A Modern Approach to Hereditary Spastic Paraplegia

Hereditary Spastic Paraplegia (HSP) is a group of inherited neurodegenerative disorders primarily affecting the long descending motor pathways, especially the corticospinal tracts. Hallmark features include progressive lower limb spasticity, weakness, hyperreflexia, gait impairment and reduced selective motor control.

The defining biological feature is usually length-dependent degeneration of the corticospinal system, meaning that the longest axons in the nervous system are usually affected first. This is why HSP is often described as a “dying-back axonopathy”, and typically manifests in the legs before the arms, and distally before proximally.

The primary symptoms my client Elizabeth was dealing with, especially with her specific sub-type of HSP - HSP type 8 - was lower limb spasticity and gait dysfunction. After trying traditional approaches including stretches and strengthening exercises, her symptoms often became worse, rather than better.

In my experience, when dealing with spasticity most rehab clinicians often oversimply things. Spasticity is not just “tight muscles”, it’s a systems-level problem involving a loss of descending inhibitory control. 

The corticospinal tract does far more than just activate muscles, it also:

• Shapes spinal reflexes

• Suppresses unwanted muscles activity

• Refines reciprocal inhibition

• Improves motor selectivity

• Regulates spinal excitability

When corticospinal input degenerates, the spinal cord becomes disinhibited. As a result, several things likely happen simultaneously:

• Increased stretch reflex gain → muscle spindle input becomes over-amplified, meaning small stretches trigger exaggerated contractions, velocity-dependent resistance, and clonus

• Reduced reciprocal inhibition → normally agonist activation suppresses antagonist activity. In HSP, antagonist co-contraction increases, movement becomes stiffened, and energy cost rises

• Persistent inward currents (PICs) → chronic loss of descending input may alter spinal motor neuron excitability. Motor neurons become ‘sticky’ and easier to activate repeatedly, contributing to sustained tone, spasms and involuntary co-contraction

• Reticulospinal dominance → as corticospinal precision declines, the nervous system increasingly relies on brainstem pathways, especially reticulospinal systems. These systems are broader, less fractioned, and more synergistic, likely contributing to mass movement patterns and difficulty with isolated distal control

In HSP, weakness and spasticity also co-exist, but weakness is not purely disuse. It’s also partly:

• Impaired descending drive

• Impaired recruitment

• Altered motor unit behaviour

• Inefficient coordination

Whilst strength can help improve force capacity, excessive load or fatigue may worsen spinal excitability, co-contraction and compensatory stiffness. This is exactly why Elzabeth’s spasticity was often worsened with strength training, meaning the key is usually to find the correct dosage for each individual.

Similar to most other neurological conditions, sensory contributions are also underappreciated in HSP. In my experience, most HSP patients also have impaired proprioception. This can significantly affect someone’s gait as the nervous system compensates with stiffness, visual dependence and co-contraction as it seeks safety and stability from a body that it feels disconnected to.

Spasticity is often partly a stability solution

This solution can be described as ‘maladaptive neuroplasticity’, as the nervous system seeks to adapt around the degradation occurring because of disease progression. From a rehab perspective, our work likely works less by ‘fixing damage’, and more by improving movement efficiency, preserving adaptability, maintaining optionality, and improving nervous system confidence. In other words, we’re creating ‘adaptive neuroplasticity’ to counteract the maladaptive compensations.

So what tends to work well clinically?

Gait Specificity

Walking is a skill. Patients need exposure, variability, context, confidence and graded challenge.

Selective Motor Control Training

Learning to move and control key areas of the body that contribute to improved functionality, especially:

• Foot to ground interaction

• Ankle-knee-hip coordination

• Trunk/pelvis control

Fatigue Management

Over-training often worsens stiffness, spasms and gait quality. The sweet spot is usually enough stimulus for adaptation, but not enough for prolonged excitability escalation.

Variability & Adaptability

Rigid systems become fragile systems.

Useful training often includes variable gait environments, perturbation, rhythm changes, sensory manipulation, and task adaptability. The goal is not perfect movement, it’s adaptable movement.

Sensory Work

Including proprioceptive cueing, tactile input, Neubie stimulation, visual dependency reduction and rhythm/auditory inputs can help the brain manage its internal and external environment better. 

Aerobic Conditioning

Huge, but often overlooked in neurorehab. Aerobic conditioning improved fatigue tolerance, autonomic regulation, movement capacity and recovery.

The Takeaway

HSP is less about tight muscles or isolated weakness, and more about a progressive loss of high-fidelity motor control. The nervous system responds by simplifying movement, increasing stiffness, reducing variability, and prioritising stability over adaptability. Spasticity and weakness are often emergent compensations, not merely pathology - and so removing spasticity and/or increasing strength does not always lead to improved movement.

If the traditional model was to “reduce tone”, and “increase strength”, the modern systems view should be to improve movement adaptability, efficiency, confidence and optionality whilst managing excitability and progressive biological constraints.

It’s this modern approach that has helped my client Elizabeth reduce stiffness, whilst improving her walking and most importantly her confidence since we started working together in January.