Spinal Muscular Atrophy (SMA)

Spinal muscular atrophy is a genetic condition in which the specialised nerve cells that drive the muscles, the lower motor neurones in the front of the spinal cord, are progressively lost. In almost all cases this stems from the loss of a working copy of a gene called SMN1, which produces a protein essential for the survival of these neurones. A second, nearly identical gene, SMN2, produces a small amount of the same protein, and the number of copies of SMN2 a person carries strongly influences how severe the disease is. When the motor neurones fail, the muscles they supply are no longer activated, and they become weak and wasted.

The condition is traditionally divided into types according to the age at which it begins and the milestones a child achieves. Type I, the most severe, begins in the first few months of life in infants who never sit unaided. Type II begins in later infancy in children who can sit but never walk independently. Type III begins after the child has learnt to walk, and type IV is a rare adult-onset form. This gradation matters enormously for the spine, because the earlier and more severe the weakness, the earlier and more aggressive the scoliosis tends to be.

Seen from the spine, the essential point is mechanical. The vertebral column is a flexible stack of bones that, in a healthy person, is held erect and balanced by a cradle of trunk muscles working constantly and almost unconsciously. In SMA those muscles are weak. The spine is left, in effect, without its scaffolding, and a young, growing, flexible spine that is not held straight will not stay straight.

Scoliosis is the dominant spinal problem in SMA, and in the more severe types it is very nearly universal. In children with type II disease, who can sit but not stand, and in many with the weaker end of type III, a progressive curvature of the spine develops in the great majority, and the published natural-history literature describes scoliosis in most non-ambulant children, often appearing within the first years of life. The weaker the child and the earlier the loss of the ability to stand and walk, the earlier and more severe the curve.

Because the deformity is driven by weakness acting on a growing spine throughout childhood, it characteristically begins early and continues to worsen so long as the child is growing, often accelerating sharply during the adolescent growth spurt. Neuromuscular curves of the kind seen in SMA arise from a global failure of trunk support, begin earlier, progress more reliably and to greater magnitudes, and very often involve the pelvis as well as the spine.

The neuromuscular curve in SMA is typically a long, sweeping, single C-shaped curve that runs through the thoracic and lumbar spine together. It is sometimes described as a collapsing curve, because the trunk has, in effect, folded to one side for want of muscular support. A defining feature of this pattern is that the curve does not stop at the spine but extends down into the pelvis. The base of the spine is no longer level, and one side of the pelvis is carried higher than the other. This is pelvic obliquity, and it is one of the most important consequences of the deformity.

Alongside the sideways curve there is often a loss of the normal front-to-back curves of the spine. The deformity is three-dimensional: the vertebrae rotate as they curve, and the rib cage on the concave side is drawn together while the ribs on the convex side splay apart. As the curve worsens, the trunk shortens and the lower ribs may come to rest close to, or against, the rim of the pelvis.

The spine and the lungs cannot be considered separately in SMA, and the respiratory dimension runs through every aspect of the spinal problem. The same weakness that fails to support the spine also weakens the muscles of breathing, particularly the intercostal muscles between the ribs, while the diaphragm is relatively spared. Children with the more severe forms therefore have a degree of respiratory weakness from the outset, quite apart from anything the spine does.

The deformity then makes this worse. As the thoracic spine curves and the vertebrae rotate, the rib cage is distorted, and the chest loses the room in which the lungs can expand. In a growing child this is compounded by the fact that the thorax is still developing, and a spine that is severely deformed during the years of growth can impair the growth of the lungs themselves, a problem sometimes described in terms of thoracic insufficiency. The published surgical literature consistently records that restrictive lung disease is common in these children and that pulmonary function tends to decline over time.

For most of the history of SMA there was no treatment that altered the underlying disease. That has changed with the arrival of disease-modifying therapies. Drugs that increase the amount of the missing SMN protein, including nusinersen, have improved survival and motor function, particularly in those treated early. More children with the severe forms now survive into and through childhood, and a direct consequence is that more children live long enough to develop, and to need attention for, a scoliosis that in earlier eras they might not have survived to develop.

The second effect is more particular. Nusinersen is given by injection into the cerebrospinal fluid in the lower back, by a lumbar puncture, repeated at intervals over years. A scoliosis distorts the anatomy and makes the puncture more difficult, and a spinal fusion, which locks the vertebrae together with bone and metal, can close off the very route the needle must take. The need to keep delivering this medicine through a spine that may need to be fused has become a genuine and recurring consideration in planning surgery.

