It has been difficult to determine the natural course of post-polio syndrome because of its slow progression with plateaus. However, weakness does lend itself to objective analysis. Mulder and colleagues (Mulder et al 1972) reported continuous progression of weakness over 12 years of follow-up. Dalakas and colleagues (Dalakas et al 1986) used Medical Research Council grading and noted both steady and stepwise progression of weakness at an average rate of only 1% per year over a mean follow-up period of 12.2 years. Some studies with shorter follow-up periods of 2 years (Ivanyi et al 1996), 2.5 years (Allen et al 1997), and 5 years (Windebank et al 1996) did not demonstrate progression of the weakness. However, the latter group did demonstrate progression when patients were followed for a much longer period of 15 years (Sorenson et al 2005). Two mechanically quantitated studies, with 4-year follow-up periods, found rates of progression of 2% per year (Agre et al 1995; Stalberg and Grimby 1995). A quantitative study of only 9 months duration found that the annual percentage decrease in strength varied from 11.2% for polio survivors in their early 40s to 17.5% for those in their late 60s and early 70s (Klein et al 2000b). Significant objective clinical weakening has been noted by others (Munsat 1991), including weakness of bulbar musculature (Sonies and Dalakas 1995). The prognosis is good when diaphragmatic paralysis is unilateral. It often goes unrecognized and, in the absence of any underlying lung disease, usually needs no treatment. Bilateral diaphragmatic paralysis can cause dyspnea, cor pulmonale, and ventilatory failure. Pulmonary function tests show a restrictive process, with symptoms worse on supine posture. In addition to chest x-ray, chest fluoroscopy, phrenic nerve conduction studies, and diaphragmatic electromyography help to confirm the diagnosis. Diaphragmatic pacing, placation of the diaphragm, tracheostomy, and mechanical ventilation might be needed alone or in combination (Qureshi 2009).
Complications of post-polio syndrome most often relate to spine disease. Arthritis (spondylosis) of the spine and herniated discs may result in radiculopathies. These complications respond somewhat to conservative treatment and more often to surgical intervention although this may be short lived. Kyphoscoliosis may occur, which may result in respiratory and cardiac failure (Lin et al 2001). At this stage, treatment is primary supportive. Bulbar muscle weakness may predispose to aspiration and pneumonia. This occurs in a small minority and can be treated with pulmonary toilet and antibiotics. Psychiatric disability may also occur (Kemp and Krause 1999). Less serious medically but more serious financially are functional decline and job loss (McNeal et al 1999).