Duane 39;s Syndrome Review Of Optometry

[Shantelle Wenske]

Thefirst step on the path to proper identification is a thorough patient history. The clinician must determine if the diplopia is monocular or binocular, as binocular diplopia may have a life-threatening cause.3,4

Diplopia that persists when one eye is covered falls into the category of monocular diplopia, or polyopia (greater than two images). Clinicians should have the patient cover each eye separately when testing for monocular diplopia. This finding is rarely due to cortex lesion and is generally attributable to causes within the eye itself. Decreased vision due to uncorrected astigmatism, dry eye and tear film deficiencies, corneal pathology or scarring, iris abnormalities, lenticular changes, vitreal opacities and macular disease are all possible causes of monocular diplopia.4,7,8 Medications (e.g., antidepressants, antihistamines, diuretics) may contribute to ocular surface dryness and induce a monocular diplopia.9

Although less frequently, certain medications can cause binocular diplopia, such as anticonvulsants, selective serotonin reuptake inhibitor antidepressants, erectile dysfunction medications, migraine therapies and other medications with anticholinergic properties. Many antidepressants may aggravate the symptoms of a convergence insufficiency by affecting accommodation.6,11

Ocular motility and alignment testing may include the cover/uncover test, alternate cover test, Maddox rod and corneal light reflex. Ocular misalignment may be caused by a tropia, and an obvious eye turn is noted. A phoria occurs when the misalignment is not obvious, and diplopia occurs only when binocularity is disrupted. A key point in alignment testing is the evaluation for comitancy, in which the size of an ocular deviation remains the same in all directions of gaze. A comitant deviation, such as a decompensating heterophoria, presents with an intermittent or gradual onset, shows full range of ocular movement in all positions of gaze and may have a history of childhood strabismus.12 In contrast, CN palsies and extraocular muscle restrictions cause non-comitant deviations with the greatest diplopia noted in the direction of action of the weakened muscle.2,3,9,10 Clinicians must examine each eye separately (ductions) to catch a subtle restriction that could be missed when evaluating both eyes together.3,5,13 To test versions, the patient fixates on a target that is slowly moving laterally while the clinician checks the medial rectus of the adducting eye and the lateral rectus of the abducting eye. The target is then moved superiorly to evaluate the superior/inferior rectus of the abducted eye and the inferior/superior oblique of the adducting eye. The test is repeated on the opposite side to test contralateral gaze.3,5,13 Forced duction testing can identify muscle restriction such as in thyroid disease or muscle entrapment by a fracture following trauma.3,5,13

Vertical diplopia assessment involves the four remaining muscles: the superior and inferior recti and the superior and inferior oblique. CN III and IV palsies, skew deviations (with or without INO), muscle restrictions and decompensated phorias can all cause vertical diplopia.9

Refractive. Misalignment of the optical centers of prescription glasses or non-prescription reading glasses, poor fitting glasses and the edges of high prescription glasses may all cause diplopia or worsen an existing heterophoria. Aniseikonia from refractive error results in differences in image size and shape in the visual cortex, causing diplopia when wearing glasses. Contact lens use often resolves the image difference in most cases of aniseikonia.1

Orbital disease. Thyroid eye disease (TED), idiopathic orbital inflammation and orbital tumors are the most common extraocular muscle and orbital diseases that cause diplopia.2,3 Orbital inflammation is usually unilateral and may affect the orbital fat, extraocular muscles, lacrimal gland, sclera or optic nerve. Onset may be sudden and painful, and the eye may appear proptotic. This has been associated with rheumatoid arthritis, sarcoidosis and, less frequently, giant cell arteritis (GCA). Testing includes rheumatoid factor, chest x-ray and ACE level for sarcoidosis and anti-nuclear antibody for systemic lupus erythematosus.

