The jaw jerk may be present or exaggerated at a time when the muscles of mastication are markedly weak. In fact, spasticity of the jaw muscles may be so pronounced that the slightest tap on the chin will evoke clonus and blinking; rarely, attempts to open the mouth elicit a “bulldog” reflex (jaw snaps shut involuntarily). Spastic weakness of the oropharyngeal muscles may be the initial manifestation of bulbar palsy and may at times surpass signs of atrophic weakness; pseudobulbar signs (pathologic laughing and crying) may reach extreme degrees. This is the only common clinical situation in which spastic and atrophic bulbar palsy coexist. Strangely, the ocular muscles always escape.
As with other forms of motor system disease, the course of bulbar palsy is inexorably progressive. Eventually the weakness spreads to the respiratory muscles and deglutition fails entirely; the patient dies of inanition and aspiration pneumonia, usually within 2 to 3 years of onset. Approximately 25 percent of cases of motor system disease begin with bulbar symptoms, but rarely, if ever, does the sporadic form of progressive bulbar palsy run its course as an independent syndrome (pure heredofamilial forms of progressive bulbar palsy in the adult are known, for example, Kennedy disease, discussed further on). In general, the earlier the onset of the bulbar involvement, the shorter the course of the disease.
This entity can be considered a special form of motor neuron disease, although many authorities classify it as a unique degenerative process. Many patients in whom the signs of corticospinal tract degeneration suggest the presence of ALS will develop indications of lower motor neuron disease within 1 year, usually earlier. Approximately 20 percent, however, have a slowly progressive corticospinal tract disorder that begins with a pure spastic paraparesis; later, the arms and oropharyngeal muscles become involved and the disease remains one solely of the upper neurons. These cases have distinctive neuropathologic features and are designated as primary lateral sclerosis (PLS), a term originally suggested by Erb in 1875. A historical review of the subject appears in the article by Pringle and colleagues.
The typical case begins insidiously in the fifth or sixth decade with a stiffness in one leg, then in the other; there is a slowing of gait, with spasticity predominating over weakness as the years go on. Walking is still possible with the help of a cane for many years after the onset, but eventually this condition acquires the characteristic features of a severe spastic paraparesis. Over the years, finger movements become slower, the arms become spastic, and, if the illness persists for decades, speech takes on a pseudobulbar lilt. There are no sensory symptoms or signs. The legs are often found to be surprisingly strong, the difficulty in locomotion being attributable to rigid spasticity. About half the patients eventually acquire spasticity of the bladder. Pringle and associates suggest that a diagnostic criterion of the disease is progression for 3 years without evidence of lower motor neuron dysfunction.
Pathologic studies in a limited number of cases have disclosed a relatively stereotyped pattern of reduced numbers of Betz cells in the frontal and prefrontal motor cortex, degeneration of the corticospinal tracts, and preservation of motor neurons in the spinal cord and brainstem (Beal and Richardson; Fisher; Pringle et al). The corticospinal tract lesions are identical to those in typical ALS. Whether some of these cases are examples of late-onset familial spastic paraplegia (see earlier) has not been extensively explored with molecular techniques.