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 ...

s (

neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

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), malnutrition, poor nourishment, poor circulatory system, circulation, loss of hormone, ...

(wasting) of various

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

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 describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

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 describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

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, mitosis ...

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 ter ...

'' gene on chromosome 5q ( locus 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, mitosi ...

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 has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

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), also known as 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 o ...

s, located at the anterior horn of spinal cord, 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 receptor potentials. This process is called sensory transduc ...

s, which are located at the

posterior horn of spinal cord The grey columns are three regions of the somewhat ridge-shaped mass of grey matter in the spinal cord. These regions present as three columns: the anterior grey column, the posterior grey column, and the lateral grey column, all of which ...

, 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 (HMSN).

References

External links

{{Diseases of the nervous system Motor neuron diseases Systemic atrophies primarily affecting the central nervous system Genetic diseases and disorders Neurological disorders Spinal muscular atrophy

Classification

See also

References

Further reading

External links