The Hmax/Mmax Ratio as a Diagnostic Tool in Assessing Spasticity in a Patient with Hereditary Spastic Paraplegia

Hereditary spastic paraplegia (HSP) refers to a group of inherited diseases caused by progressive degeneration of the corticospinal tracts. We report a case of an HSP patient with ankle spasticity treated with an intrathecal baclofen (ITB) pump, for whom the Hmax/Mmax ratio was used as a diagnostic tool for spasticity. A man in his early 30s who was born without any complications and developed normally in childhood was diagnosed with HSP when he was 29 years old. Equinus gait pattern and bilateral genu recurvatum improved after manual therapy and botulinum toxin injections in both gastrocnemius muscles; however, after a few months, his gait disturbance became more severe as a natural course of the disease. To treat ankle spasticity and clonus, he was considered a suitable candidate for an ITB therapy. However, as spasticity is a finding that is easy to recognize but difficult to evaluate, we conducted electrophysiological testing, including H-reflex and the ratio of H-reflex amplitude to compound muscle action potential amplitude. The Hmax/Mmax ratio was 75.2% on the right side and 65.2% on the left side before an ITB pump, and 37.6% on the right side and 47.0% on the left side after an ITB pump. This case illustrates the usefulness of testing electrophysiological parameters such as the Hmax/Mmax ratio to measure spasticity objectively in late-onset HSP patients. The Hmax/Mmax ratio is also a good tool for measuring the degree of improvement in spasticity after ITB or additional treatment in these patients.