Sedentary Work Exerting up to 10 pounds (4.5 kg) of force occasionally and/or a negligible amount of force frequently or constantly to lift, carry, push, pull, or otherwise move objects, including the human body. Sedentary work involves sitting most of the time, but may involve walking or standing for brief periods of time. Jobs are sedentary if walking and standing are required only occasionally and other sedentary criteria are met.

Light Work Exerting up to 20 pounds (9.1 kg) of force occasionally and/or up to 10 pounds (4.5 kg) of force frequently, and/or negligible amount of force constantly to move objects. Physical demand requirements are in excess of those for Sedentary Work. Light Work usually requires walking or standing to a significant degree. However, if the use of the arm and/or leg controls requires exertion of forces greater than that for Sedentary Work and the worker sits most the time, the job is rated Light Work.

Medium Work Exerting up to 50 (22.7 kg) pounds of force occasionally, and/or up to 25 pounds (11.3 kg) of force frequently, and/or up to 10 pounds (4.5 kg) of forces constantly to move objects.

Heavy Work Exerting up to 100 pounds (45.4 kg) of force occasionally, and/or up to 50 pounds (22.7 kg) of force frequently, and/or in excess of 20 pounds (9.1 kg) of force constantly to move objects.

Very Heavy Work Exerting in excess of 100 pounds (45.4 kg) of force occasionally, and/or in excess of 50 pounds (22.7 kg) of force frequently, and/or in excess of 20 pounds (9.1 kg) of force constantly to move objects.

Job Classification

In most duration tables, five job classifications are displayed. These job classifications are based on the amount of physical effort required to perform the work. The classifications correspond to the Strength Factor classifications described in the United States Department of Labor's Dictionary of Occupational Titles. The following definitions are quoted directly from that publication.

Sedentary Work Exerting up to 10 pounds (4.5 kg) of force occasionally and/or a negligible amount of force frequently or constantly to lift, carry, push, pull, or otherwise move objects, including the human body. Sedentary work involves sitting most of the time, but may involve walking or standing for brief periods of time. Jobs are sedentary if walking and standing are required only occasionally and other sedentary criteria are met.

Light Work Exerting up to 20 pounds (9.1 kg) of force occasionally and/or up to 10 pounds (4.5 kg) of force frequently, and/or negligible amount of force constantly to move objects. Physical demand requirements are in excess of those for Sedentary Work. Light Work usually requires walking or standing to a significant degree. However, if the use of the arm and/or leg controls requires exertion of forces greater than that for Sedentary Work and the worker sits most the time, the job is rated Light Work.

Medium Work Exerting up to 50 (22.7 kg) pounds of force occasionally, and/or up to 25 pounds (11.3 kg) of force frequently, and/or up to 10 pounds (4.5 kg) of forces constantly to move objects.

Heavy Work Exerting up to 100 pounds (45.4 kg) of force occasionally, and/or up to 50 pounds (22.7 kg) of force frequently, and/or in excess of 20 pounds (9.1 kg) of force constantly to move objects.

Very Heavy Work Exerting in excess of 100 pounds (45.4 kg) of force occasionally, and/or in excess of 50 pounds (22.7 kg) of force frequently, and/or in excess of 20 pounds (9.1 kg) of force constantly to move objects.

Myasthenia Gravis


Related Terms

  • MG
  • Myasthenia Gravis Crisis
  • Pseudoparalytica Gravis

Differential Diagnosis

Specialists

  • Clinical Psychologist
  • Neurologist
  • Physiatrist (Physical Medicine and Rehabilitation Specialist)
  • Psychiatrist
  • Rheumatologist

Comorbid Conditions

Factors Influencing Duration

Length of disability may be influenced by job requirements for individuals who perform physical labor or public speaking duties, by the degree and severity of fatigue, and by which muscle groups are affected by the disease. The type of treatment, individual's age and response to treatment, frequency of exacerbations, and residual impairments may also influence duration.

