SMA in Scientific Terms
To see the
actual medical textbook explanation of each of these types, click on the
hyperlinks below associated with each type.
Spinal Muscular Atrophy
(SMA) is one of the neuromuscular diseases. Muscles weaken and waste
away (atrophy) due to degeneration of anterior horn cells or motor
neurons which are nerve cells in the spinal cord. Normally, these motor
neurons relay signals, which they receive from the brain, to the muscle
cells. When these neurons fail to function, the muscles deteriorate. SMA
effects the voluntary muscles for activities such as crawling, walking,
head and neck control and swallowing.
SMA mainly affects the proximal muscles, or in other words the muscles
closest to the trunk of the body. Weakness in the legs is generally
greater than weakness in the arms. Some abnormal movements of the
tongue, called tongue fasciculation's may be present in patients with
Type I and some patients with Type II. The brain and the sensory nerves
(that allow us to feel sensations such as touch, temperature, pain etc.)
are not affected. Intelligence is normal. In fact it is often
observed that patients with SMA are unusually bright and sociable.
SMA, or Werdnig-Hoffman disease
spinal muscular atrophy (Werdnig-Hoffman disease) is the
most severe form of SMA. It usually becomes evident in the first six
months of life. The child is unable to roll or sit unsupported, and
the severe muscle weakness eventually causes feeing and breathing
problems. There is a general weakness in the intercostals and
accessory respiratory muscles (the muscles situated between the
ribs). The chest may appear concave due to the diaphragmatic
children usually do not live beyond about 24 months of age.
Intermediate type (this
does not have a hyperlink so it is spelled out below instead.)
What are the
features of intermediate (type 2) SMA?
A child with the
intermediate form of SMA often reaches six to twelve months of age,
sometimes later, and learns to sit unsupported, before symptoms are
noticed. Weakness of the muscles in the legs and trunk develops and
this makes it difficult for the child to crawl properly or to walk
normally, if at all. Weakness in the muscles of the arms occurs as
well although this is not as severe as in the legs. Usually the
muscles used in chewing and swallowing are not significantly
affected early on. The muscles of the chest wall are affected,
causing poor breathing function. Parents notice that the child is
"floppy" or limp, the medical term for this being
hypotonia. Tongue fasciculations are less often found in
children with Type II but a fine tremor in the outstretched fingers
is common. Children with Type II are also diaphragmatic breathers.
Physical growth continues at a
normal pace and, most importantly, mental functions is not affected.
The children are bright and alert and it is important that they
receive all the available opportunities to develop their
intellectual capacities to their fullest extent. Integration into a
normal school environment gives them the best chance to mature
intellectually and emotionally.
What does the future
The course of the
disease is quite variable, and difficult to precisely predict from
the start. Children
with the intermediate form of SMA usually sit unsupported. Weakness
of the legs and trunk usually, but not always, holds the child back
from standing and walking alone. Sometimes the muscle weakness can
seem to be non-progressive, but in most cases weakness and
disability will increase over many years. Severe illness with
prolonged periods of relative immobility, putting on excessive
weight or growth spurts may contribute to deterioration in function.
Due to weakness of the muscles
supporting the bones of the spinal column, scoliosis (curvature of
the spine) often develops in children who are wheelchair bound. If
this becomes severe it can cause discomfort and can have a bad
influence on breathing function as well. An operation can be done to
straighten the spine and prevent further deterioration.
Recurrent chest infections may
occur, because of decreased respiratory function and difficulty in
coughing. Parents will have been shown how to encourage their child
to maintain his/her maximum respiratory function as well as how they
can perform postural drainage of the chest. They should start this
as the first sign of any chest problem. Antibiotics and inhalation
therapy may also be needed. Sometimes hospitalization is required to
best manage and care for the child. The
long term outlook depends mainly on the severity of weakness of the
muscles of the chest wall and on the development of scoliosis.
Lifespan is always difficult to predict. Mildly affected children
may live into adult years. The more severely affected children may
die, due to pneumonia and other chest problems, before or in their
SMA, or Kugelberg-Welander disease
muscular atrophy (Kugelberg-Welander disease) usually has its onset
after 2 years of age. It is considerably milder than the infantile
or intermediate forms. In juvenile spinal muscular atrophy children
are able to walk, although with difficulty. The patient with
Type III can stand alone and walk, but may show difficulty with
walking and/or getting up from a sitting or bent over position. With
Type III, a fine tremor can be seen in the outstretched fingers but
tongue fasciculations are seldom seen.
Type 4 Adult Onset
Typically in the
adult form symptoms begin after age 35. It is very rare for Spinal
Muscular Atrophy to begin between the ages of 18 and 30. Adult SMA
is characterized by insidious onset and very slow progression. The
bulbar muscles, those muscles used for swallowing and respiratory
function, are rarely affected in Type IV.
Kennedy's Syndrome or Bulbo-Spinal Muscular
This form also known
as Adult Onset X-Linked SMA, occurs only in males, although 50% of
female offspring are carriers. This form of SMA is associated with a
mutation in the gene that codes for part of the androgen receptor
and therefore these male patients have feminine characteristics,
such as enlarged breasts. Also noticeably affected are the facial
and tongue muscles. Like all forms of SMA the course of the disease
is variable, but in general tends to be slowly progressive or non-progressive.