One approach is surgical. At the time of the fusion, or as a separate procedure, a small window can be left or created in the bone of the lower lumbar spine, below the end of the spinal cord and between the rods, through which a needle can later be passed into the fluid. This has been described variously as a laminotomy or laminectomy window, or as a unilateral interlaminar fenestration, and the published technical accounts report that injections can be delivered through such a window over many years while the mechanical stability of the fusion is preserved.

A second approach is to rely on image guidance to find a path that already exists. Real-time ultrasound can be used to identify a gap between the bony layers and to steer the needle into it without radiation, an approach particularly suited to children. Where the bony anatomy is more complex, computed tomography or a cone-beam scan in the procedure room can be used to plan and guide the needle, sometimes by an unconventional route through the side of the spine.

In younger children, growth-friendly techniques aim to control the curve while allowing the spine and chest to keep growing. The classic form is the growing rod, in which rods are anchored to the spine, or to the spine and the pelvis, without fusing the vertebrae in between, and are lengthened periodically as the child grows. Magnetically controlled growing rods can be lengthened non-invasively, with an external device, sparing the child repeated surgery. The published experience in SMA reports that growing rods can reduce the curve and the pelvic obliquity, improve the height of the trunk and the space available for the lungs, and allow continued growth.

In older children, in whom most growth is complete, a definitive posterior spinal fusion is the more durable solution. The spine is straightened as far as is safe, instrumented with screws, hooks and rods, and fused with bone graft so that it becomes a single solid column held permanently in the corrected position. In the non-ambulant child with a curve that reaches into the pelvis, the fusion is usually carried down to the pelvis itself. The transition from a growth-friendly construct in early childhood to a definitive fusion at maturity is a recognised pathway.

Respiratory weakness dominates the risk. The same muscle weakness that causes the deformity weakens breathing, and many children have restrictive lung disease before any operation. Respiratory complications are among the most frequent after major spine surgery in SMA. The bone is often soft, because disuse and immobility leave the bones thin, and weak bone holds screws and hooks less securely, so the fixation of metalwork into the spine and pelvis must contend with poor bone quality.

These long operations on the whole length of the spine can involve significant blood loss, and children with SMA are often slight, with limited reserves and frequently with poor nutrition and low body weight. Access for future medicine is now part of the plan: the need to deliver intrathecal therapy for years afterwards influences how low the fusion extends and whether a window for future injections is preserved. The aims are framed around the whole child rather than the angle on an X-ray: a balanced, level, comfortable sitting position, relief of pressure on the skin, free hands and arms, and support of the chest.

The outlook in the spine of a child with SMA has changed substantially in recent years. For the severe forms, the arrival of disease-modifying therapy has improved survival and function, and as a result more children grow up to face the spinal deformity that profound weakness produces. The scoliosis itself remains, in the more severe types, a very common and progressive problem. Where the deformity is treated, the surgical literature is on the whole encouraging about what can be achieved: growth-friendly rods in younger children can control the curve while allowing growth to continue, and definitive fusion at maturity reliably restores a balanced, level and durable sitting position.

The shape of any one child's outlook is a matter for careful assessment by the treating team, drawing together the neurology, the respiratory state, the growth and the deformity, rather than something that can be read from a curve angle or a category alone.

This page is general medical education and is not a substitute for individual advice. New or worsening weakness, numbness, saddle numbness or any disturbance of bladder or bowel control requires urgent assessment.