Medical Codes

ICD-9-CM:
358.00 - Myasthenia Gravis without (Acute) Exacerbation
358.01 - Myasthenia Gravis with (Acute) Exacerbation; Myasthenia Gravis in Crisis

Overview

Myasthenia gravis (MG) is a relatively rare autoimmune disorder in which communication between nerve and muscle is interrupted. The body's immune system forms antibodies against acetylcholine, a substance that is needed for proper communication between nerves and muscles. Receptors in the muscles responsible for picking up and correctly interpreting nerve impulses are destroyed. Consequently, affected muscles fail to respond or respond only weakly to nerve impulses. The disease may become life-threatening when it weakens the respiratory system (myasthenic crisis). MG is a chronic condition that follows an unpredictable course of periodic exacerbations and remissions.

The cause of MG is unknown. There is, however, a genetic predisposition to autoimmune diseases in women and in individuals with certain substances (antigens) that cause the formation of antibodies. The disease has also occasionally been triggered by a drug reaction or has been associated with organ transplantation. Since antibodies circulate in the bloodstream, mothers with myasthenia gravis may pass the disease through their placenta to the unborn child (neonatal myasthenia). Although the infant may then be born with muscle weakness, it disappears within a few weeks after birth.

Incidence and Prevalence: Two out of 100,000 people develop myasthenia gravis each year (Shah).

Source: Medical Disability Advisor



Causation and Known Risk Factors

Women were thought to be more likely to have myasthenia gravis than men, but recently it has been shown that women and men have the same chance of developing MG; women have a greater likelihood of developing the disorder after age 30, whereas men are more likely to have it between ages 60 and 70 (Shah).

Source: Medical Disability Advisor



Diagnosis

History: Myasthenia gravis (MG) is characterized by chronic fatigue and muscle weakness, especially in the face and throat. The onset of symptoms is usually gradual, with drooping of the upper eyelids (ptosis), double vision (diplopia), and weakness of the facial muscles. The weakness may then extend to other muscles, particularly the respiratory muscles. Muscular exertion, stress, and infection may aggravate symptoms, which typically vary over the course of the day.

In early stages of the disease, there may be no other symptoms. Typically, myasthenic muscles are strongest in the morning but weaken throughout the day, especially after exercise. Short rest periods may temporarily restore muscle function. As the disease progresses, symptoms may eventually include difficulty chewing, swallowing, and talking; difficulty handling objects or doing tasks with the hands and arms; and difficulty standing or walking.

Physical exam: The exam may reveal weak eye closure, drooping eyelids, double vision, slack facial muscles, and a blank facial expression. Muscle fatigue improves with rest. The individual may have difficulty chewing and swallowing. Since the gag reflex is often absent, there is a risk for inhalation (aspiration) of food, liquid, or saliva that may cause the individual to choke. The voice may have a nasal quality. Neck muscles may be too weak to hold up the head, causing it to fall onto the chest when the individual is seated. Weakened respiratory muscles may impair breathing, making the individual prone to pneumonia and other respiratory infections. The thymus gland, part of the body's immune system, is abnormally enlarged in about 75% of individuals with myasthenia gravis, and in about 15% of these cases, a tumor of the thymus gland (thymoma) is found (Newton).

Tests: The diagnosis of myasthenia gravis can be confirmed by the edrophonium challenge test. In individuals with myasthenia gravis, muscle function will improve within 30 to 60 seconds after injecting edrophonium (or neostigmine) into the vein (intravenous). Muscle function improvement lasts up to 30 minutes. Other diagnostic tests may include electrophysiologic (EP) tests to assess the transmission of neuromuscular messages. Through an electromyogram (EMG), a record of the electrical activity of muscles can show changes in response to nerve stimulation. A blood test can detect antibodies to acetylcholine (the chemical that transmits messages between nerve and muscle cells). A special x-ray, using a computer to record various views and measurements of the thymus (CT scan), may be done to rule out a tumor of the thymus gland (thymoma), especially in individuals over 40 years of age.

