From OMIM
Spinal muscular atrophy with progressive myoclonic epilepsy is an autosomal recessive neuromuscular disorder characterized by childhood onset of proximal muscle weakness and generalized muscular atrophy due to degeneration of spinal motor neurons, followed by the onset of myoclonic seizures. The disorder is progressive, and usually results in loss of ambulation and early death from respiratory insufficiency (summary by Zhou et al., 2012).  http://www.omim.org/entry/159950

From MedlinePlus Genetics
Spinal muscular atrophy with progressive myoclonic epilepsy (SMA-PME) is a neurological condition that begins in childhood. SMA-PME causes muscle weakness and wasting (atrophy) and a combination of seizures and uncontrollable muscle jerks (myoclonic epilepsy).

In individuals with SMA-PME, spinal muscular atrophy results from a loss of specialized nerve cells, called motor neurons, in the spinal cord and the part of the brain that is connected to the spinal cord (the brainstem). After a few years of normal development, affected children begin experiencing muscle weakness and atrophy in the lower limbs, causing difficulty walking and frequent falls. The muscles in the upper limbs are later affected, and soon the muscle weakness and atrophy spreads throughout the body. Once weakness reaches the muscles used for breathing and swallowing, affected individuals develop life-threatening breathing problems and increased susceptibility to pneumonia.

A few years after the muscle weakness begins, affected individuals start to experience recurrent seizures (epilepsy). Most people with SMA-PME have a variety of seizure types. In addition to myoclonic epilepsy, they may have  generalized tonic-clonic seizures (also known as grand mal seizures), which cause muscle rigidity, convulsions, and loss of consciousness. Affected individuals can also have absence seizures, which cause loss of consciousness for a short period that may or may not be accompanied by muscle jerks. In SMA-PME, seizures often increase in frequency over time and are usually not well-controlled with medication. Individuals with SMA-PME may also have episodes of rhythmic shaking (tremors), usually in the hands; these tremors are not thought to be related to epilepsy. Some people with SMA-PME develop hearing loss caused by nerve damage in the inner ear (sensorineural hearing loss).

Individuals with SMA-PME have a shortened lifespan; they generally live into late childhood or early adulthood. Near the end of their lives, affected individuals often have limited mobility, difficulty swallowing, and decline in cognitive functioning. The cause of death is often respiratory failure or pneumonia.  https://medlineplus.gov/genetics/condition/spinal-muscular-atrophy-with-progressive-myoclonic-epilepsy