Other, less common, orbital causes of diplopia include trauma and neoplasms. A blow-out fracture is an emergent situation, as the sinus can cause a negative pressure that pulls on the inferior orbital wall, trapping the inferior rectus muscle, resulting in an inability to elevate the affected eye and vertical diplopia.3,6

The orbital floor in adolescents is flexible and can quickly open and close, trapping the inferior rectus, and may present with no other obvious signs of trauma. Patients with a history of orbital trauma and a white eye (or lack of subconjunctival hemorrhage) will need emergent imaging to determine if there is entrapment of the inferior rectus, especially in children. In these cases, decompression surgery is urgently needed within 24 to 48 hours to avoid ischemia of the muscle.15 Conversely, if there is no muscle entrapment, due to orbital floor fracture, surgical intervention may be considered in two weeks.16

Neoplasms and sinus-related issues should be considered in the presence of a correlating health history. Secondary orbital tumors, lymphomas and metastatic cancers are the most common orbital neoplasms presenting with unilateral proptosis and resistance to retropulsion.3 All patients with a new onset of diplopia and a history of cancer should have urgent imaging studies.17 Rarely, a silent sinus syndrome will cause a downward displacement and enophthalmos of the eye. The obstruction of the ostium of the maxillary causes a negative pressure that pulls downward on the inferior orbital wall, resulting in a vertical diplopia.3,6,18

Giant cell arteritis. A patient with GCA can present with any CN palsy.10 GCA should be ruled out in all patients who present with diplopia, especially those older than age 60. Urgent blood work should include complete blood count, c-reactive protein (CRP) and sedimentation rate (ESR). If the platelet count, CRP and ESR are elevated or GCA is suspected based on accompanying scalp tenderness, headache, fever and generalized malaise, clinicians should refer the patient for urgent treatment.8,19 Studies indicate that GCA is the underlying cause of diplopia in anywhere from 3% to 15% of presenting cases of diplopia with biopsy proven GCA, but the risk of morbidity and mortality is too high to miss this disease.8,19

Neuromuscular junction dysfunction. MG is the classic neuromuscular junction disease that can become life threatening when it affects the muscles of respiration, causing respiratory failure. Approximately 50% to 60% of MG patients present with a ptosis and diplopia, and approximately 20% to 30% have localized ocular involvement.5,8 The most common age of onset is in the third decade for women and the seventh decade for men.5 Weakness of the medial rectus is fairly common, but diplopia can vary between horizontal, vertical and oblique. Patients report variable fatigue and ptosis of one or both eyelids that worsens with prolonged activity or toward the end of the day. However, MG can cause a fluctuating diplopia at any time of the day, even on waking.9 A recent history of weakness and difficulty walking or swallowing are found in generalized MG but absent in the ocular form. As with TED, clinicians should remain suspicious of MG in all cases, as it can mimic CN IV, VI and partial CN III palsies in addition to INO, although the pupil is never involved.2,3

Several in-office tests are available to help support the diagnosis of MG. During the Cogan lid twitch test, the patient looks down for a few seconds and the clinician then watches the lid reaction when they return to primary gaze. A 1mm to 2mm drop of eyelid elevation immediately after returning to primary gaze is a positive response. Application of ice packs for one to two minutes or a resting state for 10 minutes is an another easy in-office test, and an improvement in the ptotic eyelid is a positive response in suspected cases of MG. Fatigue in prolonged upgaze for at least two minutes with a resulting ptosis, worsened ptosis or inability to maintain upgaze is considered a positive test.2 Approximately 15% of MG patients will have thyroid changes and co-existing TED, while about 10% will have thymoma present and will be evaluated for surgery.5

Cranial nerve palsies. CN III, IV and VI palsies share many of the same underlying etiologies such as microvascular CN palsies, intracranial aneuryms and neoplasms.10,20 Trauma can impair the function of any nerve, but CN IV in particular is more susceptible to trauma. Microvascular disease accounts for many CN palsies in patients older than 50, especially in those with known microvascular disease. Pain and rapidity of onset provide less definitive clues about cause, should a cerebrovascular accident be suspected. Pain can be severe or absent in aneurysmal CN III palsies and ischemic events, though a significant headache and CN III palsy requires careful pupil testing and referral to an emergency room. Research suggests acute onset is associated with ischemic events while slow onset is associated with compressive cases.20 A CN VI palsy is the most common, followed by CN IV and CN III.7 In all cases of nerve palsies, evaluation must carefully determine if single or multiple nerves are involved, as imaging is most often warranted, particularly when multiple cranial nerves are involved.21

A decompensated congenital CN IV can be distinguished from an acute CN palsy by evaluating vertical fusional amplitudes with prism bars or the amount of ocular rotation between the eyes along with the size of the vertical deviation.10 The prism bar test is performed by measuring the range of prism that will eliminate the diplopia. Normal vertical fusional amplitudes range from one to four prism diopters, whereas patients with congenital strabismus may demonstrate up to six prism diopters of vertical fusion amplitude.10,20 A review of old photos may also help to identify those patients with longstanding congenital palsies.

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