Source: Medical Disability Advisor



Treatment

In myasthenia gravis (MG), treatment is aimed at relieving symptoms. Anticholinesterase drugs counteract fatigue and muscle weakness and allow about 80% of normal muscle function. These types of drugs may need to be taken as often as every 3 to 4 hours by mouth or injection. Unfortunately, these medications become less effective as the disease worsens. During acute relapses, treatment may also include anti-inflammatory drugs (such as corticosteroids) and drugs that suppress the immune system (such as azathioprine).

Individuals with tumors of the thymus (thymomas) require removal of the thymus gland (thymectomy).

Acute MG exacerbations causing severe respiratory difficulty (myasthenic crisis) may require critical emergency treatment. This may include an incision made into the trachea or "windpipe" (tracheotomy) so that the individual can breathe. Saliva from the mouth may need to be suctioned in order to prevent the individual from choking, and anticholinesterase drugs will have to be given by injection.

A process called plasmapheresis may be recommended when drugs do not bring relief, during a myasthenic crisis, or to prepare an individual for surgery. In plasmapheresis, blood is progressively removed from the body, cleansed of toxic substances (in this case the offending antibodies), and then returned to the body. Although plasmapheresis is a slow process, taking several hours, the individual usually improves markedly for weeks to months thereafter. Plasmapheresis can be repeated at regular intervals.

Source: Medical Disability Advisor



Prognosis

Although there is no cure for myasthenia gravis (MG), drug treatment has improved prognosis and allowed individuals to lead relatively normal lives with a nearly normal life expectancy. Without medical treatment, the death rate is 25% to 31%; with medical treatment, the death rate is only 3% to 4% (Shah). Approximately 33% of individuals experience a spontaneous remission in which all symptoms permanently disappear (Goetz).

In general, the outcome depends on the rapidity of the disease progression and the effectiveness of medications. As more and more muscles become weakened, some muscles may eventually lose function entirely. Impaired muscle strength can result in aspiration, respiratory complication, falls, and death.

According to a study done at Johns Hopkins University Hospital from 1990 to 1998, approximately 46% of individuals with myasthenia gravis who develop respiratory failure, the life-threatening complication of myasthenic crisis, get pneumonia and atelectasis (Varelas). The outcome is then influenced by whether or not they require a tracheotomy and whether it is temporary or permanent. If temporary, the opening is allowed to close, and the individual will return to breathing through the nose. If permanent, the individual will continue to breathe through this opening in the throat (trachea). Care must be taken to guard against infection of the opening.

Surgical removal of the thymus gland leads to a remission in approximately 85% of those cases (Newton). Since the thymus helps the body's immune system to recognize and combat infection, special care must be taken afterward to avoid infection. Medical attention must be sought immediately if signs of infection develop.

Source: Medical Disability Advisor



Rehabilitation

Although there is no cure for myasthenia gravis, rehabilitation alone or in combination with other forms of treatment can relieve symptoms for many people. Since myasthenia gravis often causes muscle weakness in the arms or legs, physical therapy is beneficial for long-term restoration of muscle strength.

Strengthening exercises help the individual remain as functional as possible. If significant weakness is present, active assist exercises may be necessary in which the individual generates some of the effort with the help of the therapist. When improvement is noted and the exercises are well-tolerated, the therapist then introduces active range of motion exercises. In this type of exercise, the individual performs all of the motion independently. Resistance is then added to each exercise (isotonic exercise). The primary goal of therapy is to build the individual's strength to facilitate return to work and activities of daily living. The amount of resistance is determined at a point when the individual's final repetition is difficult but doable.

In myasthenia gravis, good days alternate with bad, and the physical therapist should modify the rehabilitation program accordingly. The rehabilitation program will also vary because the intensity and progression of the exercise depends on the stage of the individual's disease and his or her overall health.

Occupational therapy may help the individual adapt to new ways of performing specific domestic and occupational tasks. Speech therapy may be used as facial and throat muscles become weak or if the individual needs to be trained to speak by forcing air in and out of the esophagus (esophageal speech) following a permanent opening in the throat or "windpipe" (tracheotomy). Vocational counseling might be necessary if present job requirements can no longer be met. Psychological interventions may be necessary to help the individual cope with the effects of the illness.

