APA (Old Order Amish, Eastern Pennsylvania)
MEN (General Swiss-German Mennonite)
HUT (Hutterite)

Inheritance: autosomal recessive
Genes: SMN1

SMA I
Infantile acute SMA
Infantile muscular atrophy
Werdnig-Hoffmann disease

Muscle weakness
Muscle atrophy
Motor weakness
Minimal limb movement
Difficulty feeding

Spinal Muscular Atrophy I

Clinical Characteristics

General description (for patients):

This is an acute, early onset, and rapidly fatal neurological disease. Infants are ‘floppy’ at birth and do not move much. They are never able to lift their heads or sit alone even in early childhood. Feeding, sucking, and swallowing are often difficult. The face may have little expression as facial muscles are weak as well. Same infants may have symptoms slightly later (sometimes 6 months) and have a somewhat better prognosis but this condition usually leads to death by the age of two years. The severe muscle weakness and wasting is the result of degeneration of nerve cells in the spinal cord.
In spite of weakness in the muscles, sensation such as touch and pain are present and intellect is normal.

Medical description:

This is a devastating and unfortunately common neurological disorder. Symptoms of motor weakness may be noted at birth but always before 6 months of age. Some neurologists have divided SMA into several categories depending upon age of onset but this seems to have little value now that we understand the genetic defect. It is true that the disease progresses more slowly in those with a later onset of symptoms but the outcome remains the same.
Profound weakness involving axial, limb, and facial musculature is evident in early childhood and most infants are never able to hold their heads or sit alone. Limb movements may be minimal and tongue fasiculations are frequently present. Some individuals develop knee and elbow contractures. Deep tendon reflexes are absent but sensory functions and intellect usually remain normal. The EMG reveals denervation but spontaneous motor unit activity often remains. Nerve conduction velocities and histology are normal. Muscle biopsies show severe atrophy of type 1 and type 2 muscle fibers.

Genetics:

This is an autosomal recessive disorder caused by homozygous mutations in the SMN1 (survival motor neuron) gene.
SMN1 has been reported in Montana and South Dakota Hutterites where the carrier frequency is extremely high. It is also known to occur among plain people (Old Order Mennonites) in Eastern Pennsylvania and in Anabaptists living in Virginia. 

Treatment:

Treatment is largely supportive. The use of ventilators and gastric feeding can prolong life to some extent. 

Prognosis:

This disorder is uniformly fatal usually before the age of 2 years.

Ancillary treatments and support: 

Assisted ventilation and gastric feeding for nutrition can prolong life but have no impact on the ultimate outcome.

Specialists and specialty centers:

Pediatricians and neurologists often make the diagnosis and provide the primary care. Nutritionists and respiratory therapists can be of great assistance. 

References:

Chong JX, Oktay AA, Dai Z, Swoboda KJ, Prior TW, Ober C. A common spinal muscular atrophy deletion mutation is present on a single founder haplotype in the US Hutterites. Eur J Hum Genet. 2011 Oct;19(10):1045-51. PubMed PMID: 21610747.

Resources:

NCBI
SMA Foundation
Families of SMA