Muscles controlling eye movements smaller in MG patients: Study
Limited eye movement linked to decreased muscle volume
The volume of the extraocular muscles controlling eye movements is decreased in Chinese patients with myasthenia gravis (MG), particularly in those with ophthalmoparesis, or limited eye movement, a study found.
Long disease duration was also associated with decreased extraocular muscle volume, but only in patients with ophthalmoparesis. A reduction in extraocular muscle volume indicates a greater degree of atrophy, or wasting, of extraocular muscle tissue in MG patients.
The results “suggest that MG patients with ophthalmoparesis and thin [extraocular muscles] may benefit from early and more intensive immunotherapy,” the researchers wrote.
Their findings were reported in the study, “Extraocular muscle volume on time-of-flight magnetic resonance angiography in patients with myasthenia gravis,” published in Muscle & Nerve.
Eye muscles often first affected
MG is a rare disease characterized by progressive muscle weakness and fatigue. While any voluntary muscles can be affected by MG, extraocular muscles are often among the first to be affected.
“Despite being a prominent feature of myasthenia gravis,” extraocular muscles have “received little attention in clinical research,” the researchers wrote.
They sought to better understand this common symptom and its relationship to clinical findings by quantifying and comparing the volume of extraocular muscles from 42 Chinese MG patients with those of individuals who had no eye movement impairments.
MG patients were further divided into two groups based on the presence of ophthalmoparesis, which was defined in the study as limited eye movements in any direction, without considering ptosis, or droopy eyelids.
The 18 patients with ophthalmoparesis did not differ from the 24 who didn’t have ophthalmoparesis in terms of age, sex distribution, disease duration, or antibody status.
The total volume of all eight extraocular muscles, deter mined based on images obtained from a special type of MRI, was significantly lower in the MG group than in the control group (3,823 mm3 vs. 4,141 mm3).
The volume of several individual extraocular muscles, including the superior rectus, the levator palpebrae, and the left inferior rectus, was also significantly lower in MG patients.
MG patients who had ophthalmoparesis had significantly lower total extraocular muscle volume compared with patients who did not have eye movement impairments (3,468mm3 vs. 4,089 mm3). The volume of the eight individual extraocular muscles was also significantly lower in the group of patients with ophthalmoparesis.
Extraocular muscle volume in MG patients who did not have ophthalmoparesis was similar to that of controls.
“Among controls and MG patients with and without ophthalmoparesis, [extraocular muscle] volume was smallest in the [group of MG patients with ophthalmoparesis],” the researchers wrote, though “the volume did not differ between controls and [MG patients without ophthalmoparesis],” the researchers wrote.
Considering all MG patients, extraocular muscle volume was not found to be correlated with disease duration. However, a moderate negative correlation was found in patients with ophthalmoparesis, indicating that those who had a longer disease course also tended to have lower total extraocular muscle volume. This is consistent with a scenario of greater muscle atrophy in patients with chronic disease.
Additional statistical analyses indicated that disease duration, along with the presence or absence of ophthalmoparesis, were the two major factors dictating extraocular muscle volume. In fact, the two accounted for 48.4% of extraocular muscle volume.
“Patients with MG show atrophy of the [extraocular muscles], especially those with ophthalmoparesis and long disease duration,” the researchers wrote, adding that long-term studies will be needed to confirm and expand on the findings.
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