A reinterpretation of certain disorders affecting the eye muscles and their tissues

Abstract

Recent discoveries about the orbital tissues prompt a re-evaluation of the way that clinicians think about disorders affecting the extraocular muscles, their nerves and motoneurons in the brainstem. The revolutionary discovery that the orbital layers of the extraocular muscles insert not onto the eyeball, but into fibromuscular pulleys that guide the orbital layers, provides explanations for the kinematic properties of eye rotations and clinical findings in some patients with strabismus. The demonstration that all extraocular fibers types, except pale global fibers, lack synaptic folding provides an explanation for why saccades may remain fast in patients with limited ocular mobility due to myasthenia gravis. More than one mechanism may account for the observation that patients with disorders affecting the eye muscles or their nerves can present with the appearance of central disorders of ocular motility, such as internuclear ophthalmoplegia. New approaches to analyzing saccades in patients with disjunctive eye movements provide the means to identify disorders affecting the peripheral or central components of the ocular motor system, or both.

Keywords: CPEO; Marfan’s syndrome; myasthenia gravis; strabismus.

Figure 1

Horizontal saccades in CPEO due to mitochondrial myopathy; these horizontal movements are slow and disjunctive (note different size of movements of the right and left eyes). Positive values indicate rightward movements. Eye movements were recorded using the magnetic search coil technique (Leigh and Zee 2006).

Figure 2

Comparison of true internuclear ophthalmoplegia due to multiple sclerosis (MS) (A and B) with pseudo-internuclear ophthalmoplegia due to ocular myasthenia gravis (MG) (C and D). Note that both patients show slower adducting movements (evident on the velocity channels of the time plots). However, on the corresponding binocular phase planes, the initial movements of abducting and adducting eyes are conjugate for the myasthenic (D – arrow) but differ from the onset of the movement for the patient with true INO (C – arrow). Positive values indicate rightward movements. Eye movements were recorded using the magnetic search coil technique (Leigh and Zee 2006).