Disease severity is associated with age at onset, with the most severe forms of the disease presenting during infancy as first described in the late nineteenth century by Werdnig and Hoffmann. These patients, termed “type 1 SMA” patients, have minimal functional use of their limbs, and by definition never achieve the ability to sit unsupported. Although SMA type 1 has the highest incidence among SMA phenotypes, it represents a smaller portion of a typical clinic population due to the limited life expectancy. Recent advances in proactive medical care has resulted in more SMA type 1 patients surviving past 2 years of age.

The intermediate form of the disease, termed “SMA type 2,” includes patients presenting between 6 and 18 months of age with hypotonia and motor delay, who are clinically classified by their ability to sit unsupported. Patients with SMA type 2 never become functional walkers but typically live into adulthood. Physical and occupational therapy are an integral part of the care of patients with SMA type 2 to maximize functional independence, integrate technology, and prevent orthopedic complications common in this cohort. Similarly, pulmonary complications are common, making proactive respiratory management essential.

The mildest SMA phenotype, type 3, also termed “Kugelberg–Welander disease,” presents after 18 months of age with symptoms of proximal weakness, mostly of the legs. Patients with SMA type 3 attain the ability to walk unaided. They complain of difficulty rising from the floor or low surfaces, running, and negotiating stairs. Fatigue is most common in this phenotype. Pulmonary complications are not common in SMA type 3, and patients typically have a normal life expectancy.

Children presenting with symmetric, proximal more than distal weakness that is greater in the legs than the arms, diminished or absent deep tendon reflexes, and preserved sensation should raise high suspicion for SMA. Tongue fasciculations and hand tremors are a common finding. The first diagnostic procedure for SMA is DNA analysis for the SMN gene deletion which has up to 95% sensitivity and 100% specificity, to detect homozygous deletions of SMN1 exon 7. If negative, electromyography (EMG) and nerve conduction studies, laboratory studies for muscle enzyme creatine kinase, and possibly muscle or nerve biopsy may be indicated to test for other motor neuron disorders, myopathies, neuropathies, or muscular dystrophies.

Once an SMA diagnosis has been confirmed, genetic inheritance of this autosomal recessive disorder should be reviewed with the family with referral for genetic counseling. Parental carrier testing and prenatal or preimplantation diagnosis are options for family planning. Testing of asymptomatic siblings is not recommended in the absence of a viable intervention. However, genetic counseling and carrier testing when unaffected siblings reach reproductive age are indicated.

Management of SMA

Family education is essential upon diagnosis. Multidisciplinary care should be implemented, ideally in a specialized neuromuscular clinic and, depending on disease severity, may include a neurologist, pulmonologist, gastroenterologist, physiatrist, physical and occupational therapists, and a communication specialist. For patients with infancy-onset SMA type 1, the prognosis should be discussed with the family so that they can make an informed choice between palliative multidisciplinary care and a more active approach which would include respiratory support and feeding through a gastrostomy tube. When palliative care is chosen for an SMA type 1 baby, the multidisciplinary care should ideally include a palliative care specialist, hospice care, and psychological support to the family, so that the patient’s and family’s suffering is minimized and a meaningful relationship and memories are encouraged. If proactive care is chosen for severe SMA type 1 patients or for all milder phenotypes, a multidisciplinary approach should be tailored to the patient and family’s individual needs. The following care areas should be considered.

Pulmonary Management

Impaired pulmonary function is the major cause of morbidity and mortality in SMA type 1 and more severe SMA type 2. The axial muscle weakness associated with SMA impairs chest wall mobility and development, resulting in poor lung growth. Impaired lung function is compounded by scoliosis, exacerbating respiratory compromise. Poor airway clearance, atelectasis, and potential swallowing dysfunction put SMA patients at high risk for recurrent infections. Proactive pulmonary care is an integral part of managing SMA patients. Routine immunizations including influenza, pneumococcal, and respiratory syncytial virus vaccines in those aged <2 years are highly recommended prophylaxis in this population. Maintaining adequate nutrition and hydration is also recommended to maximize pulmonary health.

Respiratory insufficiency is first manifest as nocturnal hypoventilation, with complaints of orthopnea, morning headaches, and daytime sleepiness. Typical assessments include pulse oximetry, polysomnography, end-tidal carbon dioxide, and transcutaneous CO₂ monitoring in infants and young children. Serum bicarbonate levels are typically normal despite respiratory insufficiency during sleep. Assessments of forced vital capacity in the seated and supine positions to monitor pulmonary function in patients aged >5 years with SMA types 2 and 3 should be performed routinely. Formal swallowing evaluations are indicated after acute unexplained respiratory failure or recurrent pneumonia.

If respiratory insufficiency is identified, chronic in-home treatment should be implemented. For poor airway clearance, the use of cough assist devices is recommended. High-frequency chest wall oscillation therapy is also used as an adjunct to help loosen and mobilize secretions. Hypoventilation is treated with noninvasive positive pressure ventilation. Tracheostomy and long-term mechanical ventilation is a treatment option in patients with respiratory failure who are not adequately maintained with noninvasive measures.

Overall, pulmonary issues are important to consider when managing SMA patients. For all SMA patients, perioperative care should include preoperative evaluation for respiratory status, infections, cough effectiveness, sleep-disordered breathing, asthma, aspiration, and reflux, because they are at high risk for post-anesthesia complications. These complications can include hypoventilation, atelectasis, impaired airway clearance, and poor cough, which can lead to prolonged intubation, infection, tracheostomy, and death. Postoperatively patients should be weaned to noninvasive ventilation as a transition to extubation with careful postoperative pain management.