Spinal muscular atrophy (SMA) is a degenerative motor neuron disease. There are several types depending on the age of onset and the best motor function achieved. Type IV SMA is the rarest, often underdiagnosed, and later form with symptoms beginning at the age of 18.
A clinical, neurophysiological and radiological study of a 20 patient-cohort with type IV SMA was conducted in Brazil. The median age at onset is 31 years old. The diagnosis suggested before the confirmation of SMA was girdle muscular dystrophy (LGMD) (60%), amyotrophic lateral sclerosis (ALS) (20%), inflammatory myopathy (10%) or inflammatory demyelinating polyneuropathy (10 %).
The most common clinical symptoms in this cohort are:
- weakness of the proximal muscles of the lower limbs (75% of cases),
- cramps / fasciculations (20%),
- asymptomatic hyperCKemia (5%).
Neurological results show:
- an absence of osteotendinous reflex (90% of cases),
- fasciculation localized to the thighs (45%),
- a distal tremor of the upper limbs (40%),
- pseudo-hypertrophy of the calves (30%).
On the neurophysiological level, there are:
- for 100% of participants, chronic denervation,
- for 60%, signs of bulbar denervation
- for 15%, signs of acute denervation.
The functional scales (6-minute walk test, RULM, HFMSE, etc.) and the MUNIX index are correlated with the duration of the disease.
Muscle MRI data show frequent involvement of the anterior compartment of the thigh, while the internal compartment is frequently spared.
Genetically, the majority of patients (80%) have a homozygous deletion of exon 7 of the SMN2 gene, and 60% have 4 copies of the SMN2 gene.
Only one participant is on non-invasive nocturnal mechanical ventilation. None have a gastrostomy and all are able to walk without assistance.
