthoptist to assess for conditions associated with limitation of movement such as Duane syndrome, Brown syndrome, congenital fibrosis syndrome, Moebius syndrome, and cranial nerve palsies. Orthoptists are also trained to recognize gaze disturbances such as horizontal and vertical gaze pareses, including internuclear ophthalmoplegia, one-and-a half syndrome, and Parinaud syndrome.
Eye movement abnormalities
Any nystagmus associated with the strabismus will be documented and described, as well as paresis of convergence or divergence. An experienced orthoptist can also identify subtle abnormalities of the oculomotor system such as ocular dysmetria, hypometria, hypermetria, myokymia, cyclic phenomena, exaggerated end-gaze nystagmus, micro-square-wave jerk nystagmus, cogwheel pursuit, and convergence-retraction nystagmus. The orthoptist is aware that these findings can assist the physician in determining the etiology of acquired adult strabismus.
Non-surgical management of adult strabismus
Prisms
Determination of the most effective prism power for treatment of diplopia can be a time-consuming process. For a busy ophthalmologist, it is helpful to have an orthoptist assist with this task. With incomitant deviations or with unstable fusion, trials with varying prism powers can be required before the final power is determined. The orthoptist can determine an effective prism power, institute a trial with press-on prisms, then re-evaluate the patient for adjustments in the prism power to arrive at the amount and direction of prism that provides the most comfortable field of single binocular vision. This process is especially time-consuming for patients with diplopia secondary to macular disease, which can sometimes require several adjustments in prism power over the course of several months.
Exercises
Not all patients are candidates for eye exercises. An orthoptist can perform careful sensory testing to assess fusion ability and suppression patterns to determine whether or not orthoptic therapy is a viable treatment option. One must be careful to avoid inadvertently breaking suppression with exercises in the absence of stable fusion ability, because this can result in unmanageable diplopia. When a patient has been determined to be a good candidate for eye exercises, the orthoptist can decide which exercises are most appropriate, instruct the patient, follow the patient for response to treatment, and re-instruct as needed. Patients with small deviations and patients with large exodeviations for whom surgery is medically contraindicated should be considered for possible orthoptic therapy with exercises designed to build fusional amplitudes.
Optical adjustments
Fixation switch diplopia is rare but can occur when a patient begins to fixate with the habitually deviated nondominant eye. Orthoptists can evaluate for fixation switch diplopia and make recommendations for treatment, with adjustments in the patient’s optical correction to return fixation to the dominant eye.
Occlusion
When fusion is not possible with prisms, occlusion is sometimes the only option for eliminating diplopia. There are a number of occlusive strategies available. As with prisms, determining the optimal occlusive strategy for a particular patient can also be time consuming. Utilizing an orthoptist for occlusive recommendations can save the ophthalmologist time that can be spent on medical aspects of an ophthalmology practice. Some patients require only partial occlusion for comfortable single vision, others require full field occlusion. The orthoptist can determine whether the patient requires an opaque occluder or whether they can achieve single vision with the more cosmetically acceptable option of tape or a Bangerter foil applied to all or part of one lens of their spectacles. The orthoptist can discuss these options with the patient and offer trials of various strategies. In addition, the orthoptist can discuss the possibility of obtaining an opaque contact lens when appropriate.
Summary
Thorough assessment of adult strabismus requires a deep understanding of the sensorimotor system and a keen awareness of medical issues that can come into play. Orthoptists' scope of training enables them to provide valuable services in the evaluation and management of adults with strabismus.