Source: Medical Disability Advisor



Complications

Possible complications of the disease include pneumonia as a result of aspiration of saliva, food, or liquids; choking as a result of inability to swallow or manage oral secretions; and increased risk of falls as a result of muscle weakness in the legs. If the respiratory muscles become too weak, the individual may lose the ability to breathe without assistance (myasthenic crisis). Excessive physical or emotional stress may trigger a crisis.

Source: Medical Disability Advisor



Ability to Work (Return to Work Considerations)

Work restrictions and special accommodations are determined on an individual basis according to the severity of symptoms. The individual may need to be placed in a job that requires less physical strength and mobility. If job performance requires physical strength for physical labor, the individual may reach a point at which it is no longer possible to work due to muscle weakness. A job that requires public speaking may be in jeopardy if facial and throat muscles are affected, resulting in speech difficulties. As muscle weakness advances, environmental work alterations may be necessary to be compliant with the Americans with Disabilities Act and to make sure that the workplace is accessible.

Source: Medical Disability Advisor



Failure to Recover

If an individual fails to recover within the expected maximum duration period, the reader may wish to consider the following questions to better understand the specifics of an individual's medical case.

Regarding diagnosis:

  • Does individual have chronic fatigue? Muscle weakness, especially in the face and throat? Does it extend to the respiratory muscles?
  • Has individual noticed he or she is stronger in the morning? Does rest restore strength?
  • Does individual have difficulty chewing, swallowing, or talking? Is the gag reflex absent?
  • Does individual have difficulty using arms, hands, or legs?
  • On physical exam, were weak eye closure, drooping eyelids, double vision, slack facial muscles, and a blank facial expression noted?
  • Is individual able to hold the head up?
  • Is voice nasal-sounding?
  • Is thymus gland enlarged? Is there a tumor?
  • Has individual had an edrophonium challenge test? EMG? CT scan? Acetylcholine antibody test?
  • Have conditions with similar symptoms been ruled out?

Regarding treatment:

  • Is individual being treated with anticholinesterase drugs?
  • For acute relapses, have corticosteroids and immunosuppressant drugs been used?
  • Was a thymectomy performed?
  • Has plasmapheresis been conducted as needed?

Regarding prognosis:

  • Is individual active in rehabilitation? Do they have a home exercise program? Have they sought psychological intervention?
  • Is individual's employer able to accommodate any necessary restrictions?
  • Does individual have any conditions that may affect the ability to recover?
  • Has individual had any complications such as pneumonia? Choking? Frequent falls?
  • Has individual had a myasthenic crisis?

Source: Medical Disability Advisor



References

Cited

Elsevier, Inc. "Myasthenia Gravis." Textbook of Clinical Neurology. 2nd ed. Philadelphia: W.B. Saunders, 2003. 1118-1120. MD Consult. 27 Dec. 2004 <http://home.mdconsult.com/das/book/43494553-2/view/1158/1.html/top>.

Newton, Edward. "Myasthenia Gravis." eMedicine. Eds. Eric Kardon, et al. 10 Dec. 2001. Medscape. 27 Dec. 2004 <http://emedicine.com/emerg/topic325.htm>.

Shah, Aashit K. "Myasthenia Gravis." eMedicine. Eds. Donald B. Sanders, et al. 2 Aug. 2004. Medscape. 27 Dec. 2004 <http://emedicine.com/neuro/topic232.htm>.

Varelas, P. N., et al. "Ventilatory Care in Myasthenia Gravis Crisis: Assessing the Baseline Adverse Event Rate." Critical Care Medicine 30 12 (2002): 2663-2668. MD Consult. Elsevier, Inc. 27 Dec. 2004 <http://home.mdconsult.com/das/journal/view/43495077-2/N/12607567?sid=327002506&source=MI>.

Source: Medical Disability Advisor






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