• —Wijngaarde CA, Brink RC, de Kort FAS, et al. Natural course of scoliosis and lifetime risk of scoliosis surgery in spinal muscular atrophy. Neurology. 2019;93(2):e149–e158.
• —Coratti G, Pera MC, Lucibello S, et al. Systematic literature review of the natural history of spinal muscular atrophy: motor function, scoliosis, and contractures. Neurology. 2024;102(1):e207878.
• —Mercuri E, Finkel RS, Muntoni F, et al. Diagnosis and management of spinal muscular atrophy: Part 1. Neuromuscul Disord. 2018;28(2):103–115.
• —Finkel RS, Mercuri E, Meyer OH, et al. Diagnosis and management of spinal muscular atrophy: Part 2. Neuromuscul Disord. 2018;28(3):197–207.
• —Fujak A, Raab W, Schuh A, Richter S, Forst R, Forst J. Natural course of scoliosis in proximal spinal muscular atrophy type II and IIIa. BMC Musculoskelet Disord. 2013;14:283.
• —Mullender M, Blom N, De Kleuver M, et al. A Dutch guideline for the treatment of scoliosis in neuromuscular disorders. Scoliosis. 2008;3:14.
• —Zebala LP, Bridwell KH, Baldus C, et al. Minimum 5-year radiographic results of long scoliosis fusion in juvenile spinal muscular atrophy patients. J Pediatr Orthop. 2011;31(5):480–488.
• —Chandran S, McCarthy J, Noonan K, et al. Early treatment of scoliosis with growing rods in children with severe spinal muscular atrophy: a preliminary report. J Pediatr Orthop. 2011;31(4):450–454.
• —McElroy MJ, Shaner AC, Crawford TO, et al. Growing rods for scoliosis in spinal muscular atrophy: structural effects, complications, and hospital stays. Spine. 2011;36(16):1305–1311.
• —Lenhart RL, Youlo S, Schroth MK, et al. Radiographic and respiratory effects of growing rods in children with spinal muscular atrophy. J Pediatr Orthop. 2017;37(8):e500–e504.
• —Modi HN, Suh SW, Hong JY, Yang JH. Posterior multilevel vertebral osteotomy for severe and rigid idiopathic and neuromuscular scoliosis. Spine. 2011;36(15):1146–1153.
• —Sucato DJ. Spine deformity in spinal muscular atrophy. J Bone Joint Surg Am. 2007;89(Suppl 1):148–154.
• —Lee BH, Hwang J, Kim SY, et al. Outcomes of primary posterior spinal fusion for scoliosis in spinal muscular atrophy. J Pediatr Orthop. 2019;39(6):e399–e405.
• —Mesfin A, Sponseller PD, Leet AI. Spinal muscular atrophy: manifestations and management. J Am Acad Orthop Surg. 2012;20(6):393–401.
• —Catteruccia M, Vuillerot C, Vaugier I, et al. Orthopaedic management of scoliosis by Garches Brace and spinal fusion in SMA type II children. J Neuromuscul Dis. 2015;2(4):453–462.
• —Veerapandiyan A, Eichinger K, Guntrum D, et al. Scoliosis in spinal muscular atrophy type 1 in the nusinersen era. Neurol Clin Pract. 2022;12(4):e60–e65.
• —Pino MS, Sugarman EA, Fasano A, et al. Spinal muscular atrophy scoliosis in the era of background therapies — a review of the literature. J Clin Med. 2024;13(11):3279.
• —Towbin RB, Schaefer CM, Kaye RD, et al. Lumbar laminotomy for the intrathecal administration of nusinersen for spinal muscular atrophy. J Neurosurg Spine. 2019;31(2):217–221.
• —Cartwright MS, Walker FO, Strickland CD, et al. Ultrasound-guided interlaminar approach for nusinersen administration in patients with spinal muscular atrophy with spinal fusion or severe scoliosis. Orphanet J Rare Dis. 2023;18(1):29.
• —Spiliopoulos S, Reppas L, Zompola C, et al. Unilateral interlaminar fenestration on the convex side provides a reliable access for intrathecal administration of nusinersen in spinal muscular atrophy. Eur Spine J. 2024.
• —Geraci AP, Black K, Jin M, et al. Surgical access for intrathecal therapy in spinal muscular atrophy with spinal fusion: long-term outcomes of lumbar laminectomy. J Clin Med. 2025;14(12):4280.
• —Stolte B, Bois JM, Bolz S, et al. Radiation dose reduction for CT-guided intrathecal nusinersen administration in adult patients with spinal muscular atrophy. Sci Rep. 2020;10:3406.
• —Nascene DR, Ozutemiz C, Estby H, McKinney AM, Rykken JB. Transforaminal thecal sac puncture for the administration of nusinersen in patients with spinal muscular atrophy and spinal fusion. AJNR Am J Neuroradiol. 2019;40(7):1252–1255.
• —Granata C, Merlini L, Magni E, Marini ML, Stagni SB. Spinal muscular atrophy: natural history and orthopaedic treatment of scoliosis. Spine. 1989;14(7):760–762.
• —Chng SY, Wong YQ, Hui JH, et al. Pulmonary function and scoliosis in children with spinal muscular atrophy types II and III. J Paediatr Child Health. 2003;39(9):673–676.

This information is for general patient education and is not a substitute for an individual consultation. Always seek personalised advice from a qualified spinal specialist.