Table 1 List of notable causes of male infertility and their relationship with impaired motility

Varicocele

Results in elevated levels of reactive oxygen species (ROS) which alters the testicular microenvironment, thereby causing reduced motility [7].

Damaged mitochondria result in abnormal expression of mitochondrial proteins, thereby decreasing ATP levels and altering calcium signalling cascade [8].

Genetic abnormalities

Defect in several genes coding for the proteins in the central axoneme apparatus, dynein proteins, as well as genes such as Spag 6, 16, 17 that are responsible for central axoneme function result in impaired motility [9,10,11].

Lifestyle choices

Lifestyle factors such as smoking, stress, and alcoholism may affect sperm parameters. Tobacco smoke containing traces of tar, carbon monoxide, polycyclic aromatic hydrocarbons, and heavy metals are known to influence motility [12].

Radiation

Causes genetic abnormalities and production of ROS. Catsper genes are known to be mutated due to exposure to radiation, thereby causing impaired calcium metabolism [13].

Heat exposure

Heat exposure downregulates mitochondrial activity and decreases ATP levels. Heat exposure also decreases antioxidant levels, alters protein expression, and causes mitochondrial degeneration [14].

Environmental Factors

Environmental chemicals, such as pesticides, polychlorinated biphenyls, bisphenol A, glycol ethers, perfluoronated compounds, dioxins and dioxin-like compounds, phthalates, heavy metals, dichloro-diphenyl-trichloroethane, and plasticizers are known to affect sperm motility [12].

Infections

Bacterial infections, leucocyte accumulation (leukocytospermia), antibody buildup, inflammation and oxidative stress are known to impair fertility. Infections reduce mitochondrial membrane potential and increases apoptosis [15].

Psychological stress

Hormones such as corticosterone suppress testosterone and inhibin, thereby altering the testicular microenvironment. Hormonal changes are known to affect motility [12].