This is most often caused by an ocular myopathy (the mitochondrial disorder known as progressive external ophthalmoplegia). The mitochondrial defect may show a mendelian inheritance pattern, as occurs with POLG1 and twinkle gene mutations, or may be the result or a mutation in mitochondrial DNA and show maternal inheritance only (see Chap. 45), Other causes include myasthenia gravis or Lambert-Eaton syndrome. We have encountered instances of the Lambert-Eaton myasthenic syndrome that caused an almost complete ophthalmoplegia (but not as an initial sign, as it may be in myasthenia) and a patient with paraneoplastic brainstem encephalitis similar to the case reported by Crino and colleagues, but both of these are certainly rare as causes of complete loss of eye movements. The congenital myopathies are typically named for the morphologic characteristic of the affected limb musculature, and may include the central core, myotubular, and nemaline types. Another cause is the slow channel congenital myasthenic syndrome (see Chap. 46). Among the chronic conditions, progressive supranuclear palsy may ultimately produce complete ophthalmoplegia, after first affecting vertical gaze. Thyroid ophthalmopathy as a cause of chronic ophthalmoparesis is discussed below.
The Duane retraction syndrome takes one of several forms, depending upon the pattern of ocular muscles affected. In the most common presentation, there is impaired abduction with retraction of the globe and narrowing of the palpebral fissure that is elicited by attempted adduction. These features occur because the lateral rectus is aberrantly innervated by branches of the third nerve. Cocontraction of the medial and lateral recti results in retraction of the globe.
Several causes of a pseudoparalysis of ocular muscles that are due to mechanical restriction of the ocular muscles are distinguished from the neuromuscular and brainstem diseases discussed previously. Processes that infiltrate the orbit, such as lymphoma, carcinoma and granulomatosis may limit the range of motion of individual or all the ocular muscles. In thyroid disease, a swollen and tight inferior or superior rectus muscle may limit upward and downward gaze; involvement of the medial rectus limits abduction. The frequency of involvement of the ocular muscles is given by Wiersinga and colleagues as inferior rectus 60 percent; medical rectus 50 percent; and superior rectus 40 percent. In most instances of thyroid ophthalmopathy, diagnosis is clear as there is an associated proptosis, but in the absence of the latter sign, and particularly if the ocular muscles are affected on one side predominantly, there may be difficulty. The extraocular muscle enlargement can be demonstrated by CT scans and ultrasonography. This disorder is discussed further in Chap. 45. In a significant number of cases, 10 percent according to Bahn and Heufelder, there are no signs of hyperthyroidism.
The mechanical restriction of motion is confirmed by forced duction tests in which the eye is physically pulled or pushed over by the examiner. In the past, the insertions of the extraocular muscles were anesthetized and grasped by toothed forceps and attempts to move the globe are palpably restricted; more often, a cotton swab applied to the sclera is used to manipulate the globe.
We have already considered two types of neural paralysis of the extraocular muscles: paralysis of conjugate movements (gaze) and paralysis of individual ocular muscles. Here we discuss a third, more complex one—namely, mixed gaze and ocular muscle paralysis. The mixed type is always a sign of an intrapontine or mesencephalic lesion that may have a variety of causes.