Woman Diagnosed With Ocular MG, Motor Neuropathy in Case Report

Margarida Maia, PhD avatar

by Margarida Maia, PhD |

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A woman was diagnosed with both ocular myasthenia gravis (MG) and a hereditary form of motor neuropathy, a condition affecting the nerves controlling the muscles in the body, according to a case report from China.

The report, “Myasthenia gravis coexisting with HINT1-related motor axonal neuropathy without neuromyotonia: a case report,” was published in the journal BMC Neurology.

MG occurs when the body makes antibodies that attack its own receptors for acetylcholine, a chemical made by nerve cells that sends signals to other cells, including muscle cells. This causes episodes of weakness in the muscles controlling voluntary movement, especially those of the eyes, mouth, throat, arms and legs.

Visual problems, including drooping eyelids (ptosis) and double vision (diplopia), are usually among the earliest and most common symptoms of MG. When the muscles controlling eye and eyelid movements are the only ones affected, patients are said to have ocular MG.

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Patients who have had symptoms of ocular MG for some years are unlikely to develop muscle weakness elsewhere. However, its diagnosis can be difficult when other conditions are present.

A team from Central South University in Hunan reported the case of a 32-year-old woman with ocular MG coexisting with a hereditary form of motor neuropathy.

When the woman went to the hospital, she reported having recurrent ptosis in both eyes for eight years and diplopia for two years. She also reported having muscle weakness in her legs and feet for about a year and a half. Because of this, “she couldn’t take care of herself independently, and couldn’t walk on a level road without support,” the researchers wrote.

A physical examination revealed ophthalmoplegia, or paralysis of the muscles controlling eye movements. She also had weakness and wasting of the muscles in her legs, feet, arms, and hands.

An examination revealed no reflexes in the knee nor in the Achilles’ tendon in her heel.

An intramuscular (into-the-muscle) injection of 1 mg of neostigmine eased both ptosis and diplopia. Neostigmine is an anticholinesterase medication that improves signal transmission to muscles by increasing the levels of acetylcholine.

Blood testing showed she had high levels of antibodies against acetylcholine receptors, the most common type of self-reactive antibodies seen in MG. A nerve conduction study also found slowed signals to two shoulder muscles, the right deltoid muscle and the left trapezius muscle. Based on these findings, she was diagnosed with ocular MG.

Electromyography, a test measuring the electrical activity of a muscle in response to a nerve’s signals, revealed complex repetitive discharges, which occur when the muscle contracts spontaneously and repeatedly.

“The diagnosis of MG was established, and a concomitant hereditary motor axonal neuropathy was also suspected,” the scientists wrote.

Genetic testing revealed she carried a mutation, called c.278G>T (G93V), in both copies of the HINT1 gene, which provides instructions for making a protein called histidine triad nucleotide-binding protein 1 (HINT1). Her biological parents and healthy younger brother carried the same mutation in one copy of the HINT1 gene.

The HINT1 protein plays a role in nerve cells (neurons), and mutations in its gene cause hereditary motor axonal neuropathy. Patients with this condition experience weakness in the muscles of the feet, legs, and hands. The majority also experience continuous muscle tensing or delayed relaxation after tensing (neuromyotonia). The woman, however, did not show signs of neuromyotonia.

“This is the first report of ocular MG concurrent with HINT1-related hereditary motor axonal neuropathy,” the scientists wrote.

She was treated with Mestinon (pyridostigmine), an anticholinesterase medication. She also received oral prednisolone, a corticosteroid, and the immunosuppressant azathioprine. After half a year of treatment, her ptosis and diplopia eased, and she retained only a mild inability to move her right eye toward the center. There was little recovery in her leg weakness.

“Detection of ptosis with or without ophthalmoplegia, distribution of limb weakness, and reflex can help in recognizing [concurrent] MG and peripheral neuropathy,” the researchers wrote. “Early diagnosis is important for initial treatment and prognosis.”