Spinal muscular atrophies (SMAs) are a genetically and clinically heterogeneous group of rare debilitating disorders characterised by the degeneration of
lower motor neuron
Lower motor neurons (LMNs) are motor neurons located in either the anterior grey column, anterior nerve roots (spinal lower motor neurons) or the cranial nerve nuclei of the brainstem and cranial nerves with motor function (cranial nerve lower mo ...
s (
neuron
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
al cells situated in the
anterior horn of the spinal cord) and subsequent
atrophy
Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations (which can destroy the gene to build up the organ), poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply t ...
(wasting) of various
muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
groups in the body.
While some SMAs lead to early infant death, other diseases of this group permit normal adult life with only mild weakness.
Classification
Based on the type of muscles affected, spinal muscular atrophies can be divided into:
* ''Proximal spinal muscular atrophies'', i.e., conditions that affect primarily
proximal
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
muscles;
* ''Distal spinal muscular atrophies'' (which significantly overlap with
distal hereditary motor neuronopathies) where they affect primarily
distal
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
muscles.
When taking into account
prevalence
In epidemiology, prevalence is the proportion of a particular population found to be affected by a medical condition (typically a disease or a risk factor such as smoking or seatbelt use) at a specific time. It is derived by comparing the number o ...
, spinal muscular atrophies are traditionally divided into:
* ''
Autosomal recessive proximal spinal muscular atrophy'', responsible for 90-95% of cases and usually called simply ''spinal muscular atrophy'' (SMA) – a disorder associated with a
genetic mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitos ...
on the ''
SMN1
Survival of motor neuron 1 (''SMN1''), also known as component of gems 1 or ''GEMIN1'', is a gene that encodes the SMN protein in humans.
Gene
''SMN1'' is the telomeric copy of the gene encoding the SMN protein; the centromeric copy is term ...
'' gene on
chromosome 5
Chromosome 5 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 5 spans about 181 million base pairs (the building blocks of DNA) and represents almost 6% of the total DNA in cells. Ch ...
q (
locus
Locus (plural loci) is Latin for "place". It may refer to:
Entertainment
* Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front
* ''Locus'' (magazine), science fiction and fantasy magazine
** ''Locus Award' ...
5q13), diagnosed predominantly in young children and in its most severe form being the most common genetic cause of infant death if left untreated;
* ''Localised spinal muscular atrophies'' – much more rare conditions, in some instances described in but a few patients in the world, which are associated with
mutations
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
of genes other than ''SMN1'' and for this reason sometimes termed simply ''non-5q spinal muscular atrophies''; none has currently a causal treatment.
A more detailed classification is based on the
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
associated with the condition (where identified) and is presented in table below.
In all forms of SMA (with an exception of
X-linked spinal muscular atrophy type 1), only
motor neuron
A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectl ...
s, located at the
anterior horn of spinal cord
The anterior grey column (also called the anterior cornu, anterior horn of spinal cord, motor horn or ventral horn) is the front column of grey matter in the spinal cord. It is one of the three grey columns. The anterior grey column contains motor ...
, are affected;
sensory neuron
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...
s, which are located at the
posterior horn of spinal cord
The posterior grey column (posterior cornu, dorsal horn, spinal dorsal horn, posterior horn, sensory horn) of the spinal cord is one of the three grey columns of the spinal cord. It receives several types of sensory information from the body, incl ...
, are not affected. By contrast, hereditary disorders that cause both weakness due to motor denervation along with ''sensory'' impairment due to sensory denervation are known as
hereditary motor and sensory neuropathies
Hereditary motor and sensory neuropathies (HMSN) is a name sometimes given to a group of different neuropathy, neuropathies which are all characterized by their impact upon both Afferent nerve fiber, afferent and Efferent nerve fiber, efferent neur ...
(HMSN).
See also
*
Distal hereditary motor neuropathies Distal hereditary motor neuronopathies (distal HMN, dHMN), sometimes also called distal hereditary motor neuropathies, are a genetically and clinically heterogeneous group of motor neuron diseases that result from genetic mutations in various genes ...
*
Motor neuron disease
Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease (MND) or Lou Gehrig's disease, is a neurodegenerative disease that results in the progressive loss of motor neurons that control voluntary muscles. ALS is the most comm ...
*
Polyneuropathy in dogs and cats
Polyneuropathy in dogs and cats is a collection of peripheral nerve disorders that often are breed-related in these animals. Polyneuropathy indicates that multiple nerves are involved, unlike mononeuropathy. Polyneuropathy usually involves motor ...
References
Further reading
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External links
{{Medicine
Motor neuron diseases
Genetic disorders by system
Systemic atrophies primarily affecting the central nervous system
Genetic diseases and disorders
Neurological disorders
Spinal muscular atrophy