Somairle's final campaign appears similar to later Norwegian-backed invasions of the Firth of Clyde conducted by his descendants in the thirteenth century (1230 and 1263). In fact, the Viking sack of Alt Clut in 870 may parallel these invasions, since it is possible that Dublin-based Vikings destroyed the fortress of Alt Clut (Dumbarton Castle) in an effort to nullify a threat posed by the Strathclyde Britons. Another factor that may have spurned Somairle to attack the Scots could have been Malcolm's increasingly poor health. Certainly, the "Chronicle of Melrose" states that the Scottish king was stricken with a "great sickness" in 1163, and it is possible that he never fully recovered. The fact that the "Annals of Ulster" accords Malcolm the epithet ""Cennmor"" ("Big Head") upon his death could be evidence that he suffered from Paget's disease. One possibility is that the king's impairment was opportunistically seized upon by Somairle, who overestimated a weakening in Scottish royal power.