Primary lateral sclerosis (PLS) is an idiopathic non-familial neurodegenerative disorder of upper motor neurones. It presents as a slowly progressive pyramidal tract syndrome, sometimes with marked pseudobulbar symptoms.

The diagnosis is based on exclusion of other causes. Pringle and colleagues¹ proposed diagnostic criteria in 1992 for PLS: adult onset; negative family history; duration of at least three years (to exclude motor neurone disease (ALS)); and normal blood, CSF, EMG, and MRI results. However, it remains an enigmatic, heterogeneous syndrome of uncertain nosology, for some clinically similar patients have varied patterns of symptom progression and physiology.² MRS may show a reduction of N–acetylaspartate/creatinine in the motor cortex. Modern technology can exclude other disorders with an accuracy of about 90%. It is now frequently accepted^3–5 as a benign ALS variant, yet some disagree.²

Often confused with ALS, its existence as an entity has been debated since the time of Pierre Marie. In his classic book, Lectures on diseases of the spinal cord,⁶ Pierre Marie observed:

“In 1875, Erb⁷ and a few months later Charcot described an affection with pronounced ‘spasmodic paresis’ [In the nineteenth century the word spasmodic was often used to mean spastic, not episodic.] in the extremities, without loss of sensation. Erb believe this was due to primary degeneration of the lateral columns of the cord. Charcot was of the same opinion. This idea as to the nature of spasmodic spinal paralysis (name given by Erb) or spasmodic tabes dorsalis (name given by Charcot), thus at once gave it the right of being included in nervous pathology, but owing to a singular misfortune the autopsies of adult patients in whom the diagnosis...had been made, there was found either disseminated sclerosis or transverse, or focal myelitis, or perhaps amyotrophic lateral sclerosis; not a single case in fact occurred in which primary degeneration of the pyramidal tract existed...thus during the last few years the diagnosis of spasmodic tabes dorsalis has scarcely ever been made.” p84

Marie went on to say: “The diagnosis should be excluded from a ‘nosological catalogue’ with the exception of congenital spastic rigidity of the limbs”, which he recognised as Little’s disease.

It remains mandatory to exclude: compressive lesions at the foramen magnum and cervical cord, MS, Chiari malformation, and human immunodeficiency virus or human T-lymphotrophic virus type I. In PLS and in ALS morphometry shows small pyramidal cells in the precentral gyrus; and in PLS, quantitative histopathology shows the neuronal degeneration confined to the corticospinal system, without involvement of lower motor neurones.¹ Its place in neurological taxonomy is still unclear.