Congenital Insensitivity to Pain (HSAN IV)

Congenital insensitivity to pain with anhidrosis is one of a small family of conditions, the hereditary sensory and autonomic neuropathies, in which the sensory and autonomic nervous systems are abnormal from birth. In the type designated HSAN IV the fault lies in the NTRK1 gene, which carries the instructions for a receptor called TrkA. This receptor sits on the surface of particular nerve cells and receives a chemical messenger, nerve growth factor, that tells those cells to grow, mature and survive. When the receptor does not work, the nerves that carry pain and temperature sensation never properly form, and the nerves that drive sweat glands fail, the feature termed anhidrosis.

It is worth being clear about what is preserved and what is lost. The sense of light touch, of vibration and of joint position is usually largely intact, because these travel by different, larger nerve fibres. What is missing is the protective sense, the sharp signal that says an action is harmful and must stop. Pain is the mechanism by which a normal spine protects itself, the reason a person shifts position before a joint is overloaded, guards a strained back, rests a healing fracture and seeks help when something is seriously wrong. Remove that signal and the spine loses its capacity for self-protection.

The most characteristic and serious spinal manifestation is a destructive arthropathy known as a neuropathic spine, or Charcot spine. It is the spinal expression of a principle seen throughout the body in conditions that abolish protective sensation: a joint that cannot feel pain is a joint that is destroyed by its own use.

The mechanism is one of repetitive, unperceived microtrauma. Each movement and load that a normal spine would moderate through the discomfort it produces is, in the insensate spine, taken to and beyond its limit without restraint. The small joints and the discs, and the bone of the vertebrae themselves, are subjected again and again to forces that would ordinarily be guarded against. Over months and years the cartilage wears, the bone fragments, the discs collapse, and the architecture of one or more spinal segments breaks down. The body's attempt at repair is disordered, producing new bone, debris and fluid around a joint that continues to be loaded, so that the segment becomes simultaneously destroyed and grossly disorganised.

The radiological picture is often dramatic and out of all proportion to the symptoms. Because the person feels little or nothing, a vertebral segment may be largely destroyed, with collapse, fragmentation, sclerosis and abnormal new bone, before the problem declares itself. It frequently comes to attention not through pain but through a deformity that becomes visible, a swelling, an audible or palpable clunking as the unstable segment moves, a loss of height or function, or, in the most serious cases, the gradual development of pressure on the spinal cord or nerves.

In the general population a vertebral fracture announces itself with pain, and that pain is what causes a person to rest, to protect the back and to seek assessment, all of which give the fracture the chance to heal in a good position. In congenital insensitivity to pain none of that happens. A fracture of a vertebra may occur during ordinary activity, or after a minor stumble, and produce no signal at all. The person continues to move, to bear weight and to load the injured bone exactly as before.

A fracture that is never rested is a fracture that continues to be displaced, that may not heal, or that heals in a deformed position. Repeated loading of an unhealed vertebral injury can merge with the neuropathic process described above, so that what begins as a single unnoticed fracture becomes the seed of a wider segmental collapse and destruction.

Deformity of the spine is common in congenital insensitivity to pain. Scoliosis, a sideways curvature with rotation, and kyphosis, an excessive forward bend, are both described, and they arise through more than one route. In part, deformity is the visible end result of the destructive and fracturing processes already described: when segments collapse, fragment or heal in a deformed position, the overall alignment of the spine is thrown out. In part, it reflects the loss of the normal, sensation-guided muscular control that keeps the growing spine balanced.

What sets deformity in this condition apart is its tendency to progress in the setting of insensate, fragile bone and disordered healing, and the fact that the usual safeguards against worsening are absent. A curve that would be guarded, braced and monitored against a background of normal sensation here progresses in a spine that gives no warning.

The first difficulty is the bone and the segment itself. The neuropathic, Charcot process leaves bone that is fragmented, disorganised and abnormally remodelled, a poor foundation into which to anchor screws and across which to achieve a solid fusion. The destructive process that necessitated surgery does not switch off because an operation has been performed; the same insensate loading that destroyed the segment continues to act on the reconstruction.

The second difficulty is healing. The case series describe high rates of non-union, or pseudarthrosis, where the bone simply fails to bridge. Where fusion does not take, the metalwork is left to carry load indefinitely and eventually loosens, breaks or pulls out of the bone, a sequence documented again and again in reports of repeated hardware failure and migration.

The third difficulty is infection. These patients are prone to wound problems and to deep infection, a tendency rooted both in the autonomic and skin abnormalities of the condition, including the absence of sweating, and in the difficulty of detecting an infection that produces no pain. The orthopaedic literature on spinal deformity correction in this group specifically notes high rates of infection and reoperation.

The fourth difficulty is the cycle of recurrent breakdown. Because the underlying driver, the insensate, unprotected loading of the spine, persists after any operation, reconstructions are at constant risk of breaking down again at the same or an adjacent level, so that surgery in this condition is frequently not a single definitive event but a sequence of interventions over years.

Assessment rests heavily on imaging, precisely because the usual clinical signal of pain is absent. Plain X-rays, including standing and bending views, are the foundation, used to document deformity and to reveal destruction, collapse, fragmentation and abnormal new bone. Computed tomography gives the detailed bony picture needed to understand the extent of destruction. Magnetic resonance imaging is used to assess the soft tissues, the discs, and above all whether the spinal cord or nerves are being compressed by an unstable or collapsing segment. Because infection is both common and difficult to detect by symptoms in this group, imaging and laboratory assessment also carry a particular weight in identifying it.

The evidence base for managing the spine in congenital insensitivity to pain is built almost entirely on individual case reports, small case series and reviews of those reports. The literature describes a tension at the heart of management. On one hand, a destroyed, unstable or deforming spinal segment, particularly one threatening the spinal cord or nerves, may appear to demand surgical stabilisation. On the other, the same reports document just how poorly this spine tolerates surgery.

Where surgery is described, the principles that emerge from the case literature are those of robust, often extensive fixation across the destroyed segment, attention to achieving fusion in a hostile biological environment, vigilance for infection, and an expectation that revision may be needed. Several long-term reports document the reality that a single operation rarely settles the matter and that ongoing destruction may require staged or repeated intervention.

The outlook is shaped, above all, by the fact that the underlying cause is permanent. The genetic fault that abolishes protective sensation does not change, and so the tendency to unperceived injury, destruction and deformity persists throughout life. That said, the picture is not uniformly bleak, and a great deal depends on the circumstances of the individual: how early the spine's vulnerability is recognised, whether deformity and destruction can be limited before they become severe, and, where surgery is undertaken, whether a fusion can be achieved and held.

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.

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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.